1859 2026
This is an AI modernization of On the Origin of Species into contemporary English. The original ebook is available from Standard Ebooks.
Lamarck was the first person whose conclusions on this subject attracted serious attention. This deservedly famous naturalist first published his views in 1801, expanded them significantly in 1809 in his Philosophie Zoologique, and later, in 1815, in the introduction to his Natural History of Invertebrates. In these works he argues that all species, including humans, are descended from other species. He performed the great service of drawing attention to the likelihood that all change in the living world — just as in the non-living world — results from natural laws rather than miraculous intervention. Lamarck seems to have been led to his conclusion about the gradual change of species mainly by the difficulty of telling species apart from varieties, by the nearly seamless gradation of forms in certain groups, and by the analogy with domesticated organisms. As for the means of change, he credited some influence to the direct effects of environmental conditions, some to the crossing of existing forms, and a great deal to use and disuse — that is, to the effects of habit. He seems to have attributed all the beautiful adaptations we see in nature to this last mechanism — for example, the giraffe's long neck for reaching tree branches. But he also believed in a law of progressive development, and since all life forms tend to advance, he argued that the reason simple organisms still exist today is that they are being spontaneously generated all the time.
Geoffroy Saint-Hilaire, according to the biography written by his son, suspected as early as 1795 that what we call species are really just different versions of the same basic type. But it wasn't until 1828 that he published his belief that the same forms have not been perpetuated unchanged since the beginning of things. Geoffroy seems to have relied mainly on environmental conditions — what he called the "monde ambiant" — as the cause of change. He was cautious in his conclusions and didn't believe that today's species are currently being modified. As his son adds (originally in French): "This is a problem to be left entirely for the future — assuming the future will even be able to solve it."
In 1813, Dr. W. C. Wells read a paper before the Royal Society titled "An Account of a White Female, Part of Whose Skin Resembles That of a Negro," though it wasn't published until 1818, when it appeared alongside his famous essays on dew and single vision. In this paper, he clearly recognizes the principle of natural selection — the first recognition of it that anyone has pointed out — but he applies it only to human populations and only to certain traits. After noting that Black and mixed-race people have immunity to certain tropical diseases, he observes, first, that all animals tend to vary to some degree, and second, that farmers improve their domesticated animals through selective breeding. Then he adds that what breeders accomplish through deliberate effort "seems to be done with equal effectiveness, though more slowly, by nature, in producing varieties of humanity fitted for the country they inhabit. Among the chance variations that would occur in the first scattered inhabitants of central Africa, some individuals would be better suited than others to withstand the region's diseases. This population would multiply, while the others would decline — not only because they couldn't resist disease, but because they couldn't compete with their more vigorous neighbors. The color of this vigorous population would, I take for granted from what I've already said, be dark. But since the tendency to produce variations would still exist, darker and darker populations would appear over time. And since the darkest would be the best fitted for the climate, they would eventually become the most common, if not the only population, in the region where they originated." He then extends these same ideas to the white inhabitants of colder climates. I'm grateful to Mr. Rowley of the United States, who drew my attention to this passage in Dr. Wells' work through Mr. Brace.
The Honorable and Reverend W. Herbert, later Dean of Manchester, in the fourth volume of the Horticultural Transactions (1822) and in his work on the Amaryllidaceae (1837, pages 19 and 339), declares that "horticultural experiments have established beyond any possibility of refutation that botanical species are merely a higher and more permanent class of varieties." He extends the same view to animals. The Dean believed that single species of each genus were created in an originally very flexible state, and that these produced all our existing species — mainly through crossbreeding, but also through variation.
In 1826, the zoologist Professor Grant, in the concluding paragraph of his well-known paper on the freshwater sponge Spongilla (Edinburgh Philosophical Journal, vol. XIV, page 283), clearly states his belief that species descend from other species and improve through the process of change. He expressed the same view in his Fifty-fifth Lecture, published in the Lancet in 1834.
In 1831, Mr. Patrick Matthew published his work on Naval Timber and Arboriculture, in which he presents precisely the same view on the origin of species as the one put forward by Mr. Wallace and me in the Linnean Journal, and as the one developed at length in the present book. Unfortunately, Matthew stated this view only briefly, in scattered passages in an appendix to a book on a different subject, so it went unnoticed until Matthew himself called attention to it in the Gardeners' Chronicle on April 7, 1860. The differences between Matthew's views and mine aren't very significant: he seems to think that the world was nearly depopulated at successive periods and then restocked, and he suggests as an alternative that new forms might be generated "without the presence of any mold or germ of former aggregates." I'm not sure I fully understand some of his passages, but he seems to give a lot of weight to the direct effects of environmental conditions. He clearly saw, however, the full force of the principle of natural selection.
The celebrated geologist and naturalist Von Buch, in his excellent Physical Description of the Canary Islands (1836, page 147), clearly expresses his belief that varieties slowly become permanent species that are no longer able to interbreed.
Rafinesque, in his New Flora of North America, published in 1836, wrote (page 6): "All species might have been varieties once, and many varieties are gradually becoming species by taking on constant and distinctive characters." But further on (page 18) he adds, "except the original types or ancestors of the genus."
In 1843–44, Professor Haldeman (Boston Journal of Natural History of the United States, vol. IV, page 468) ably presented the arguments for and against the hypothesis that species develop and change over time. He seems to lean toward the side of change.
The Vestiges of Creation appeared in 1844. In the tenth and greatly improved edition (1853), the anonymous author says (page 155): "The proposition, arrived at after much consideration, is that the various series of living things, from the simplest and oldest up to the most complex and most recent, are — under the providence of God — the results, first, of an impulse imparted to living forms, advancing them over definite periods, through reproduction, up through levels of organization ending in the highest flowering plants and vertebrates (these levels being few in number and generally separated by gaps in biological character that make it hard to trace relationships); and second, of another impulse connected with the life force, tending over generations to reshape organisms in response to external conditions like food, habitat, and weather — these being the 'adaptations' of the natural theologian." The author apparently believes that organisms advance in organization through sudden leaps, while the effects of environmental conditions are gradual. He argues forcefully, on general grounds, that species are not fixed and unchanging. But I can't see how the two supposed "impulses" explain, in any scientific way, the countless beautiful mutual adaptations we see throughout nature. I can't see that we gain any real understanding of how, for instance, a woodpecker became so perfectly suited to its particular way of life. The book, thanks to its powerful and brilliant style — though the early editions showed little accurate scientific knowledge and a serious lack of scientific caution — immediately reached a very wide audience. In my opinion, it did excellent service in this country by drawing attention to the subject, removing prejudice, and preparing the ground for similar views.
In 1846, the veteran geologist M. J. d'Omalius d'Halloy published a short but excellent paper (Bulletins de l'Académie Royale de Bruxelles, vol. XIII, page 581) arguing that it is more likely that new species have been produced by descent with modification than that they were separately created. He had first put forward this opinion in 1831.
Professor Owen, in 1849 (Nature of Limbs, page 86), wrote: "The archetypal idea was made manifest in living form under various modifications on this planet, long before the animal species that actually display it came into existence. What natural laws or secondary causes may govern the orderly succession and progression of such organic phenomena, we are as yet ignorant." In his address to the British Association in 1858, he speaks (page li) of "the axiom of the continuous operation of creative power, or of the ordained becoming of living things." Later in the same address (page xc), after discussing geographical distribution, he adds: "These facts shake our confidence in the conclusion that the Apteryx of New Zealand and the Red Grouse of England were distinct creations made in and for those islands respectively. It is also worth bearing in mind that when the zoologist says 'creation,' he means 'a process he knows not what.'" He develops this idea further by adding that when cases like the Red Grouse are "cited by the zoologist as evidence of a distinct creation of the bird in and for such islands, he is mainly expressing that he doesn't know how the Red Grouse came to be there, and there exclusively — and by putting it that way, he is also expressing his belief that both the bird and the islands owe their origin to a great first Creative Cause." If we read these sentences from the same address in light of one another, it seems that this eminent thinker felt in 1858 that his confidence was shaken — that the Apteryx and the Red Grouse first appeared in their respective homes by some process "he knew not what."
This address was delivered after the papers by Mr. Wallace and me on the origin of species, which I'll discuss shortly, had been read before the Linnean Society. When the first edition of this book was published, I was so completely taken in — as were many others — by phrases like "the continuous operation of creative power" that I listed Professor Owen along with other paleontologists as firmly convinced that species don't change. But it turns out (Anatomy of Vertebrates, vol. III, page 796) that this was a ridiculous error on my part. In the last edition of this book, I concluded — and the conclusion still seems perfectly fair to me — from a passage beginning with the words "no doubt the type-form," etc. (vol. I, page xxxv), that Professor Owen acknowledged that natural selection may have played some role in forming new species. But this too, it turns out (vol. III, page 798), is inaccurate and unsupported. I also quoted some extracts from a correspondence between Professor Owen and the editor of the London Review, which seemed clearly — both to the editor and to me — to show that Professor Owen was claiming to have proposed the theory of natural selection before I did. I expressed my surprise and satisfaction at this claim. But as far as I can make sense of certain recently published passages (vol. III, page 798), I have either partly or entirely fallen into error once again. It's some comfort to me that other people find Professor Owen's polemical writings just as hard to understand and to square with each other as I do. In any case, as far as simply stating the principle of natural selection goes, it doesn't really matter whether Professor Owen came before me or not, because both of us — as this historical sketch shows — were preceded long ago by Dr. Wells and Mr. Matthew.
Isidore Geoffroy Saint-Hilaire, in his lectures of 1850 (summarized in the Revue et Magazine de Zoologie, January 1851), briefly explains why he believes that the defining traits of each species (originally in French) "remain fixed as long as the species persists under the same conditions; they change if the surrounding conditions change. In short, observation of wild animals already demonstrates the limited variability of species. Experiments on wild animals that have become domesticated, and on domesticated animals that have returned to the wild, demonstrate it even more clearly. These same experiments prove, moreover, that the differences produced can be significant enough to constitute differences at the genus level." In his General Natural History (vol. II, page 430, 1859) he develops similar conclusions further.
From a circular recently issued, it appears that Dr. Freke, in 1851 (Dublin Medical Press, page 322), proposed the doctrine that all living things have descended from one original form. His reasoning and his treatment of the subject are entirely different from mine. But since Dr. Freke has now (1861) published his essay on the Origin of Species by Means of Organic Affinity, any attempt on my part to summarize his views would be unnecessary.
Herbert Spencer, in an essay originally published in the Leader in March 1852 (and republished in his Essays in 1858), compared the theories of creation and evolutionary development of living things with remarkable skill and force. He argues from the analogy of domesticated organisms, from the changes that embryos of many species undergo during development, from the difficulty of distinguishing species from varieties, and from the principle of general gradation, that species have been modified. He attributes these modifications to changed environmental conditions. In 1855, Spencer also applied the principle of gradual acquisition through stages to psychology — arguing that each mental power and capacity is built up step by step.
In 1852, the distinguished botanist M. Naudin expressly stated in an admirable paper on the origin of species (Revue Horticole, page 102; later partly republished in the Nouvelles Archives du Muséum, vol. I, page 171) his belief that species form in a way analogous to how varieties are produced under cultivation — and that the latter process depends on the breeder's power of selection. But he doesn't show how selection works in nature. Like Dean Herbert, he believes that species were more flexible in their early stages than they are now. He puts great emphasis on what he calls the principle of finality — (originally in French) "a mysterious, undefined power; fate for some people, divine providence for others, whose unceasing action on living things determines, at every period of the world's existence, the form, the size, and the lifespan of each organism, according to its destiny in the order of things to which it belongs. It is this power that harmonizes each part with the whole, by fitting it to the function it must fulfill in the general system of nature — a function that is its reason for being."
In 1853, the celebrated geologist Count Keyserling (Bulletin de la Société Géologique, 2nd series, vol. X, page 357) suggested that just as new diseases, supposedly caused by some infectious agent, have arisen and spread across the world, so at certain periods the seeds of existing species may have been chemically altered by particular molecules in the surrounding environment, giving rise to new forms.
That same year, 1853, Dr. Schaaffhausen published an excellent pamphlet (Proceedings of the Natural History Society of the Prussian Rhineland, etc.) in which he argues for the gradual development of life on Earth. He concludes that many species have remained unchanged for long periods, while a few have been modified. He explains the distinctness of species by the destruction of intermediate, graded forms. "Thus living plants and animals are not separated from extinct ones by new creations, but should be regarded as their descendants through continuous reproduction."
The well-known French botanist M. Lecoq writes in 1854 (Studies in Botanical Geography, vol. I, page 250; originally in French): "We can see that our research on whether species are fixed or variable leads us directly to the ideas put forward by two justly famous men, Geoffroy Saint-Hilaire and Goethe." Some other passages scattered through Lecoq's large work make it somewhat unclear how far he extends his views on the modification of species.
The Reverend Baden Powell treated the "Philosophy of Creation" in a masterful way in his Essays on the Unity of Worlds (1855). Nothing could be more striking than the way he shows that the appearance of new species is "a regular, not a random phenomenon," or as Sir John Herschel puts it, "a natural as opposed to a miraculous process."
The third volume of the Journal of the Linnean Society contains papers read on July 1, 1858, by Mr. Wallace and me, in which — as noted in the introductory remarks to that volume — the theory of natural selection is presented by Mr. Wallace with admirable force and clarity.
Von Baer, the great embryologist toward whom all zoologists feel deep respect, expressed around 1859 (see Professor Rudolph Wagner, Zoological-Anthropological Investigations, 1861, page 51) his conviction — based mainly on the laws of geographical distribution — that forms now completely distinct have descended from a single parent form.
In June 1859, Professor Huxley gave a lecture at the Royal Institution on "Persistent Types of Animal Life." Discussing such cases, he remarks: "It is difficult to make sense of facts like these if we suppose that each species of animal and plant, or each major type of organization, was formed and placed on the surface of the globe at long intervals by a distinct act of creative power — and it's worth remembering that such an assumption is as unsupported by tradition or revelation as it is contrary to the general pattern of nature. If, on the other hand, we view 'Persistent Types' in light of the hypothesis that species alive at any given time are the result of gradual modification of earlier species — a hypothesis which, though unproven and badly damaged by some of its supporters, is still the only one that physiology gives any support to — then their existence seems to show that the amount of change living things have undergone during geological time is actually very small compared to the whole series of changes they have been through."
In December 1859, Dr. Hooker published his Introduction to the Australian Flora. In the first part of this important work, the botanist Joseph Hooker accepts the truth of descent with modification and supports the doctrine with many original observations.
The first edition of this work was published on November 24, 1859, and the second edition on January 7, 1860.
When I was serving as the naturalist aboard H.M.S. Beagle, I was deeply struck by certain facts about the distribution of living things across South America, and by the geological connections between the continent's present and past inhabitants. These facts, as we'll see in the later chapters of this book, seemed to shed some light on the origin of species — that "mystery of mysteries," as one of our greatest philosophers has called it. After I returned home, it occurred to me in 1837 that I might be able to make some progress on this question by patiently collecting and thinking over every kind of fact that could possibly have any bearing on it. After five years of this work, I let myself begin to speculate on the subject and drafted some short notes. In 1844, I expanded these into a sketch of the conclusions that seemed likely to me. From that time to the present day, I have steadily pursued the same goal. I hope I may be forgiven for going into these personal details — I share them to show that I haven't been hasty in reaching my conclusions.
My work is now (1859) nearly finished; but since it will take me many more years to complete it, and since my health is far from strong, I've been urged to publish this summary. I was especially motivated to do so because Alfred Russel Wallace, who is currently studying the natural history of the Malay Archipelago, has independently arrived at almost exactly the same general conclusions that I have about the origin of species. In 1858, he sent me a paper on the subject, asking me to forward it to the geologist Sir Charles Lyell. Lyell sent it to the Linnean Society, and it was published in the third volume of that society's journal. Sir Charles Lyell and the botanist Dr. Joseph Hooker, who both knew about my work — Hooker had actually read my sketch of 1844 — did me the honor of arranging to publish, alongside Wallace's excellent paper, some brief extracts from my manuscripts.
This summary, which I now publish, is necessarily imperfect. I can't provide references and citations for all my statements here, and I have to ask the reader to place some trust in my accuracy. No doubt errors will have crept in, though I hope I've always been careful to rely only on good authorities. I can give here only the general conclusions I've reached, with a few illustrative facts that I hope will be sufficient in most cases. No one feels more strongly than I do the need to eventually publish, in full detail, all the facts and references on which my conclusions are based — and I hope to do this in a future work. I'm well aware that there is hardly a single point discussed in this book where facts can't be brought forward that seem to lead to conclusions directly opposite to mine. A fair result can only be reached by fully stating and weighing the facts and arguments on both sides of each question, and that simply isn't possible here.
I deeply regret that lack of space prevents me from acknowledging the generous help I've received from a great many naturalists, some of them personally unknown to me. I can't let this opportunity pass, however, without expressing my deep gratitude to Dr. Hooker, who for the last fifteen years has helped me in every possible way with his vast knowledge and excellent judgment.
When considering the origin of species, it's entirely possible that a naturalist — reflecting on the relationships between living things, their embryological connections, their geographic distribution, their geological history, and other such facts — might conclude that species haven't been independently created, but have descended, like varieties, from other species. Yet even if such a conclusion were well-founded, it would still be unsatisfying until someone could show how the countless species in this world have been modified to achieve that perfection of structure and mutual adaptation that so rightly amazes us. Naturalists constantly point to external conditions — climate, food, and so on — as the only possible cause of variation. In one limited sense, as we'll see later, this may be true. But it's absurd to attribute to mere external conditions something like the structure of the woodpecker, with its feet, tail, beak, and tongue so beautifully adapted to catching insects beneath the bark of trees. Or take the mistletoe: it draws its nourishment from certain trees, its seeds must be carried by certain birds, and its flowers have separate sexes that absolutely require certain insects to carry pollen from one flower to another. It's equally absurd to explain the structure of this parasite, with its relationships to several completely different organisms, as the result of external conditions, or of habit, or of the plant's own will.
It is, therefore, critically important to understand the means of modification and mutual adaptation. When I first began my observations, it seemed to me that a careful study of domesticated animals and cultivated plants would offer the best chance of solving this difficult problem. And I wasn't disappointed. In this and in every other puzzling case, I've consistently found that our knowledge of variation under domestication — imperfect as it is — provided the best and most reliable guide. I'll venture to say that I'm convinced these kinds of studies are extremely valuable, even though naturalists have very commonly neglected them.
With this in mind, I'll devote the first chapter of this book to variation under domestication. We'll see that a large amount of hereditary change is at least possible, and — what is equally or even more important — we'll see how great is the power of humans to accumulate small, successive variations through selective breeding. I'll then move on to the variability of species in a state of nature, though I'll unfortunately be forced to treat this subject far too briefly, since doing it justice would require long catalogs of facts. We will, however, be able to discuss what circumstances are most favorable to variation. In the next chapter, I'll consider the struggle for existence among all living things throughout the world — a struggle that inevitably follows from the high rate at which they multiply. This is the doctrine of Malthus, applied to the entire animal and plant kingdoms. Since far more individuals of each species are born than can possibly survive, and since there is therefore a constantly recurring struggle for existence, it follows that any organism — if it varies even slightly in any way that's beneficial to itself, under the complex and sometimes changing conditions of life — will have a better chance of surviving, and will thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to pass on its new and modified form.
This fundamental subject of natural selection will be covered at some length in the fourth chapter. We'll then see how natural selection almost inevitably causes widespread extinction of less well-adapted forms of life, and leads to what I've called divergence of character. In the next chapter, I'll discuss the complex and poorly understood laws of variation. In the five chapters after that, I'll address the most obvious and serious difficulties with accepting the theory: first, the problem of transitions — how a simple organism or a simple organ can be changed and perfected into a highly developed organism or an elaborately constructed organ; second, the subject of instinct, or the mental abilities of animals; third, hybridism, or the infertility of species and the fertility of varieties when crossbred; and fourth, the imperfection of the geological record. In the chapter after those, I'll consider the geological succession of organisms through time; in the twelfth and thirteenth chapters, their geographic distribution across space; in the fourteenth, their classification and relationships to one another, both as adults and in their embryonic stages. In the last chapter, I'll give a brief summary of the whole work, along with a few concluding remarks.
No one should be surprised that much remains unexplained about the origin of species and varieties, if we remember how profoundly ignorant we are about the relationships among the many organisms that live around us. Who can explain why one species ranges widely and is very common, while a closely related species has a narrow range and is rare? Yet these relationships are of the highest importance, because they determine the present welfare and, I believe, the future success and evolution of every living thing on this planet. We know even less about the relationships among the world's countless inhabitants during the many past geological ages of its history. Although much remains unclear, and will remain unclear for a long time, I have no doubt — after the most careful study and unbiased judgment I'm capable of — that the view most naturalists held until recently, and that I once held myself — namely, that each species was independently created — is wrong. I'm fully convinced that species are not unchangeable; rather, those belonging to what we call the same genera are direct descendants of some other, generally extinct species, in the same way that the recognized varieties of any one species are the descendants of that species. Furthermore, I'm convinced that natural selection has been the most important, but not the only, means of modification.
When we compare individuals of the same variety or sub-variety of our long-cultivated plants and animals, one of the first things that strikes us is that they generally differ from each other more than individuals of any one species or variety in the wild. And if we consider the vast diversity of the plants and animals that have been cultivated and domesticated — varying across all ages, under the most different climates and treatments — we're driven to conclude that this great variability comes from our domestic organisms having been raised under conditions of life less uniform than, and somewhat different from, those their parent species experienced in nature. There is also some support for the view proposed by Andrew Knight that this variability may be partly connected with an excess of food. It seems clear that organisms must be exposed for several generations to new conditions before any great amount of variation appears, and that once an organism has begun to vary, it generally continues varying for many generations. No case is on record of a variable organism ceasing to vary under cultivation. Our oldest cultivated plants, such as wheat, still produce new varieties. Our oldest domesticated animals are still capable of rapid improvement or modification.
As far as I can judge, after long attention to this subject, the conditions of life appear to act in two ways — directly on the whole organism or on certain parts alone, and indirectly by affecting the reproductive system. Regarding the direct effects, we must keep in mind that in every case, as Professor Weismann has recently insisted, and as I have incidentally shown in my work on *Variation Under Domestication*, there are two factors at play: the nature of the organism and the nature of the conditions. The organism itself seems to be much more important. Nearly similar variations sometimes arise under what appear to be quite different conditions, and on the other hand, very different variations arise under conditions that seem nearly uniform. The effects on offspring are either definite or indefinite. They may be considered definite when all or nearly all the offspring of individuals exposed to certain conditions over several generations are modified in the same way. It is extremely difficult to determine the extent of changes that have been produced in this definite manner. There can, however, be little doubt about many slight changes — such as size from the amount of food, color from the type of food, thickness of skin and hair from climate, and so on. Each of the endless variations we see in the plumage of our domestic fowls must have had some effective cause. And if the same cause were to act uniformly over a long series of generations on many individuals, all of them would probably be modified in the same way. Consider how a tiny drop of poison injected by a gall-producing insect can trigger complex and extraordinary growths — this shows us what remarkable modifications might result in plants from a chemical change in the nature of their sap.
Indefinite variability is a much more common result of changed conditions than definite variability, and has probably played a more important role in the formation of our domestic breeds. We see indefinite variability in the endless slight differences that distinguish individuals of the same species — differences that can't be traced to inheritance from either parent or from some more remote ancestor. Even strongly marked differences occasionally appear among the young of the same litter, or in seedlings from the same seed capsule. At long intervals of time, out of millions of individuals raised in the same country and fed on nearly the same food, deviations of structure so strongly pronounced as to deserve being called monstrosities arise. But monstrosities can't be separated by any clear line from slighter variations. All such changes of structure, whether extremely slight or strongly marked, which appear among many individuals living together, may be considered the indefinite effects of the conditions of life on each individual organism — in much the same way that a chill affects different people in an indefinite manner, according to their state of health or constitution, causing coughs or colds, rheumatism, or inflammation of various organs.
As for what I have called the indirect action of changed conditions — namely, through the reproductive system being affected — we can infer that variability is produced this way, partly from the fact that this system is extremely sensitive to any change in conditions, and partly from the similarity, as Kolreuter and others have noted, between the variability that follows from crossing distinct species and the variability seen when plants and animals are raised under new or unnatural conditions. Many facts clearly show just how remarkably sensitive the reproductive system is to very slight changes in its surroundings. Nothing is easier than taming an animal, and few things are harder than getting it to breed freely in captivity, even when male and female do mate. How many animals refuse to breed, even when kept in an almost free state in their native country! This is generally, but mistakenly, blamed on corrupted instincts. Many cultivated plants grow with the greatest vigor and yet rarely or never set seed! In some few cases, it has been discovered that a very minor change — such as a little more or less water at some particular stage of growth — determines whether or not a plant will produce seeds. I can't go into all the details I've collected and published elsewhere on this curious subject. But to show how strange the laws are that govern reproduction in captive animals, I'll mention that carnivorous mammals, even those from the tropics, breed pretty freely in this country under confinement — with the exception of the plantigrades, or bear family, which seldom produce young. Carnivorous birds, on the other hand, with the rarest exceptions, hardly ever lay fertile eggs. Many exotic plants produce utterly worthless pollen, in the same condition as the most sterile hybrids. When, on one hand, we see domesticated animals and plants, though often weak and sickly, breeding freely under confinement, and when, on the other hand, we see individuals taken young from the wild, perfectly tamed, long-lived, and healthy (of which I could give numerous examples), yet having their reproductive system so seriously affected by unnoticed causes that it fails entirely — we shouldn't be surprised that this system, when it does function in captivity, acts irregularly and produces offspring somewhat unlike their parents. I might add that since some organisms breed freely under the most unnatural conditions — for instance, rabbits and ferrets kept in hutches — showing that their reproductive organs aren't easily affected, so too will some animals and plants withstand domestication or cultivation and vary very little, perhaps hardly more than in the wild.
Some naturalists have argued that all variations are connected with the act of sexual reproduction. But this is certainly wrong, for I have given in another work a long list of what gardeners call "sporting plants" — that is, plants that have suddenly produced a single bud with a new and sometimes very different character from that of the other buds on the same plant. These bud variations, as they may be called, can be propagated by grafts, offsets, and similar means, and sometimes by seed. They occur rarely in nature but are far from rare under cultivation. Since a single bud out of the many thousands produced year after year on the same tree under uniform conditions has been known suddenly to take on a new character — and since buds on different trees, growing under different conditions, have sometimes yielded nearly the same variety (for instance, buds on peach trees producing nectarines, and buds on common roses producing moss roses) — we can clearly see that the nature of the conditions is of secondary importance compared with the nature of the organism itself in determining each particular form of variation. The conditions may be no more important than the nature of the spark that ignites a mass of combustible material is in determining the nature of the flames.
Changed habits produce inherited effects, as when the flowering time of plants shifts after they are transported from one climate to another. In animals, the increased use or disuse of parts has had a more marked influence. For example, I find that in the domestic duck, the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than the same bones in the wild duck. This change can safely be attributed to the domestic duck flying much less and walking more than its wild ancestors. The greatly developed udders in cows and goats in countries where they are habitually milked, compared with these organs in other countries, is probably another example of the effects of use. Not one of our domestic animals can be named that hasn't developed drooping ears in some country. The suggestion that the drooping is due to disuse of the ear muscles — from the animals being seldom alarmed — seems a reasonable one.
Many laws regulate variation, a few of which can be dimly perceived and will be briefly discussed later. I will here only mention what may be called correlated variation. Important changes in the embryo or larva will probably lead to changes in the mature animal. In monstrosities, the correlations between quite distinct body parts are very curious, and many examples are given in Isidore Geoffroy St. Hilaire's great work on this subject. Breeders believe that long limbs are almost always accompanied by an elongated head. Some examples of correlation are downright whimsical: cats that are entirely white and have blue eyes are generally deaf, though it has recently been stated by Mr. Tait that this is confined to males. Color and constitutional traits go together, and many remarkable cases could be given among animals and plants. From facts collected by Heusinger, it appears that white sheep and pigs are injured by certain plants, while dark-colored individuals escape. Professor Wyman recently shared a good illustration of this with me: on asking some farmers in Virginia how it was that all their pigs were black, they told him that the pigs ate the paint-root (*Lachnanthes*), which colored their bones pink and caused the hooves of all but the black varieties to drop off. One of the "crackers" (that is, Virginia squatters) added, "We select the black members of a litter for raising, as they alone have a good chance of living." Hairless dogs have imperfect teeth. Long-haired and coarse-haired animals tend to have, it is said, long or many horns. Pigeons with feathered feet have skin between their outer toes. Pigeons with short beaks have small feet, and those with long beaks have large feet. So if a breeder keeps selecting and thus amplifying any particular trait, he will almost certainly modify other parts of the animal unintentionally, owing to the mysterious laws of correlation.
The results of the various, unknown, or only dimly understood laws of variation are infinitely complex and diverse. It is well worth carefully studying the treatises on some of our old cultivated plants — the hyacinth, potato, even the dahlia — and it is really surprising to note the endless points of structure and constitution in which the varieties and sub-varieties differ slightly from each other. The whole organism seems to have become plastic, departing in slight degrees from the parental type.
Any variation that is not inherited is unimportant for our purposes. But the number and diversity of inheritable deviations of structure, both those of slight and those of considerable physiological importance, are endless. Dr. Prosper Lucas's treatise, in two large volumes, is the fullest and best on this subject. No breeder doubts how strong the tendency to inheritance is. "Like produces like" is the breeder's fundamental belief — doubts have been raised about this principle only by theoretical writers. When a deviation of structure often appears, and we see it in both parent and child, we can't be sure it isn't due to the same cause acting on both. But when among individuals apparently exposed to the same conditions, some very rare deviation — due to some extraordinary combination of circumstances — appears in a parent (say, once among several million individuals), and then reappears in the child, the laws of probability almost force us to attribute its reappearance to inheritance. Everyone must have heard of cases of albinism, prickly skin, hairy bodies, and so on appearing in several members of the same family. If strange and rare deviations of structure are truly inherited, then less strange and more common deviations can freely be admitted as inheritable. Perhaps the correct way of looking at this whole subject would be to regard the inheritance of every character as the rule, and non-inheritance as the exception.
The laws governing inheritance are for the most part unknown. No one can say why the same trait in different individuals of the same species, or in different species, is sometimes inherited and sometimes not. Or why a child often reverts to traits of its grandfather or grandmother or a more remote ancestor. Or why a trait is often transmitted from one sex to both sexes, or to one sex alone — more commonly but not exclusively to the same sex. It is a fact of some importance to us that traits appearing in the males of our domestic breeds are often transmitted either exclusively or predominantly to males alone. A much more important rule, and one I think can be trusted, is that whatever age a trait first appears, it tends to reappear in the offspring at a corresponding age — though sometimes earlier. In many cases this couldn't be otherwise: inherited traits in the horns of cattle, for example, could appear only when the offspring is nearly mature. Traits in the silkworm appear at the corresponding caterpillar or cocoon stage. But hereditary diseases and some other facts make me believe the rule has a wider reach — that when there's no obvious reason why a trait should appear at any particular age, it still tends to appear in the offspring at the same stage at which it first showed up in the parent. I believe this rule to be of the highest importance in explaining the laws of embryology. These remarks are, of course, about the first appearance of the trait, not the original cause which may have acted on the egg or on the male reproductive element. In much the same way, the increased length of horns in the offspring of a short-horned cow crossed with a long-horned bull, though appearing late in life, is clearly due to the male parent's contribution.
Having mentioned the subject of reversion, I may here address a claim often made by naturalists — that our domestic varieties, when allowed to run wild, gradually but inevitably revert to the character of their original wild ancestors. From this, it has been argued that no conclusions can be drawn from domestic breeds about species in the wild. I have tried in vain to discover what decisive facts this claim rests on. It would be very difficult to prove it true. We can safely conclude that many of the most strongly marked domestic varieties could not possibly survive in the wild. In many cases we don't know what the original wild ancestor was, so we couldn't tell whether nearly perfect reversion had occurred. To prevent the effects of crossbreeding, only a single variety would need to be released into its new home. Still, since our varieties do occasionally revert in some of their characters to ancestral forms, it seems possible to me that if we could succeed in establishing, or were to cultivate over many generations, the several varieties of the cabbage, for instance, in very poor soil — though in that case some effect would have to be attributed to the direct action of the poor soil — they would largely, or even entirely, revert to the wild ancestral type. Whether or not the experiment would succeed is not critical to our argument, because the experiment itself changes the conditions of life. If it could be shown that our domestic varieties manifested a strong tendency to reversion — that is, to lose their acquired characters while kept under the same conditions and in a large enough population that free crossbreeding might check, by blending, any slight deviations in their structure — in that case, I grant that we could deduce nothing from domestic varieties about species. But there is not a shadow of evidence for this view. To assert that we could not breed our cart horses and racehorses, long- and short-horned cattle, and poultry of various breeds, and edible vegetables, for an unlimited number of generations, would go against all experience.
Character of Domestic Varieties; Difficulty of Distinguishing Between Varieties and Species; Origin of Domestic Varieties from One or More Species
When we look at the hereditary varieties or breeds of our domestic animals and plants and compare them with closely allied species, we generally see in each domestic breed, as I've already noted, less uniformity of character than in true species. Domestic breeds often have a somewhat exaggerated character — by which I mean that although they differ from each other and from other species of the same genus in several minor ways, they often differ to an extreme degree in some one feature, both when compared with one another and especially when compared with the wild species to which they are most closely related. With these exceptions (and with the exception of the perfect fertility of varieties when crossed — a subject I'll discuss later), domestic breeds of the same species differ from each other in the same manner as closely allied species of the same genus in the wild, but the differences are usually less in degree. This must be accepted as true, because the domestic breeds of many animals and plants have been classified by some competent judges as descendants of originally distinct species, and by other equally competent judges as mere varieties. If any clear-cut distinction existed between a domestic breed and a species, this kind of doubt would not keep arising. It has often been stated that domestic breeds don't differ from each other in characters of generic importance. It can be shown that this statement is incorrect. But naturalists differ greatly in deciding which characters have generic importance — all such judgments being, at present, based on experience rather than firm rules. When I explain how genera originate in nature, it will be seen that we have no right to expect often to find a genus-level amount of difference in our domesticated breeds.
In trying to estimate the amount of structural difference between related domestic breeds, we quickly run into doubt, because we don't know whether they descend from one or several parent species. If this could be cleared up, it would be very interesting. If, for instance, it could be shown that the greyhound, bloodhound, terrier, spaniel, and bulldog — which we all know breed true to type — were the offspring of a single species, then such facts would carry great weight in making us doubt the supposed unchangeability of the many closely related natural species (for instance, the many foxes inhabiting different parts of the world). I don't believe, as we shall soon see, that the full extent of difference between the various dog breeds was produced under domestication. I believe that a small part of the difference comes from their descending from distinct species. In the case of strongly marked breeds of some other domesticated species, there is strong or even conclusive evidence that all are descended from a single wild ancestor.
It has often been assumed that humans chose for domestication those animals and plants having an extraordinary inherent tendency to vary, and also an ability to withstand diverse climates. I don't dispute that these capacities have added greatly to the value of most of our domesticated organisms. But how could an early human possibly have known, when first taming an animal, whether it would vary in later generations, or whether it would tolerate other climates? Has the limited variability of the donkey and goose, or the poor tolerance of warmth by the reindeer, or of cold by the common camel, prevented their domestication? I have no doubt that if other animals and plants, equal in number to our domesticated species and belonging to equally diverse groups and countries, were taken from the wild and could be made to breed for an equal number of generations under domestication, they would on average vary as much as the parent species of our existing domesticated organisms have varied.
For most of our anciently domesticated animals and plants, it is not possible to reach any definite conclusion about whether they descend from one or several wild species. The main argument relied on by those who believe in the multiple origin of our domestic animals is that we find great diversity in the breeds depicted on the monuments of ancient Egypt and in the lake-dwellings of Switzerland, and that some of these ancient breeds closely resemble, or are even identical with, breeds still existing today. But this only pushes the history of civilization much further back and shows that animals were domesticated at a much earlier period than was previously supposed. The lake-dwelling people of Switzerland cultivated several kinds of wheat and barley, the pea, the poppy for oil, and flax. They possessed several domesticated animals. They also carried on trade with other nations. All this clearly shows, as Heer has noted, that they had already advanced considerably in civilization. And this in turn implies a long prior period of less advanced civilization, during which domesticated animals kept by different tribes in different districts might have varied and given rise to distinct breeds. Since the discovery of flint tools in surface formations across many parts of the world, all geologists believe that early humans existed at an enormously remote period. And we know that at the present day, there is hardly a tribe so primitive as not to have domesticated at least the dog.
The origin of most of our domestic animals will probably forever remain uncertain. But I may state here that, looking at the domestic dogs of the whole world, I have — after a laborious collection of all known facts — come to the conclusion that several wild species of Canidae have been tamed, and that their blood, in some cases mingled together, flows in the veins of our domestic breeds. Regarding sheep and goats, I can form no definite opinion. From facts communicated to me by Mr. Blyth about the habits, voice, constitution, and structure of the humped Indian cattle, it is almost certain that they descend from a different ancestral stock than our European cattle. Some competent judges believe that European cattle have had two or three wild ancestors, whether or not these deserve to be called separate species. This conclusion, as well as the finding that humped and common cattle are specifically distinct, may indeed be considered established by the admirable research of Professor Rutimeyer. Regarding horses, for reasons I can't detail here, I'm inclined — though with some doubt and against the views of several authors — to believe that all the breeds belong to the same species. Having kept nearly all the English breeds of fowl alive, having bred and crossed them, and examined their skeletons, it seems to me almost certain that all are descendants of the wild Indian fowl, *Gallus bankiva*. This is also the conclusion of Mr. Blyth and others who have studied this bird in India. Regarding ducks and rabbits, some breeds of which differ considerably from each other, the evidence is clear that they all descend from the common wild duck and wild rabbit.
The doctrine that our several domestic breeds originated from several distinct wild species has been carried to an absurd extreme by some authors. They believe that every breed that breeds true, no matter how slight the distinguishing characters, has had its own wild ancestor. At this rate, there must have existed at least twenty species of wild cattle, as many sheep, and several goats in Europe alone, and several even within Great Britain. One author believes that there formerly existed eleven wild species of sheep unique to Britain! When we consider that Britain today has not one unique mammal, and France has few distinct from those of Germany, and so on with Hungary, Spain, and others, yet each of these countries possesses several unique breeds of cattle, sheep, and so on — we must admit that many domestic breeds must have originated in Europe, for where else could they have come from? The same is true for India. Even in the case of dog breeds throughout the world, which I accept are descended from several wild species, it can't be doubted that there has been an immense amount of inherited variation. For who would believe that animals closely resembling the Italian greyhound, the bloodhound, the bulldog, the pug, or the Blenheim spaniel — so unlike all wild Canidae — ever existed in the wild? It has often been loosely stated that all our dog breeds were produced by crossing a few ancestral species. But by crossing we can only get forms that are to some degree intermediate between their parents. If we try to account for our various domestic breeds this way, we must accept the former existence of the most extreme forms — such as the Italian greyhound, bloodhound, bulldog, and so on — in the wild state. Moreover, the possibility of creating distinct breeds by crossing has been greatly exaggerated. Many cases are on record showing that a breed can be modified by occasional crosses, if aided by careful selection of individuals showing the desired trait. But to produce a breed intermediate between two quite distinct breeds would be very difficult. Sir J. Sebright experimented with exactly this goal and failed. The offspring from the first cross between two pure breeds is fairly — and sometimes, as I have found with pigeons, quite — uniform in character, and everything seems simple enough. But when these hybrids are crossed with one another for several generations, hardly two of them are alike, and then the difficulty of the task becomes clear.
Believing that it is always best to study some specific group in depth, I have, after careful thought, taken up domestic pigeons. I have kept every breed I could buy or obtain, and have been very generously supplied with skins from several parts of the world — most notably by the Honorable W. Elliot from India and the Honorable C. Murray from Persia. Many treatises in different languages have been published on pigeons, and some of them are very important, being of considerable antiquity. I have associated with several eminent pigeon fanciers and have been permitted to join two of the London Pigeon Clubs. The diversity of the breeds is simply astonishing.
Compare the English carrier and the short-faced tumbler, and see the wonderful difference in their beaks, with corresponding differences in their skulls. The carrier, especially the male, is also remarkable for the wonderful development of the warty skin around the head, accompanied by greatly elongated eyelids, very large nostril openings, and a wide gape of the mouth. The short-faced tumbler has a beak in outline almost like that of a finch, and the common tumbler has the remarkable inherited habit of flying at great height in a compact flock and tumbling head over heels through the air. The runt is a bird of great size, with a long, massive beak and large feet. Some sub-breeds of runts have very long necks, others very long wings and tails, still others strikingly short tails. The barb is related to the carrier but, instead of a long beak, has a very short and broad one. The pouter has a much elongated body, wings, and legs, and its enormously developed crop, which it glories in inflating, can hardly fail to provoke astonishment and even laughter. The turbit has a short, conical beak with a line of reversed feathers running down the breast, and it has the habit of continually expanding, slightly, the upper part of the esophagus. The Jacobin has its feathers so thoroughly reversed along the back of the neck that they form a hood, and it has, proportional to its size, elongated wing and tail feathers. The trumpeter and laugher, as their names suggest, utter a very different coo from the other breeds. The fantail has thirty or even forty tail feathers, instead of the twelve or fourteen that are normal in all members of the great pigeon family. These feathers are kept spread and carried so erect that in good birds the head and tail touch. The oil gland is completely absent. Several other less distinct breeds could be mentioned.
In the skeletons of the various breeds, the development of the bones of the face — in length, breadth, and curvature — differs enormously. The shape, breadth, and length of the lower jaw varies in a highly remarkable manner. The tail and sacral vertebrae vary in number, as does the number of ribs, along with their relative breadth and the presence of bony processes. The size and shape of the openings in the breastbone are highly variable, as is the degree of divergence and relative size of the two arms of the wishbone. The proportional width of the gape of the mouth, the proportional length of the eyelids, of the nostril openings, of the tongue (not always strictly correlated with the length of beak), the size of the crop and of the upper part of the esophagus, the development or absence of the oil gland, the number of the primary wing and tail feathers, the relative length of the wing and tail to each other and to the body, the relative length of the leg and foot, the number of scales on the toes, the development of skin between the toes — all of these are variable. The age at which the adult plumage is acquired varies, as does the condition of the down that clothes the nestlings when they hatch. The shape and size of the eggs vary. The manner of flight, and in some breeds the voice and temperament, differ remarkably. Finally, in certain breeds, the males and females have come to differ slightly from each other.
All told, at least twenty pigeons could be chosen which, if shown to a bird expert and he were told they were wild birds, would certainly be classified by him as well-defined species. Moreover, I don't believe any ornithologist would in this case place the English carrier, the short-faced tumbler, the runt, the barb, pouter, and fantail in the same genus — especially since in each of these breeds, several truly inherited sub-breeds (or species, as he would call them) could be shown to him.
Great as the differences between the breeds of pigeon are, I'm fully convinced that the common opinion among naturalists is correct — that all are descended from the rock pigeon (*Columba livia*), including under this term several geographic races or sub-species that differ from each other in only the most trivial ways. Since several of the reasons that have led me to this belief apply to some degree in other cases, I'll briefly lay them out here.
If the various breeds are not varieties and did not descend from the rock pigeon, they must have descended from at least seven or eight ancestral species — because it is impossible to produce the present domestic breeds by crossing any fewer number. How, for instance, could a pouter be produced by crossing two breeds unless one of the parent stocks already possessed that characteristically enormous crop? The supposed ancestral species must all have been rock pigeons — that is, they did not breed in or willingly perch on trees. But besides *C. livia* with its geographic sub-species, only two or three other species of rock pigeons are known, and these have none of the characters of the domestic breeds. So the supposed ancestral species must either still exist in the countries where they were originally domesticated, yet be unknown to ornithologists — which, considering their size, habits, and remarkable features, seems unlikely — or they must have gone extinct in the wild. But birds that breed on cliff faces and are strong fliers are unlikely to be wiped out. The common rock pigeon, which has the same habits as the domestic breeds, has not been exterminated even on several of the smaller British islands or on the shores of the Mediterranean. The supposed extinction of so many species with habits similar to the rock pigeon seems a very rash assumption. What's more, the various domesticated breeds I've named have been transported to all parts of the world, and some of them must therefore have been carried back to their native country. Yet not one has gone wild or feral — though the dovecote pigeon, which is the rock pigeon in a very slightly altered state, has gone feral in several places. Again, all recent experience shows that it is difficult to get wild animals to breed freely under domestication. Yet under the theory of multiple origins for our pigeons, we would have to assume that at least seven or eight species were so thoroughly domesticated in ancient times, by people at an early stage of civilization, as to be completely fertile in confinement.
An argument of great weight, and one applicable in several other cases, is this: the breeds I've described, while generally agreeing with the wild rock pigeon in constitution, habits, voice, coloring, and most parts of their structure, are certainly highly abnormal in other respects. We may look in vain throughout the whole great family Columbidae for a beak like that of the English carrier, or the short-faced tumbler, or the barb — for reversed feathers like those of the Jacobin — for a crop like that of the pouter — for tail feathers like those of the fantail. So we would have to assume not only that early humans succeeded in thoroughly domesticating several species, but that they intentionally or by chance picked out extraordinarily abnormal species — and further, that these very species have since all gone extinct or become unknown. So many strange coincidences are improbable in the highest degree.
Some facts about the coloring of pigeons deserve careful attention. The rock pigeon is slaty blue with white lower back feathers, though the Indian sub-species, *C. intermedia* of Strickland, has this part bluish. The tail has a dark terminal bar, with the outer feathers edged at the base with white on the outside. The wings have two black bars. Some semi-domestic breeds and some truly wild breeds have, besides the two black bars, the wings checkered with black. These several markings do not occur together in any other species of the entire family. Now, in every one of the domestic breeds, taking thoroughly well-bred birds, all of the above markings — even down to the white edging of the outer tail feathers — sometimes appear perfectly developed. What's more, when birds belonging to two or more distinct breeds are crossed, none of which are blue or have any of the marks I've described, the hybrid offspring are very likely to suddenly acquire these characters.
To give one example out of several I have observed: I crossed some white fantails, which breed very true, with some black barbs — and it so happens that blue varieties of barbs are so rare that I have never heard of one in England. The hybrids were black, brown, and mottled. I also crossed a barb with a spot (a white bird with a red tail and red spot on the forehead, which breeds very true). The hybrids were dusky and mottled. I then crossed one of the barb-fantail hybrids with a barb-spot hybrid, and they produced a bird of as beautiful a blue color, with the white lower back, double black wing bar, and barred and white-edged tail feathers, as any wild rock pigeon! We can understand these facts on the well-known principle of reversion to ancestral characters, if all the domestic breeds descend from the rock pigeon. But if we deny this, we must accept one of two highly improbable alternatives. Either, first, that all the supposed original wild species were colored and marked like the rock pigeon — although no other existing species is colored and marked this way — so that in each separate breed there would be a tendency to revert to the very same colors and markings. Or second, that each breed, even the purest, has within a dozen, or at most within a score, of generations been crossed with the rock pigeon. I say within a dozen or twenty generations because no case is known of crossed descendants reverting to an ancestor of foreign blood removed by a greater number of generations. In a breed that has been crossed only once, the tendency to revert to any character derived from that cross will naturally become less and less with each succeeding generation, as there will be less of the foreign blood. But when there has been no cross, and there is a tendency in the breed to revert to a character lost during some former generation, this tendency — for all we can see to the contrary — may be transmitted undiminished for an indefinite number of generations. These two distinct cases of reversion are often confused by those who have written on inheritance.
Finally, the hybrids from crosses between all the breeds of pigeon are perfectly fertile, as I can state from my own observations, made deliberately on the most distinct breeds. Now, hardly any cases have been established with certainty of hybrids from two quite distinct species of animals being perfectly fertile. Some authors believe that long-continued domestication eliminates this strong tendency toward sterility between species. From the history of the dog and some other domestic animals, this conclusion is probably correct when applied to species closely related to each other. But to extend it so far as to suppose that species originally as distinct as carriers, tumblers, pouters, and fantails now are should yield perfectly fertile offspring when crossed with each other — that seems to me rash in the extreme.
From these several reasons — the improbability that early humans made seven or eight supposed pigeon species breed freely under domestication; these supposed species being completely unknown in the wild; their never having gone feral anywhere; these species displaying certain very abnormal characters compared with all other Columbidae, though so like the rock pigeon in most other respects; the occasional reappearance of the blue color and various black markings in all the breeds, both when kept pure and when crossed; and finally, the hybrid offspring being perfectly fertile — from all these reasons taken together, we can safely conclude that all our domestic breeds descend from the rock pigeon, or *Columba livia*, with its geographic sub-species.
In further support of this view, I may add: first, that the wild *C. livia* has been found capable of domestication in Europe and in India, and that it agrees in habits and in a great number of structural details with all the domestic breeds. Second, that although an English carrier or a short-faced tumbler differs immensely in certain characters from the rock pigeon, by comparing the several sub-breeds of these two types — especially those brought from distant countries — we can construct an almost perfect series of gradations between them and the rock pigeon. We can do the same in some other cases, though not with all breeds. Third, those characters that are mainly distinctive of each breed are in each case eminently variable: for instance, the wattle and beak length of the carrier, the shortness of the tumbler's beak, and the number of tail feathers in the fantail. The explanation of this fact will be obvious when we come to discuss selection. Fourth, pigeons have been watched, tended, and loved by many people with the utmost care. They have been domesticated for thousands of years in several parts of the world. The earliest known record of pigeons is from the fifth Egyptian dynasty, about 3000 BC, as was pointed out to me by Professor Lepsius, though Mr. Birch informs me that pigeons appear in a bill of fare from the previous dynasty. In the time of the Romans, as we learn from Pliny, immense prices were given for pigeons: "Indeed, they have come to such a point that they can reckon up their pedigree and lineage." Pigeons were much valued by Akbar Khan in India, around the year 1600; never less than 20,000 pigeons were taken with the court. "The monarchs of Iran and Turan sent him some very rare birds," and, continues the courtly historian, "His Majesty, by crossing the breeds — a method never practiced before — has improved them astonishingly." Around the same period, the Dutch were as passionate about pigeons as the ancient Romans had been. The paramount importance of these considerations in explaining the immense amount of variation that pigeons have undergone will also be obvious when we discuss selection. We shall then also see how it is that the various breeds so often have a somewhat exaggerated character. It is also a most favorable circumstance for the production of distinct breeds that male and female pigeons can easily be mated for life, and thus different breeds can be kept together in the same aviary.
I have discussed the probable origin of domestic pigeons at some length — yet not nearly enough — because when I first kept pigeons and observed the various breeds, knowing full well how truly they breed, I felt just as much difficulty in believing that since domestication they had all come from a common ancestor as any naturalist could feel in reaching a similar conclusion about the many species of finches, or other groups of birds, in nature. One thing has struck me particularly: nearly all the breeders of various domestic animals, and the growers of plants, with whom I have spoken or whose writings I have read, are firmly convinced that the several breeds each has attended to are descended from as many originally distinct species. Ask, as I have asked, a celebrated raiser of Hereford cattle whether his cattle might not have descended from Longhorns, or both from a common ancestor, and he will laugh you to scorn. I have never met a pigeon, poultry, duck, or rabbit fancier who was not fully convinced that each main breed descended from a distinct species. Van Mons, in his treatise on pears and apples, shows how completely he disbelieves that the various types — for instance, a Ribston Pippin or a Codlin apple — could ever have come from seeds of the same tree. Countless other examples could be given. The explanation, I think, is simple. From long-continued study, these breeders are deeply impressed by the differences between the various breeds. And though they know well that each breed varies slightly — for they win their prizes by selecting such slight differences — they ignore all general arguments and refuse to add up in their minds the slight differences accumulated over many successive generations. May not those naturalists who, knowing far less of the laws of inheritance than the breeder, and knowing no more than he does of the intermediate links in the long lines of descent, yet accept that many of our domestic breeds descend from the same parents — may they not learn a lesson in caution, when they dismiss the idea that species in the wild are direct descendants of other species?
Now let's briefly consider the steps by which domestic breeds have been produced, whether from one or from several related species. Some effect may be attributed to the direct and definite action of external conditions of life, and some to habit. But it would take a bold person to account by such causes for the differences between a draft horse and a racehorse, a greyhound and a bloodhound, a carrier and a tumbler pigeon. One of the most remarkable features of our domesticated breeds is that we see in them adaptation — not to the animal's or plant's own good, but to human use or fancy. Some variations useful to us have probably arisen suddenly, in a single step. Many botanists, for instance, believe that the fuller's teasel, with its hooks that no mechanical device can rival, is only a variety of the wild *Dipsacus* — and this amount of change may have arisen suddenly in a seedling. So it probably was with the turnspit dog, and this is known to have been the case with the Ancon sheep. But when we compare the draft horse and the racehorse, the dromedary and the camel, the various breeds of sheep suited to either cultivated land or mountain pasture — with the wool of one breed good for one purpose and that of another breed for another — when we compare the many breeds of dogs, each useful to us in different ways — when we compare the gamecock, so stubborn in battle, with other breeds so little quarrelsome, with "everlasting layers" that never want to sit on their eggs, and with the bantam so small and elegant — when we compare the vast array of agricultural, culinary, orchard, and flower-garden varieties of plants, most useful to us at different seasons and for different purposes, or so beautiful in our eyes — we must, I think, look beyond mere variability. We can't suppose that all the breeds were suddenly produced as perfect and useful as we now see them. Indeed, in many cases we know that is not their history. The key is humanity's power of accumulative selection: nature gives successive variations; humans add them up in certain directions useful to them. In this sense, we may be said to have made for ourselves useful breeds.
The great power of this principle of selection is not hypothetical. It is certain that several of our eminent breeders have, even within a single lifetime, modified their breeds of cattle and sheep to a large extent. To fully appreciate what they have done, it is almost necessary to read the many treatises devoted to this subject and to inspect the animals. Breeders routinely speak of an animal's constitution as something plastic, which they can shape almost as they please. If I had space, I could quote numerous passages to this effect from highly competent authorities. Youatt, who was probably better acquainted with the works of agriculturalists than almost anyone and was himself an excellent judge of animals, speaks of the principle of selection as "that which enables the agriculturalist, not only to modify the character of his flock, but to change it altogether. It is the magician's wand, by means of which he may summon into life whatever form and mold he pleases." Lord Somerville, speaking of what breeders have done for sheep, says: "It would seem as if they had chalked out upon a wall a form perfect in itself, and then had given it existence." In Saxony, the importance of the principle of selection in merino sheep is so fully recognized that men follow it as a trade: the sheep are placed on a table and studied like a painting by a connoisseur. This is done three times at intervals of months, and the sheep are each time marked and graded, so that the very best may ultimately be selected for breeding.
What English breeders have actually achieved is proved by the enormous prices given for animals with a good pedigree, and these have been exported to almost every part of the world. The improvement is by no means generally due to crossing different breeds — all the best breeders are strongly opposed to this practice, except sometimes among closely related sub-breeds. And when a cross has been made, the closest selection is far more essential even than in ordinary cases. If selection consisted merely in separating out some very distinct variety and breeding from it, the principle would be so obvious as to hardly be worth mentioning. But its importance lies in the great effect produced by the accumulation, in one direction over successive generations, of differences absolutely invisible to an untrained eye — differences which I for one have tried in vain to detect. Not one person in a thousand has accuracy of eye and judgment sufficient to become an eminent breeder. If gifted with these qualities and willing to study the subject for years and devote a lifetime to it with tireless perseverance, such a person will succeed and may make great improvements. If lacking any of these qualities, they will certainly fail. Few would readily believe the natural ability and years of practice required to become even a skilled pigeon fancier.
The same principles are followed by plant breeders, though the variations are often more abrupt. No one supposes that our finest plant varieties were produced by a single variation from the wild ancestor. We have proof that this is not so in several cases where exact records have been kept. To give a very minor example, the steadily increasing size of the common gooseberry may be cited. We see astonishing improvement in many flower varieties when today's flowers are compared with drawings made only twenty or thirty years ago. When a plant variety is fairly well established, the seed growers don't pick out the best plants but merely go over their seed beds and pull up the "rogues," as they call plants that deviate from the proper standard. With animals, this kind of selection is likewise followed — for hardly anyone is so careless as to breed from their worst animals.
Regarding plants, there is another way to observe the accumulated effects of selection — by comparing the diversity of flowers in the different varieties of the same species in the flower garden with the diversity of leaves, pods, tubers, or whatever part is valued in the kitchen garden, and with the diversity of fruit in the orchard. See how different the leaves of the cabbage are, yet how extremely alike the flowers. How unlike the flowers of the pansy are, yet how alike the leaves. How much the fruit of different kinds of gooseberries differs in size, color, shape, and hairiness, yet the flowers show very slight differences. It is not that the varieties which differ greatly in one feature don't differ at all in others — this is hardly ever, perhaps never, the case (I speak from careful observation). The law of correlated variation, whose importance should never be overlooked, will ensure some differences. But as a general rule, there can be no doubt that the continued selection of slight variations — whether in the leaves, the flowers, or the fruit — will produce breeds differing from each other chiefly in those characters.
Someone might object that the principle of selection has been practiced as a systematic method for scarcely more than three-quarters of a century. It has certainly received more attention in recent years, and many treatises have been published on the subject, and the results have been correspondingly rapid and important. But it is very far from true that the principle is a modern discovery. I could cite several references to works of great antiquity in which the full importance of the principle is recognized. In early and rough periods of English history, prized animals were often imported and laws were passed to prevent their export. The destruction of horses under a certain size was ordered — which may be compared to the "roguing" of plants by nurserymen. I find the principle of selection clearly stated in an ancient Chinese encyclopedia. Explicit rules are laid down by some of the Roman classical writers. From passages in Genesis, it is clear that the color of domestic animals was attended to at that early period. Indigenous peoples sometimes cross their dogs with wild canine species to improve the breed, and they formerly did so, as attested by passages in Pliny. The indigenous peoples of South Africa match their draft cattle by color, as do some of the Inuit their teams of dogs. Livingstone states that good domestic breeds are highly valued by the people in the interior of Africa who have not had contact with Europeans. Some of these facts do not demonstrate actual selection, but they show that the breeding of domestic animals was carefully attended to in ancient times and is still attended to by the most isolated peoples. It would indeed have been strange if attention had not been paid to breeding, for the inheritance of good and bad qualities is so obvious.
At the present time, eminent breeders try, through methodical selection with a distinct goal in mind, to create a new strain or sub-breed superior to anything of the kind in the country. But for our purposes, a form of selection that may be called unconscious — resulting from everyone trying to own and breed from the best individual animals — is more important. A man who intends keeping pointers naturally tries to get the best dogs he can and afterwards breeds from his own best dogs, but he has no wish or expectation of permanently altering the breed. Yet we may infer that this process, continued over centuries, would improve and modify any breed, in the same way as Bakewell, Collins, and others, by this very same process only carried out more methodically, greatly modified the forms and qualities of their cattle even within their own lifetimes. Slow and imperceptible changes of this kind could never be recognized unless actual measurements or careful drawings of the breeds had been made long ago for comparison. In some cases, however, unchanged or little-changed individuals of the same breed exist in less developed districts where the breed has been less improved. There is reason to believe that King Charles's spaniel has been unconsciously modified to a large extent since the time of that monarch. Some highly competent authorities are convinced that the setter descends directly from the spaniel and has probably been slowly altered from it. It is known that the English pointer has been greatly changed within the last century, and in this case the change is believed to have been mainly achieved by crosses with the foxhound. But what concerns us is that the change was made unconsciously and gradually, yet so effectively that although the old Spanish pointer certainly came from Spain, Mr. Borrow, as he informed me, has not seen any native dog in Spain resembling our pointer.
By a similar process of selection, and by careful training, English racehorses have come to surpass in speed and size the Arabian horses from which they descend, so that the latter, by the regulations for the Goodwood Races, are given a weight advantage. Lord Spencer and others have shown how the cattle of England have increased in weight and in early maturity compared with the stock formerly kept in this country. By comparing the accounts given in various old treatises of the former and present state of carrier and tumbler pigeons in Britain, India, and Persia, we can trace the stages through which they have gradually passed and come to differ so greatly from the rock pigeon.
Youatt gives an excellent illustration of the effects of a course of selection that may be considered unconscious, since the breeders could never have expected or even wished to produce the result that followed — namely, the development of two distinct strains. The two flocks of Leicester sheep kept by Mr. Buckley and Mr. Burgess, as Youatt remarks, "have been purely bred from the original stock of Mr. Bakewell for upwards of fifty years. There is not a suspicion in the mind of anyone at all acquainted with the subject that the owner of either flock has deviated in any one instance from the pure blood of Mr. Bakewell's flock, and yet the difference between the sheep possessed by these two gentlemen is so great that they have the appearance of being quite different varieties."
If there exist peoples so primitive as never to think about the inherited character of the offspring of their domestic animals, yet any one animal particularly useful to them for any special purpose would be carefully preserved during famines and other disasters — and such choice animals would thus generally leave more offspring than the inferior ones. In this case, there would be a kind of unconscious selection going on. We see the value set on animals even by the most isolated peoples — the inhabitants of Tierra del Fuego, for example, killed and ate their old women in times of famine, considering them of less value than their dogs.
In plants, the same gradual process of improvement through the occasional preservation of the best individuals — whether or not distinct enough to be ranked as separate varieties at their first appearance, and whether or not two or more species or varieties have become blended together by crossing — can plainly be seen in the increased size and beauty we now see in the varieties of the pansy, rose, pelargonium, dahlia, and other plants, compared with the older varieties or with their wild ancestors. No one would ever expect to get a first-rate pansy or dahlia from the seed of a wild plant. No one would expect to raise a first-rate melting pear from the seed of a wild pear, though they might succeed from a poor seedling growing wild if it had come from a garden stock. The pear, though cultivated in classical times, appears from Pliny's description to have been a fruit of very inferior quality. I have seen great surprise expressed in horticultural works at the wonderful skill of gardeners in having produced such splendid results from such poor materials. But the art has been simple, and as far as the final result is concerned, has been followed almost unconsciously. It has consisted in always cultivating the best known variety, sowing its seeds, and when a slightly better variety chanced to appear, selecting it, and so on. But the gardeners of the classical period, who cultivated the best pears they could get, never imagined what splendid fruit we would eat — though we owe our excellent fruit in some small degree to their having naturally chosen and preserved the best varieties they could find anywhere.
A large amount of change, slowly and unconsciously accumulated in this way, explains, as I believe, the well-known fact that in many cases we can't identify, and therefore don't know, the wild ancestors of the plants that have been cultivated longest in our flower and kitchen gardens. If it has taken centuries or thousands of years to improve or modify most of our plants to their present standard of usefulness, we can understand why neither Australia, the Cape of Good Hope, nor any other region inhabited by peoples without agriculture has provided us a single plant worth cultivating. It is not that these countries, so rich in species, don't by some strange chance possess the wild ancestors of any useful plants, but that the native plants have not been improved by continued selection to a standard of perfection comparable with that achieved by plants in countries with long histories of civilization.
Regarding the domestic animals kept by peoples without agriculture, it should not be overlooked that these animals almost always have to find their own food, at least during certain seasons. And in two very differently situated countries, individuals of the same species having slightly different constitutions or structures would often succeed better in one country than in the other. Thus, by a process of "natural selection" — as will be more fully explained later — two sub-breeds might be formed. This perhaps partly explains why the varieties kept by isolated peoples, as some authors have noted, have more of the character of true species than the varieties kept in more developed countries.
Given the important role that human selection has played, as I've described it here, it becomes immediately obvious how it is that our domestic breeds show adaptation in their structure or habits to human needs or fancies. We can, I think, also understand the frequently exaggerated character of our domestic breeds, and likewise why the differences between them are so great in external characters yet relatively slight in internal parts or organs. A breeder can hardly select, or only with great difficulty, any deviation of structure except one that is externally visible — and indeed rarely cares about what is internal. They can never act by selection except on variations that nature first provides in some slight degree. No one would ever try to create a fantail until they saw a pigeon with a tail developed in some slight degree in an unusual manner, or a pouter until they saw a pigeon with a crop of somewhat unusual size. And the more abnormal or unusual any character was when it first appeared, the more likely it would be to catch attention. But to use an expression like "trying to make a fantail" is, I have no doubt, in most cases utterly wrong. The person who first selected a pigeon with a slightly larger tail never dreamed what the descendants of that pigeon would become through long-continued, partly unconscious and partly methodical selection. Perhaps the ancestor of all fantails had only fourteen tail feathers, somewhat expanded, like the present Java fantail, or like individuals of other distinct breeds in which as many as seventeen tail feathers have been counted. Perhaps the first pouter pigeon did not inflate its crop much more than the turbit now expands the upper part of its esophagus — a habit that all fanciers ignore, as it is not one of the valued features of the breed.
Nor should we think that some great deviation of structure would be necessary to catch the fancier's eye. A fancier perceives extremely small differences, and it is human nature to value any novelty, however slight, in one's own possession. Nor should the value that would formerly have been placed on any slight differences in individuals of the same species be judged by the value placed on them now, after several breeds have been firmly established. It is known that with pigeons, many slight variations now occasionally appear, but these are rejected as faults or deviations from the standard of perfection in each breed. The common goose has not given rise to any marked varieties — hence the Toulouse and the common breed, which differ only in color (that most fleeting of characters), have recently been exhibited as distinct at our poultry shows.
These views seem to explain something that has sometimes been noticed — that we know hardly anything about the origin or history of any of our domestic breeds. But in fact, a breed, like a dialect of a language, can hardly be said to have a distinct origin. A person preserves and breeds from an individual with some slight deviation of structure, or takes more care than usual in matching their best animals, and thus improves them. The improved animals slowly spread in the immediate neighborhood. But they will as yet hardly have a distinct name, and from being only slightly valued, their history will have been ignored. When further improved by the same slow and gradual process, they spread more widely and will be recognized as something distinct and valuable, and will then probably first receive a local name. In countries with less developed communication, the spread of a new sub-breed will be a slow process. As soon as the points of value are recognized, the principle of unconscious selection, as I have called it, will always tend — perhaps more at one period than at another, as the breed rises or falls in fashion, and perhaps more in one district than another, depending on the state of civilization of the inhabitants — slowly to add to the characteristic features of the breed, whatever they may be. But the chance will be infinitely small that any record was preserved of such slow, varying, and imperceptible changes.
I will now say a few words about the circumstances, favorable or unfavorable, to our power of selection. A high degree of variability is obviously favorable, as it freely provides the raw materials for selection to work on — not that mere individual differences are insufficient, with extreme care, to allow the accumulation of a large amount of modification in almost any desired direction. But since variations that are clearly useful or pleasing to us appear only occasionally, the chance of their appearing will be much increased by keeping a large number of individuals. Therefore, numbers are of the highest importance for success. On this principle, Marshall once remarked of the sheep of part of Yorkshire: "As they generally belong to poor people, and are mostly in small lots, they never can be improved." On the other hand, nurseries, from keeping large stocks of the same plant, are generally far more successful than amateurs in raising new and valuable varieties. A large number of individuals of an animal or plant can be raised only where conditions for its propagation are favorable. When individuals are scarce, all will be allowed to breed whatever their quality, and this will effectively prevent selection. But probably the most important factor is that the animal or plant should be so highly valued by people that the closest attention is paid to even the slightest deviations in its qualities or structure. Unless such attention is paid, nothing can be achieved. I have seen it seriously remarked that it was most fortunate the strawberry began to vary just when gardeners began to pay attention to this plant. No doubt the strawberry had always varied since it was cultivated, but the slight varieties had been neglected. As soon as gardeners picked out individual plants with slightly larger, earlier, or better fruit, raised seedlings from them, and again picked out the best seedlings and bred from them, then (with some help from crossing distinct species) those many admirable varieties of the strawberry were raised which have appeared during the last half century.
With animals, the ability to prevent unwanted crosses is an important factor in the formation of new breeds — at least in a country that is already stocked with other breeds. In this respect, the enclosure of land plays a part. Wandering peoples or the inhabitants of open plains rarely possess more than one breed of the same species. Pigeons can be mated for life, and this is a great convenience for the fancier, since many breeds can be improved and kept true even though kept together in the same aviary. This circumstance must have greatly favored the formation of new breeds. Pigeons, I might add, can be bred in great numbers and at a very quick rate, and inferior birds may be freely rejected, since when killed they serve for food. Cats, on the other hand, because of their nocturnal roaming habits, can't be easily paired. And although they are so much valued by women and children, we rarely see a distinct breed maintained for long — such breeds as we do sometimes see are almost always imported from another country. Although I don't doubt that some domestic animals vary less than others, the rarity or absence of distinct breeds of the cat, the donkey, the peacock, the goose, and so on may be attributed mainly to selection not having been applied: in cats, because of the difficulty of pairing them; in donkeys, because only a few are kept by poor people with little attention paid to their breeding (for recently, in certain parts of Spain and the United States, this animal has been surprisingly improved by careful selection); in peacocks, because they are not easily reared and large flocks are not kept; in geese, because they are valued only for two purposes — food and feathers — and especially because no pleasure has been taken in displaying distinct breeds. But the goose, under the conditions to which it is exposed when domesticated, seems to have a remarkably inflexible constitution, though it has varied to a slight extent, as I have described elsewhere.
Some authors have maintained that the amount of variation in our domestic organisms is soon reached and can never afterwards be exceeded. It would be somewhat rash to assert that the limit has been attained in any one case, for almost all our animals and plants have been greatly improved in many ways within a recent period — and this implies variation. It would be equally rash to assert that characters now increased to their utmost limit could not, after remaining fixed for many centuries, vary again under new conditions of life. No doubt, as Wallace has remarked with much truth, a limit will eventually be reached. For instance, there must be a limit to the speed of any land animal, since this will be determined by the friction to be overcome, the weight of the body to be carried, and the power of contraction in the muscle fibers. But what concerns us is that the domestic varieties of the same species differ from each other in almost every character that humans have attended to and selected for more than do the distinct species of the same genera. Isidore Geoffroy St. Hilaire has proved this for size, and the same holds for color, and probably for the length of hair. Regarding speed, which depends on many bodily characters, the racehorse Eclipse was far fleeter, and a draft horse is comparably stronger, than any two natural species belonging to the same genus. The same is true for plants: the seeds of the different varieties of bean or maize probably differ more in size than do the seeds of distinct species in any one genus in those same two families. The same holds for the fruit of the several varieties of the plum, and even more strikingly with the melon, as well as in many other similar cases.
To sum up on the origin of our domestic breeds of animals and plants: changed conditions of life are of the highest importance in causing variability, both by acting directly on the organism and indirectly by affecting the reproductive system. It is not likely that variability is an inherent and necessary feature under all circumstances. The greater or lesser force of inheritance and reversion determine whether variations will endure. Variability is governed by many unknown laws, of which correlated traits are probably the most important. Something — but how much we don't know — may be attributed to the direct action of the conditions of life. Some effect, perhaps a great one, may be attributed to the increased use or disuse of parts. The final result is thus infinitely complex. In some cases, the crossbreeding of originally distinct species appears to have played an important role in the origin of our breeds. Once several breeds have been formed in any country, their occasional crossbreeding, with the aid of selection, has no doubt greatly aided in the formation of new sub-breeds. But the importance of crossing has been much exaggerated, both for animals and for plants that are propagated by seed. For plants that are propagated by cuttings, buds, and the like, the importance of crossing is immense, because the cultivator can here disregard the extreme variability of both hybrids and crossbreeds, and the sterility of hybrids. But plants not propagated by seed are of little importance to us, since their endurance is only temporary. Over all these causes of change, the accumulative action of selection — whether applied methodically and quickly, or unconsciously and slowly but more effectively — seems to have been the dominant force.
Before applying the principles from the last chapter to organisms in the wild, we need to briefly discuss whether wild species are actually subject to variation. To treat this subject properly, I'd need to present a long catalog of dry facts, but I'll save those for a future work. I also won't get into the various definitions that have been given of the term "species." No single definition has satisfied all naturalists, yet every naturalist knows roughly what he means when he uses the word. Generally the term includes an unspoken assumption of a distinct act of creation. The term "variety" is almost equally hard to define, but here shared descent is almost always implied, even though it can rarely be proved. We also have what are called monstrosities, but these grade into varieties. By a monstrosity I mean some major deviation of structure that is generally harmful, or at least not useful to the species. Some authors use the term "variation" in a technical sense, meaning a change caused directly by the physical conditions of life, and "variations" in this sense are supposedly not inherited. But who can say that the stunted shells in the brackish waters of the Baltic, or the dwarfed plants on Alpine summits, or the thicker fur of an animal from the far north, wouldn't be inherited for at least a few generations? And in that case, the form would be called a variety.
It's doubtful whether sudden and major deviations of structure, like those we occasionally see in our domesticated organisms -- especially in plants -- are ever permanently passed on in the wild. Almost every part of every organism is so beautifully fitted to its complex conditions of life that it seems just as unlikely that any part could have been suddenly produced in a perfect state as that a complex machine could have been invented by humans in a perfect state. Under domestication, monstrosities sometimes occur that resemble normal structures in very different animals. For example, pigs have occasionally been born with a kind of trunk, and if any wild species of the same genus had naturally possessed a trunk, it might have been argued that this appeared as a monstrosity. But after diligent searching, I've failed to find cases of monstrosities resembling normal structures in closely related species -- and those are the only cases that bear on the question. If monstrous forms of this kind ever do appear in the wild and are capable of reproducing (which isn't always the case), since they occur rarely and as isolated individuals, their survival would depend on unusually favorable circumstances. They would also, during the first and succeeding generations, cross with the ordinary form, and their abnormal character would almost inevitably be lost. But I'll return in a later chapter to the preservation and continuation of single or occasional variations.
The many slight differences that appear in offspring from the same parents -- or which we can reasonably assume arose this way, from being observed among individuals of the same species living in the same confined area -- may be called individual differences. No one supposes that all individuals of the same species are cast in the same actual mold. These individual differences are extremely important for us, because they are often inherited, as everyone knows, and they provide the raw material for natural selection to act on and accumulate, in the same way that a breeder accumulates individual differences in his domesticated organisms in whatever direction he chooses. These individual differences generally affect what naturalists consider unimportant parts. But I could show, with a long catalog of facts, that parts which must be called important -- whether viewed from a physiological or a classification standpoint -- sometimes vary among individuals of the same species. I'm convinced that even the most experienced naturalist would be surprised at the number of cases of variability, even in important parts of structure, that he could collect from reliable sources, as I have collected over a course of years. It's worth remembering that systematists are far from happy when they find variability in important characters, and that not many people will painstakingly examine internal and important organs and compare them across many specimens of the same species. No one would have expected that the branching of the main nerves close to the great central ganglion of an insect would vary within the same species. You'd think that changes of this kind could only happen through slow degrees. Yet the naturalist Sir John Lubbock has shown a degree of variability in these main nerves in Coccus that can almost be compared to the irregular branching of the stem of a tree. This keen naturalist, I should add, has also shown that the muscles in the larvae of certain insects are far from uniform. Authors sometimes argue in a circle when they claim that important organs never vary -- because these same authors practically define as "important" (as some few naturalists have honestly admitted) only those parts that don't vary. Under this reasoning, no instance of an important part varying will ever be found. But under any other definition, many instances can certainly be given.
There is one point connected with individual differences that is extremely puzzling: I'm referring to those genera that have been called "protean" or "polymorphic," in which species show an extraordinary amount of variation. For many of these forms, hardly any two naturalists agree whether to rank them as species or as varieties. Good examples include Rubus, Rosa, and Hieracium among plants, several genera of insects, and Brachiopod shells. In most polymorphic genera, some of the species do have fixed and definite characters. Genera that are polymorphic in one country seem to be, with a few exceptions, polymorphic in other countries as well -- and likewise, judging from Brachiopod shells, in former periods of time. These facts are very puzzling, because they seem to show that this kind of variability is independent of the conditions of life. I suspect that in at least some of these polymorphic genera, we're seeing variations that are of no benefit or harm to the species, and which therefore have not been seized on and made definite by natural selection, as I'll explain later.
Individuals of the same species often show, as everyone knows, great differences of structure that are independent of variation -- as in the two sexes of various animals, in the two or three castes of sterile females or workers among insects, and in the immature and larval stages of many lower animals. There are also cases of dimorphism and trimorphism, in both animals and plants. For example, Alfred Russel Wallace, who has recently drawn attention to this subject, showed that the females of certain butterfly species in the Malay Archipelago regularly appear in two or even three strikingly distinct forms, not connected by intermediate varieties. Fritz Muller described similar but even more extraordinary cases in the males of certain Brazilian crustaceans: the male of a Tanais regularly occurs in two distinct forms -- one with strong and differently shaped pincers, and the other with antennae much more richly supplied with scent-detecting hairs. Although in most of these cases the two or three forms, in both animals and plants, are not currently connected by intermediate gradations, it's possible that they once were. Wallace, for instance, describes a certain butterfly on one island that shows a great range of varieties connected by intermediate links, and the extreme forms of this chain closely resemble the two forms of a related dimorphic species living in another part of the Malay Archipelago. Similarly with ants: the several worker castes are generally quite distinct, but in some cases, as we'll see later, the castes are connected by finely graded varieties. The same is true, as I've observed myself, with some dimorphic plants. It certainly seems remarkable at first that the same female butterfly should have the power to produce, at the same time, three distinct female forms and a male -- and that a hermaphrodite plant should produce from the same seed capsule three distinct hermaphrodite forms, bearing three different kinds of females and three or even six different kinds of males. But these cases are really just exaggerations of the common fact that a female produces offspring of two sexes, which sometimes differ from each other in striking ways.
The forms that have a considerable degree of species-like character, but that are so closely similar to other forms -- or are so closely linked to them by intermediate gradations -- that naturalists hesitate to rank them as distinct species: these are in several respects the most important for us. We have every reason to believe that many of these doubtful and closely related forms have permanently retained their characters for a long time -- as long, so far as we know, as have fully recognized species. In practice, when a naturalist can connect two forms by means of intermediate links, he treats one as a variety of the other, ranking the most common form (but sometimes the one first described) as the species, and the other as the variety. But cases of great difficulty sometimes arise in deciding whether or not to rank one form as a variety of another, even when they are closely connected by intermediate links. Nor will the commonly assumed hybrid nature of the intermediate forms always resolve the difficulty. In very many cases, however, one form is ranked as a variety of another not because intermediate links have actually been found, but because analogy leads the observer to suppose that they either exist somewhere now or may have existed in the past. And here a wide door is opened for doubt and guesswork.
So in deciding whether a form should be ranked as a species or a variety, the opinion of naturalists with sound judgment and wide experience seems the only guide to follow. We must, however, in many cases decide by majority opinion, since few well-marked and well-known varieties can be named that haven't been ranked as species by at least some competent judges.
That doubtful varieties of this kind are far from uncommon cannot be disputed. Compare the various floras of Great Britain, France, or the United States, compiled by different botanists, and see what a surprising number of forms have been ranked by one botanist as good species and by another as mere varieties. The botanist H. C. Watson, to whom I'm deeply grateful for assistance of all kinds, has identified for me 182 British plants that are generally considered varieties but have all been ranked by some botanists as species. In making this list he left out many minor varieties that have nonetheless been ranked by some botanists as species, and he entirely omitted several highly polymorphic genera. Under these genera, including the most polymorphic forms, Mr. Babington lists 251 species, while Mr. Bentham lists only 112 -- a difference of 139 doubtful forms! Among animals that mate for each birth and are highly mobile, doubtful forms ranked by one zoologist as a species and by another as a variety can rarely be found within the same country, but are common in separated areas. How many of the birds and insects in North America and Europe that differ very slightly from each other have been ranked by one eminent naturalist as undoubted species, and by another as varieties, or, as they are often called, geographical races! Wallace, in several valuable papers on the various animals -- especially the Lepidoptera -- inhabiting the islands of the great Malay Archipelago, shows that they can be sorted into four categories: variable forms, local forms, geographical races or subspecies, and true representative species. The variable forms vary widely within a single island. The local forms are moderately constant and distinct on each separate island, but when specimens from all the islands are compared, the differences are so slight and gradual that it's impossible to define or describe them -- even though the extreme forms are distinct enough. The geographical races or subspecies are local forms that are completely fixed and isolated, but since they don't differ from each other in strongly marked and important characters, "there is no possible test but individual opinion to determine which of them shall be considered as species and which as varieties." Finally, representative species fill the same ecological role on each island as do the local forms and subspecies, but since they differ from each other more than the local forms and subspecies do, they are almost universally ranked by naturalists as true species. Still, no reliable criterion can possibly be given by which variable forms, local forms, subspecies, and representative species can be distinguished.
Many years ago, when comparing -- and watching others compare -- the birds from the closely neighboring islands of the Galapagos Archipelago with one another and with those from the American mainland, I was struck by how entirely vague and arbitrary the distinction between species and varieties really is. On the islets of the little Madeira group there are many insects described as varieties in Mr. Wollaston's admirable work, which would certainly be ranked as distinct species by many entomologists. Even Ireland has a few animals, now generally regarded as varieties, that have been ranked as species by some zoologists. Several experienced ornithologists consider the British red grouse to be only a strongly marked variety of a Norwegian species, while the majority rank it as an undoubted species unique to Great Britain. A wide distance between the homes of two doubtful forms leads many naturalists to rank them as distinct species. But what distance will suffice? If the distance between America and Europe is enough, will the distance between Europe and the Azores, or Madeira, or the Canaries, or between the individual islets of these small archipelagos, also be enough?
The distinguished American entomologist B. D. Walsh described what he calls phytophagic varieties and phytophagic species. Most plant-feeding insects live on one kind of plant or on one group of plants; some feed on many kinds without varying as a result. In several cases, however, Walsh observed that insects found living on different plants show, in their larval or adult stage or both, slight but consistent differences in color, size, or in the nature of their secretions. In some instances only the males differ in this way; in others, both males and females. When the differences are somewhat more strongly marked, and when both sexes and all ages are affected, the forms are ranked by all entomologists as good species. But no observer can determine for another -- even if he can do so for himself -- which of these phytophagic forms should be called species and which should be called varieties. Walsh ranks the forms that would presumably interbreed freely as varieties, and those that appear to have lost this ability as species. Since the differences depend on the insects having fed for a long time on distinct plants, we can't expect to find intermediate links connecting the various forms. The naturalist thus loses his best guide for determining whether to rank doubtful forms as varieties or species. This same problem necessarily arises with closely related organisms living on distinct continents or islands. On the other hand, when an animal or plant ranges over the same continent or inhabits many islands in the same archipelago and takes different forms in different areas, there is always a good chance that intermediate forms will be discovered linking the extremes -- and the forms are then downgraded to the rank of varieties.
A few naturalists maintain that animals never have varieties. But these same naturalists rank the slightest difference as having species-level significance, and when the same identical form is found in two distant countries or in two geological formations, they believe that two distinct species are hiding under the same appearance. Used this way, the term "species" becomes a meaningless abstraction, implying and assuming a separate act of creation. Many forms that highly competent judges consider to be varieties so completely resemble species in their characters that other equally competent judges have ranked them as species. But arguing about whether they should be called species or varieties, before any definition of these terms has been generally accepted, is beating the air.
Many of the cases of strongly marked varieties or doubtful species deserve careful attention, and several interesting lines of argument -- from geographical distribution, analogical variation, hybridism, and so on -- have been brought to bear in trying to determine their rank. But space doesn't permit me to discuss them here. Close investigation will no doubt bring naturalists to agreement on many of these doubtful forms. Yet it must be admitted that it is in the best-known countries where we find the greatest number of them. I've been struck by the fact that if any animal or plant in the wild is highly useful to humans, or for any reason closely attracts our attention, varieties of it will almost always be found on record. These varieties, moreover, will often be ranked by some authors as species. Take the common oak: how closely it has been studied, yet a German author makes more than a dozen species out of forms that are almost universally considered by other botanists to be varieties. And in this country, the highest botanical authorities and practical experts can be cited on both sides of the question of whether the sessile and pedunculated oaks are good and distinct species or mere varieties.
I should mention here a remarkable paper recently published by the botanist Alphonse de Candolle on the oaks of the whole world. No one ever had more ample materials for distinguishing species, or could have worked on them with more dedication and skill. He first gives in detail all the many points of structure that vary across the different species, estimating numerically how frequently each variation occurs. He identifies more than a dozen characters that may be found varying even on the same branch -- sometimes according to age or development, sometimes without any clear reason. Such characters are of course not useful for defining species, but they are, as the botanist Asa Gray has pointed out in commenting on this paper, the kind that generally do enter into species definitions. De Candolle then says that he gives the rank of species to forms that differ by characters which never vary on the same tree and are never found connected by intermediate states. After this discussion -- the result of so much labor -- he emphatically remarks: "They are mistaken, who repeat that the greater part of our species are clearly limited, and that the doubtful species are in a feeble minority. This seemed to be true, so long as a genus was imperfectly known, and its species were founded upon a few specimens, that is to say, were provisional. Just as we come to know them better, intermediate forms flow in, and doubts as to specific limits augment." He also adds that it is the best-known species that present the greatest number of spontaneous varieties and subspecies. Thus Quercus robur has twenty-eight varieties, all of which, except six, are clustered around three subspecies: Q. pedunculata, sessiliflora, and pubescens. The forms connecting these three subspecies are comparatively rare. And as Asa Gray again observes, if these connecting forms -- which are now rare -- were to become totally extinct, the three subspecies would hold exactly the same relation to each other as do the four or five provisionally accepted species that closely surround the typical Quercus robur. In the end, De Candolle admits that out of the 300 species to be listed in his Prodromus as belonging to the oak family, at least two-thirds are provisional species -- meaning they don't strictly meet the definition of a true species given above. It should be added that De Candolle no longer believes species are immutable creations, but concludes that the theory of descent is the most natural one, "and the most accordant with the known facts in palaeontology, geographical botany and zoology, of anatomical structure and classification."
When a young naturalist begins studying a group of organisms entirely new to him, he is at first very puzzled about which differences to consider species-level and which to consider variety-level -- because he knows nothing about the amount and kind of variation the group is subject to. This shows, at the very least, how common variation is. But if he limits his attention to one group within one country, he'll soon make up his mind about most of the doubtful forms. His general tendency will be to create many species, because he'll become impressed -- just like the pigeon or poultry fancier I mentioned earlier -- with the amount of difference in the forms he's constantly studying. And he has little general knowledge of analogical variation in other groups and other countries to correct his first impressions. As he extends his range of observations, he'll encounter more difficult cases, because he'll meet a greater number of closely related forms. But if his observations are wide enough, he'll generally be able to make up his own mind in the end -- though he'll succeed only by accepting a great deal of variation, and the truth of this conclusion will often be disputed by other naturalists. When he comes to study related forms brought from countries that are no longer connected -- where he can't hope to find intermediate links -- he'll be forced to rely almost entirely on analogy, and his difficulties will reach a peak.
Certainly no clear line of demarcation has yet been drawn between species and subspecies -- that is, the forms which in some naturalists' opinion come very close to, but don't quite reach, the rank of species. Nor has a clear line been drawn between subspecies and well-marked varieties, or between lesser varieties and individual differences. These differences blend into each other in an unbroken series, and the series gives the impression of an actual passage from one to the next.
This is why I regard individual differences, though of little interest to the systematist, as extremely important for us -- they are the first step toward the slight varieties that are barely thought worth recording in works on natural history. And I regard varieties that are in any degree more distinct and permanent as steps toward more strongly marked and permanent varieties, and those in turn as leading to subspecies, and then to species. The passage from one stage of difference to another may, in many cases, simply result from the nature of the organism and the different physical conditions it has long been exposed to. But for the more important and adaptive characters, the passage from one stage to another can safely be attributed to the cumulative action of natural selection (to be explained later) and to the effects of increased use or disuse of parts. A well-marked variety may therefore be called an incipient species -- but whether this belief is justified must be judged by the weight of the various facts and arguments presented throughout this work.
It shouldn't be assumed that all varieties or incipient species reach the rank of species. They may go extinct, or they may persist as varieties for very long periods, as Mr. Wollaston has shown with certain fossil land-shells in Madeira, and as Gaston de Saporta has shown with plants. If a variety were to flourish so as to exceed the parent species in numbers, it would then rank as the species, and the original species as the variety. Or it might come to replace and drive the parent species to extinction. Or both might coexist and both rank as independent species. But we'll return to this subject later.
From these remarks you can see that I regard the term "species" as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other -- and that it does not essentially differ from the term "variety," which is given to less distinct and more fluctuating forms. The term "variety," in turn, compared with mere individual differences, is also applied arbitrarily, for convenience.
Guided by theoretical considerations, I thought that some interesting results might be obtained about the nature and relationships of the species that vary most, by tabulating all the varieties in several well-studied floras. At first this seemed like a simple task. But H. C. Watson, to whom I'm much indebted for valuable advice and help on this subject, soon convinced me that there were many difficulties -- as did the botanist Joseph Hooker afterward, in even stronger terms. I'll save the discussion of these difficulties, and the tables of proportional numbers of varying species, for a future work. Hooker allows me to add that after carefully reading my manuscript and examining the tables, he thinks the following statements are fairly well established. The whole subject, however, treated as briefly as it necessarily is here, is rather confusing, and I can't avoid making references to the "struggle for existence," "divergence of character," and other topics to be discussed later.
Alphonse de Candolle and others have shown that plants with very wide ranges generally have varieties -- and this might have been expected, since they are exposed to diverse physical conditions and come into competition (which, as we'll see later, is a far more important factor) with different sets of organisms. But my tables further show that, in any limited country, the species that are most common -- that is, most abundant in individuals -- and the species that are most widely spread within their own country (which is a different thing from having a wide overall range, and to some extent different from mere commonness) most often give rise to varieties well-marked enough to have been recorded in botanical works. So it is the most flourishing species -- or, as they may be called, the dominant species, those that range widely, are the most spread out in their own country, and are the most numerous in individuals -- that most often produce well-marked varieties, or, as I consider them, incipient species. And this might have been expected: since varieties, in order to become permanent to any degree, necessarily have to compete with the other inhabitants of the country, the species that are already dominant will be the most likely to produce offspring that, though somewhat modified, still inherit the advantages that enabled their parents to become dominant over their neighbors. In these remarks about dominance, I should clarify that I'm referring only to forms that compete with each other, and especially to members of the same genus or class with nearly similar ways of life. When it comes to the number of individuals or commonness of species, the comparison of course applies only to members of the same group. A flowering plant may be called dominant if it is more numerous in individuals and more widely spread than the other plants of the same country that live under nearly the same conditions. Such a plant is not less dominant because some green alga in the water or some parasitic fungus is far more numerous in individuals and more widely spread. But if the alga or the parasitic fungus exceeds its own relatives in these respects, it will then be dominant within its own group.
If the plants listed in any country's flora are divided into two equal groups -- all those belonging to the larger genera (that is, genera containing many species) on one side, and all those in the smaller genera on the other -- the larger genera will be found to include a somewhat greater number of the very common, widely spread, or dominant species. This might have been expected, since the mere fact that many species of the same genus live in any country shows that something in the conditions there -- organic or inorganic -- is favorable to that genus. And so we might have expected to find, in the larger genera, a greater proportion of dominant species. But so many factors tend to obscure this result that I'm surprised my tables show even a small majority on the side of the larger genera. I'll mention just two sources of confusion. Freshwater and salt-loving plants generally have very wide ranges and are widely spread, but this seems connected with the nature of the habitats they occupy, and has little or no relation to the size of the genera they belong to. Again, plants that are lower in organizational complexity are generally much more widely spread than more complex plants, and here again there is no close relation to the size of the genera. The cause of simpler plants ranging widely will be discussed in my chapter on geographical distribution.
Because I regard species as nothing more than strongly marked and well-defined varieties, I was led to expect that the species of larger genera in each country would more often show varieties than the species of smaller genera. The reasoning is this: wherever many closely related species (that is, species of the same genus) have been formed, many varieties or incipient species ought, as a general rule, to be forming now. Where many large trees grow, we expect to find saplings. Where many species of a genus have been formed through variation, circumstances have been favorable for variation, and so we might expect those circumstances to still be favorable. On the other hand, if we regard each species as a special act of creation, there is no apparent reason why more varieties should occur in a group with many species than in one with few.
To test this expectation, I arranged the plants of twelve countries, and the beetles of two districts, into two nearly equal groups -- species of the larger genera on one side, species of the smaller genera on the other. It has invariably proved to be the case that a larger proportion of the species in the larger genera show varieties than do the species in the smaller genera. Moreover, the species of the large genera that do show varieties invariably show a larger average number of varieties than do the species of the small genera. Both these results hold when another division is made, and when all the smallest genera -- those with only one to four species -- are excluded from the tables entirely. These facts have a clear meaning if species are only strongly marked and permanent varieties: wherever many species of the same genus have been formed -- where, if we may use the expression, the species factory has been active -- we should generally find the factory still in operation. This is especially true since we have every reason to believe that the process of manufacturing new species is a slow one. And this certainly holds true if varieties are viewed as incipient species, for my tables clearly show, as a general rule, that wherever many species of a genus have been formed, the species of that genus show a number of varieties -- that is, incipient species -- beyond the average. This is not to say that all large genera are now varying rapidly and thus increasing in the number of their species, or that no small genera are now varying and increasing. If that were so, it would have been fatal to my theory, since geology plainly tells us that small genera have, over time, often grown greatly in size, and that large genera have often reached their peak, declined, and disappeared. All I want to show is that, where many species of a genus have been formed, on average many are still forming -- and this certainly holds true.
Many of the Species Included Within the Larger Genera Resemble Varieties in Being Very Closely, but Unequally, Related to Each Other, and in Having Restricted Ranges
There are other relationships between the species of large genera and their recorded varieties that deserve notice. We've seen that there is no infallible criterion for distinguishing species from well-marked varieties, and when intermediate links between doubtful forms have not been found, naturalists are forced to decide based on the amount of difference between them, judging by analogy whether the difference is enough to raise one or both to the rank of species. The amount of difference is therefore one very important criterion in deciding whether two forms should be ranked as species or varieties. Now, the botanist Fries noted with regard to plants, and the entomologist Westwood with regard to insects, that in large genera the amount of difference between species is often extremely small. I've tried to test this numerically using averages, and as far as my imperfect results go, they confirm this view. I've also consulted some sharp and experienced observers, and after deliberation, they agree. In this respect, then, the species of larger genera resemble varieties more than the species of smaller genera do. Or to put it another way: in the larger genera, where a greater-than-average number of varieties or incipient species are now being produced, many of the species already formed still resemble varieties to some extent, because they differ from each other by less than the usual amount.
Moreover, the species of the larger genera are related to each other in the same way that the varieties of any one species are related to each other. No naturalist claims that all the species of a genus are equally distinct from each other; they can generally be divided into subgenera, sections, or smaller groups. As Fries has well observed, little groups of species are generally clustered like satellites around other species. And what are varieties but groups of forms, unequally related to each other, clustered around certain forms -- that is, around their parent species? There is undoubtedly one very important difference between varieties and species: the amount of difference between varieties, when compared with each other or with their parent species, is much less than that between species of the same genus. But when we come to discuss what I call the principle of divergence of character, we'll see how this can be explained, and how the smaller differences between varieties tend to increase into the greater differences between species.
There is one more point worth noting. Varieties generally have very restricted ranges. This statement is really almost a truism, since if a variety were found to have a wider range than its supposed parent species, their names would be swapped. But there is reason to believe that species which are very closely related to other species, and which in that way resemble varieties, often have restricted ranges too. For instance, H. C. Watson has marked for me in the well-vetted London Catalogue of Plants (4th edition) sixty-three plants that are listed as species but that he considers so closely related to other species as to be of doubtful value. These sixty-three supposed species range, on average, over 6.9 of the provinces into which Watson has divided Great Britain. In that same catalog, fifty-three acknowledged varieties are recorded, and these range over 7.7 provinces -- whereas the species to which these varieties belong range over 14.3 provinces. So the acknowledged varieties have very nearly the same restricted average range as the closely related forms that Watson has flagged for me as doubtful species, but which are almost universally ranked by British botanists as good and true species.
In the end, varieties cannot be distinguished from species -- except, first, by the discovery of intermediate linking forms, and second, by a certain indefinite amount of difference between them. Two forms that differ very little are generally ranked as varieties, even when they can't be closely connected. But the amount of difference needed to give any two forms the rank of species can't be defined. In genera with more than the average number of species in any country, the species of those genera have more than the average number of varieties. In large genera, the species tend to be closely but unequally related, forming little clusters around other species. Species very closely related to other species apparently have restricted ranges. In all these respects, the species of large genera show a strong similarity to varieties. And we can clearly understand these similarities if species once existed as varieties, and originated that way -- whereas these similarities are utterly inexplicable if species are independent creations.
We've also seen that it is the most flourishing or dominant species of the larger genera within each group that, on average, produce the greatest number of varieties. And varieties, as we'll see later, tend to become converted into new and distinct species. So the larger genera tend to become still larger, and throughout nature the forms of life that are now dominant tend to become even more dominant by leaving many modified and dominant descendants. But, by steps to be explained later, the larger genera also tend to break up into smaller genera. And so the forms of life throughout the world become divided into groups subordinate to groups.
Before getting into the subject of this chapter, I need to make a few preliminary remarks to show how the struggle for existence connects to natural selection. As we saw in the last chapter, there is some individual variability among organisms in the wild -- indeed, I'm not aware that anyone has ever disputed this. It doesn't matter for our purposes whether a multitude of doubtful forms are called species, subspecies, or varieties -- what rank, for instance, the two or three hundred doubtful forms of British plants deserve to hold, so long as we admit that some well-marked varieties exist. But the mere existence of individual variability and a few well-marked varieties, though necessary as the foundation for the work, doesn't help us much in understanding how species arise in nature. How have all those exquisite adaptations of one part of an organism to another part, and to the conditions of life, and of one living thing to another, been perfected? We see these beautiful co-adaptations most clearly in the woodpecker and the mistletoe, and only a little less clearly in the humblest parasite clinging to the hairs of a mammal or the feathers of a bird; in the structure of the beetle that dives through water; in the plumed seed carried by the gentlest breeze. In short, we see beautiful adaptations everywhere and in every part of the living world.
Again, we might ask: how is it that varieties, which I have called incipient species, eventually become converted into fully distinct species, which in most cases obviously differ from each other far more than the varieties of the same species do? How do those groups of species -- what we call distinct genera, which differ from each other more than species of the same genus -- arise? All these results, as we'll see more fully in the next chapter, follow from the struggle for life. Because of this struggle, variations, however slight and whatever their cause, if they are in any degree useful to the individuals of a species -- in their infinitely complex relationships to other organisms and to their physical conditions -- will tend to be preserved in those individuals and will generally be inherited by their offspring. The offspring will thus have a better chance of surviving, because of the many individuals of any species that are born in each generation, only a small number can survive. I have called this principle -- by which each slight variation, if useful, is preserved -- natural selection, to highlight its relationship to humanity's power of selective breeding. But the expression often used by Herbert Spencer, survival of the fittest, is more accurate and sometimes equally useful. We have seen that humans, through selective breeding, can certainly produce great results and can adapt organisms to our own uses by accumulating slight but useful variations that nature provides. But natural selection, as we'll see later, is a force constantly ready for action, and is as immeasurably superior to our feeble efforts as the works of nature are to those of human craft.
Now let's discuss the struggle for existence in more detail. In my future work, this subject will be treated at the greater length it deserves. The elder Augustin de Candolle and the geologist Charles Lyell have shown extensively and thoughtfully that all living things are exposed to severe competition. When it comes to plants, no one has treated this subject with more energy and skill than the botanist W. Herbert, Dean of Manchester -- clearly the result of his great horticultural knowledge. Nothing is easier than to admit in words the truth of the universal struggle for life, or harder -- at least I found it so -- than to constantly keep this conclusion in mind. Yet unless it is thoroughly ingrained in your thinking, the whole web of nature, with every fact about distribution, rarity, abundance, extinction, and variation, will be only dimly seen or completely misunderstood. We look at the face of nature bright with gladness; we often see a superabundance of food. We don't see, or we forget, that the birds singing idly around us mostly live on insects or seeds, and are constantly destroying life. Or we forget how heavily these songsters, or their eggs, or their nestlings are destroyed by predatory birds and mammals. We don't always keep in mind that although food may be superabundant right now, it is not so at all seasons of every year.
I should explain upfront that I use this term in a broad and metaphorical sense, including the dependence of one organism on another, and including -- which is more important -- not only the life of the individual but success in leaving offspring. Two wolves in a time of famine may truly be said to struggle with each other over which one gets food and lives. But a plant on the edge of a desert is said to struggle for life against the drought, though it would be more accurate to say it depends on moisture. A plant that annually produces a thousand seeds, of which on average only one reaches maturity, may more truly be said to struggle with the plants of the same and other species that already cover the ground. The mistletoe depends on apple trees and a few other species, but it can only in a stretched sense be said to struggle with those trees -- though if too many of these parasites grow on the same tree, it weakens and dies. But several seedling mistletoes growing close together on the same branch may more truly be said to struggle with each other. Since the mistletoe is spread by birds, its existence depends on them; and it may metaphorically be said to struggle with other fruit-bearing plants by tempting birds to eat and thus scatter its seeds. In all these senses, which blend into each other, I use the general term "struggle for existence" for convenience.
A struggle for existence inevitably follows from the high rate at which all organisms tend to increase. Every creature that produces several eggs or seeds during its natural lifetime must suffer destruction at some period of its life, and during some season or occasional year. Otherwise, on the principle of geometrical increase, its numbers would quickly become so extraordinarily large that no country could support them. So, since more individuals are produced than can possibly survive, there must in every case be a struggle for existence -- either one individual against another of the same species, or against individuals of different species, or against the physical conditions of life. It is the doctrine of Malthus applied with tremendous force to the whole animal and plant kingdoms. In nature there can be no artificial increase of food and no deliberate restraint from breeding. Although some species may currently be increasing in numbers, more or less rapidly, all cannot do so -- the world simply would not hold them.
There is no exception to the rule that every organism naturally increases at so high a rate that, if not destroyed, the earth would soon be covered by the descendants of a single pair. Even slow-breeding humans have doubled in population every twenty-five years, and at this rate, in less than a thousand years there would literally not be standing room for our descendants. Linnaeus calculated that if an annual plant produced only two seeds -- and no plant is this unproductive -- and their seedlings the next year produced two, and so on, then in twenty years there would be a million plants. The elephant is considered the slowest breeder of all known animals, and I have taken some trouble to estimate its probable minimum rate of natural increase. It is safest to assume that it begins breeding at thirty years old, continues until ninety, produces six young in that interval, and lives to a hundred. If so, after a period of 740 to 750 years, there would be nearly nineteen million elephants alive, all descended from the first pair.
But we have better evidence than mere theoretical calculations -- namely, the numerous recorded cases of the astonishingly rapid increase of various animals in the wild, when conditions have been favorable for two or three consecutive seasons. Even more striking is the evidence from domestic animals of many kinds that have gone wild in various parts of the world. If the reports of the rate of increase of slow-breeding cattle and horses in South America, and more recently in Australia, had not been well documented, they would have been incredible. So it is with plants: cases could be given of introduced plants that have become common throughout whole islands in less than ten years. Several plants, such as the cardoon and a tall thistle, which now are the most common over the wide plains of La Plata -- covering square leagues of land to the near-exclusion of every other plant -- were introduced from Europe. And there are plants that now range in India, as I hear from Dr. Falconer, from Cape Comorin to the Himalayas, which have been imported from America since its discovery. In such cases -- and endless others could be given -- no one supposes that the fertility of these animals or plants has been suddenly and temporarily increased to any noticeable degree. The obvious explanation is that conditions have been highly favorable, and that consequently there has been less destruction of the old and young, and nearly all the young have been able to breed. Their geometrical ratio of increase, the result of which is always surprising, simply explains their extraordinarily rapid spread and expansion in their new homes.
In the wild, almost every full-grown plant annually produces seeds, and among animals there are very few that do not breed every year. So we can confidently say that all plants and animals tend to increase at a geometrical ratio -- that all would rapidly fill every habitat where they could possibly exist, and that this geometrical tendency to increase must be checked by destruction at some period of life. Our familiarity with larger domestic animals tends, I think, to mislead us: we see no great destruction falling on them and forget that thousands are slaughtered annually for food. In the wild, an equal number would somehow have to be eliminated.
The only difference between organisms that annually produce eggs or seeds by the thousands and those that produce very few is that the slow breeders would need a few more years to populate a whole district, no matter how large, under favorable conditions. The condor lays a couple of eggs and the ostrich a score, yet in the same country the condor may be the more numerous of the two. The fulmar petrel lays just one egg, yet it is believed to be the most numerous bird in the world. One fly deposits hundreds of eggs, and another, like the hippobosca, just a single one. But this difference does not determine how many individuals of the two species a district can support. A large number of eggs matters most for species that depend on a fluctuating food supply, because it lets them increase rapidly in number. But the real importance of producing large numbers of eggs or seeds is to make up for heavy destruction at some period of life -- and in the great majority of cases, that period is an early one. If an animal can somehow protect its own eggs or young, a small number may be produced and the population can still be fully maintained. But if many eggs or young are destroyed, many must be produced or the species will go extinct. It would be enough to maintain the full population of a tree that lived on average for a thousand years if just a single seed were produced once in a thousand years -- assuming that seed were never destroyed and could be guaranteed to germinate in a suitable place. So in all cases, the average number of any animal or plant depends only indirectly on the number of its eggs or seeds.
When looking at nature, it is absolutely essential to keep these considerations in mind -- never to forget that every single organism may be said to be striving with all its might to increase in numbers; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old during each generation or at recurring intervals. Ease any check, reduce the destruction even slightly, and the number of that species will almost instantly increase to any amount.
The causes that check the natural tendency of each species to increase are extremely obscure. Look at the most vigorous species: the more it swarms in numbers, the more it will tend to increase still further. We don't know exactly what the checks are even in a single instance. Nor will this surprise anyone who considers how ignorant we are on this subject even regarding humanity, although we are incomparably better understood than any other animal. This subject of the checks to increase has been ably treated by several authors, and I hope in a future work to discuss it at considerable length, especially regarding the wild animals of South America. Here I will make only a few remarks, just to remind the reader of some of the key points. Eggs or very young animals generally seem to suffer the most, though this is not always the case. With plants there is enormous destruction of seeds, but from some observations I've made, it appears that seedlings suffer most from germinating in ground already thickly stocked with other plants. Seedlings are also destroyed in vast numbers by various enemies. For instance, on a piece of ground three feet long and two feet wide, dug and cleared -- where there could be no choking from other plants -- I marked all the seedlings of our native weeds as they came up, and out of 357, no fewer than 295 were destroyed, mainly by slugs and insects. If turf that has long been mowed -- and the same would apply to turf closely grazed by livestock -- is allowed to grow freely, the more vigorous plants gradually kill the less vigorous ones, even though they are fully grown. Out of twenty species growing on a small plot of mowed turf (three feet by four), nine species died out because the other species were allowed to grow up unchecked.
The amount of food for each species, of course, sets the extreme limit to which it can increase. But very often it is not obtaining food but serving as prey to other animals that determines the average population of a species. There seems to be little doubt, for example, that the numbers of partridges, grouse, and hares on any large estate depend mainly on the destruction of predators. If not a single game bird were shot during the next twenty years in England, and at the same time no predators were killed, there would in all probability be less game than at present -- even though hundreds of thousands of game animals are now shot annually. On the other hand, in some cases, as with the elephant, none are killed by predators; even the tiger in India very rarely dares to attack a young elephant protected by its mother.
Climate plays an important role in determining the average numbers of a species, and periodic seasons of extreme cold or drought seem to be the most effective of all checks. I estimated -- mainly from the greatly reduced number of nests in the spring -- that the winter of 1854-55 destroyed four-fifths of the birds on my own property. This is a tremendous death toll, when we remember that ten percent is an extraordinarily severe mortality from epidemics in humans. The effect of climate seems at first glance quite independent of the struggle for existence. But insofar as climate mainly acts by reducing food, it triggers the most severe struggle between individuals -- whether of the same or different species -- that depend on the same kind of food. Even when climate, for instance extreme cold, acts directly, it will be the least vigorous individuals, or those that got the least food through the advancing winter, that will suffer most. When we travel from south to north, or from a wet region to a dry one, we always see some species gradually getting rarer and rarer, and finally disappearing. Since the change of climate is obvious, we're tempted to attribute the whole effect to its direct action. But this is a false view: we forget that each species, even where it is most abundant, is constantly suffering enormous destruction at some period of its life from enemies or competitors for the same place and food. If those enemies or competitors are even slightly favored by any change of climate, they will increase in numbers. And since each area is already fully stocked with inhabitants, the other species must decrease. When we travel southward and see a species declining in numbers, we can be sure that the cause lies just as much in other species being favored as in this one being harmed. The same is true when we travel northward, but to a somewhat lesser degree, because the number of species of all kinds -- and therefore of competitors -- decreases as you go north. So in going northward, or ascending a mountain, we far more often meet with stunted forms caused by the directly harmful action of climate than we do in heading southward or descending a mountain. When we reach the Arctic regions, or snow-capped summits, or absolute deserts, the struggle for life is almost exclusively against the elements.
That climate acts mainly in an indirect way, by favoring other species, we can clearly see from the enormous number of plants that can perfectly well endure our climate in our gardens but never become established in the wild, because they cannot compete with our native plants or withstand destruction by our native animals.
When a species, due to highly favorable conditions, increases excessively in numbers in a small area, epidemics -- at least this seems generally to happen with our game animals -- often follow. Here we have a check independent of the struggle for life. But even some of these so-called epidemics appear to be caused by parasitic worms, which have been disproportionately favored for some reason -- possibly in part because they spread easily among crowded animals. And here comes a sort of struggle between the parasite and its prey.
On the other hand, in many cases a large population of individuals of the same species, relative to the numbers of its enemies, is absolutely necessary for its survival. We can easily grow plenty of corn and rapeseed in our fields because the seeds vastly outnumber the birds that feed on them. Nor can the birds, though having a superabundance of food at this one season, increase in proportion to the seed supply, since their numbers are kept in check during winter. But anyone who has tried knows how troublesome it is to get seed from a few wheat or similar plants in a garden. I have in this case lost every single seed. This view -- that a large population of the same species is necessary for its survival -- explains, I believe, some curious facts in nature: such as that very rare plants are sometimes extremely abundant in the few spots where they do exist, and that some social plants are social -- that is, abundant in individuals -- even at the extreme edge of their range. For in such cases, we may believe that a plant could exist only where conditions were so favorable that many could survive together, thus saving the species from total destruction. I should add that the benefits of crossbreeding and the harmful effects of inbreeding no doubt play a role in many of these cases, but I won't go into that here.
Many cases are on record showing how complex and unexpected are the checks and relationships between organisms that have to struggle together in the same region. I will give just a single example, which, though a simple one, interested me. In Staffordshire, on the estate of a relative, where I had plenty of opportunity to investigate, there was a large and extremely barren heath that had never been touched by human hands. But several hundred acres of exactly the same type of land had been enclosed twenty-five years earlier and planted with Scotch fir. The change in the native vegetation of the planted portion of the heath was most remarkable -- more dramatic than you'd typically see in moving from one type of soil to a completely different one. Not only were the proportions of the heath plants completely changed, but twelve species of plants (not counting grasses and sedges) flourished in the plantations that could not be found on the open heath. The effect on insects must have been even greater, because six insect-eating bird species were very common in the plantations that were nowhere to be seen on the heath, while the heath was home to two or three different insect-eating birds. Here we see how powerful the effect of introducing a single tree species has been, with nothing else changed except that the land had been fenced so cattle couldn't enter. But how important fencing is, I saw clearly near Farnham, in Surrey. There are extensive heaths there, with a few clumps of old Scotch firs on the distant hilltops. Within the last ten years, large areas have been fenced off, and self-sown firs are now springing up in huge numbers, so close together that not all can survive. When I learned that these young trees had not been sown or planted, I was so surprised at their numbers that I went to several vantage points where I could survey hundreds of acres of the unfenced heath, and I literally could not see a single Scotch fir except the old planted clumps. But on looking closely between the stems of the heather, I found a multitude of seedlings and little trees that had been perpetually browsed down by cattle. In one square yard, at a point some hundred yards from one of the old clumps, I counted thirty-two little trees. One of them, with twenty-six rings of growth, had tried for many years to raise its head above the heather stems and had failed. No wonder that as soon as the land was fenced, it became thickly clothed with vigorously growing young firs. Yet the heath was so extremely barren and so extensive that no one would ever have imagined that cattle could have so closely and effectively searched it for food.
Here we see that cattle absolutely determine the existence of the Scotch fir. But in several parts of the world, insects determine the existence of cattle. Perhaps Paraguay offers the most curious example, for there neither cattle nor horses nor dogs have ever gone wild, even though they swarm in a feral state to the south and north. The naturalists Azara and Rengger showed that this is caused by the greater number in Paraguay of a certain fly that lays its eggs in the navels of these animals at birth. The increase of these flies, numerous as they are, must in turn be kept in check by some means -- probably by other parasitic insects. So if certain insect-eating birds were to decrease in Paraguay, the parasitic insects would probably increase, and this would reduce the number of the navel-infesting flies -- then cattle and horses would go wild, and this would certainly greatly alter the vegetation (as indeed I've observed in parts of South America). This in turn would greatly affect the insects, and this -- as we just saw in Staffordshire -- the insect-eating birds, and so onward in ever-increasing circles of complexity. Not that in nature the relationships are ever as simple as this. Battle within battle must be continually recurring with varying success. Yet in the long run the forces are so nicely balanced that the face of nature remains uniform for long periods of time -- though the merest trifle would certainly give the victory to one organism over another. Nevertheless, so deep is our ignorance, and so great our presumption, that we are astonished when we hear of the extinction of a living creature; and because we don't see the cause, we invoke catastrophes to devastate the world, or invent laws governing the lifespan of species!
I am tempted to give one more example showing how plants and animals, far apart in the scale of nature, are bound together by a web of complex relationships. I will later have occasion to show that the exotic Lobelia fulgens is never visited in my garden by insects and consequently, because of its particular structure, never sets a seed. Nearly all our orchid species absolutely require visits from insects to remove their pollen masses and thus fertilize them. I have found through experiments that bumblebees are almost essential to the fertilization of the heartsease (Viola tricolor), because other bees do not visit this flower. I have also found that bee visits are necessary for the fertilization of some kinds of clover. For instance, twenty heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but twenty other heads protected from bees produced not one. Again, 100 heads of red clover (T. pratense) produced 2,700 seeds, but the same number of protected heads produced not a single seed. Only bumblebees visit red clover, because other bees cannot reach the nectar. It has been suggested that moths may fertilize the clovers, but I doubt they could do so in the case of red clover, since their weight is not enough to push down the wing petals. So we can infer with high probability that if the whole group of bumblebees became extinct or very rare in England, the heartsease and red clover would become very rare or disappear entirely. The number of bumblebees in any district depends in large part on the number of field mice, which destroy their combs and nests. Colonel Newman, who has long studied the habits of bumblebees, believes that "more than two-thirds of them are thus destroyed all over England." Now the number of mice depends largely, as everyone knows, on the number of cats. And Colonel Newman says, "Near villages and small towns I have found the nests of bumblebees more numerous than elsewhere, which I attribute to the number of cats that destroy the mice." So it is entirely plausible that the presence of cats in large numbers in a district might determine, through the chain of mice and then bees, the abundance of certain flowers in that district!
In the case of every species, many different checks, acting at different periods of life and during different seasons or years, probably come into play. Some one check or some few are generally the most important, but all work together to determine the average number, or even the existence, of the species. In some cases it can be shown that very different checks act on the same species in different regions. When we look at the plants and bushes clothing a tangled bank, we're tempted to attribute their proportional numbers and kinds to what we call chance. But how false a view this is! Everyone has heard that when an American forest is cut down, a very different vegetation springs up. But it has been observed that ancient Indian ruins in the southern United States, which must formerly have been cleared of trees, now display the same beautiful diversity and proportion of species as the surrounding virgin forests. What a struggle must have gone on during long centuries between the several kinds of trees, each annually scattering its seeds by the thousand! What war between insect and insect -- between insects, snails, and other animals with birds and predators -- all striving to increase, all feeding on each other, or on the trees, their seeds and seedlings, or on the other plants that first covered the ground and thus checked the growth of the trees. Throw up a handful of feathers, and all fall to the ground according to definite laws. But how simple is the problem of where each feather will fall, compared to the action and reaction of the innumerable plants and animals that have determined, over the course of centuries, the proportional numbers and kinds of trees now growing on the old Indian ruins!
The dependence of one organism on another, as of a parasite on its host, generally occurs between organisms far apart in the scale of nature. This is sometimes also the case with those that may strictly be said to struggle with each other for existence, as with locusts and grass-eating mammals. But the struggle will almost always be most severe between individuals of the same species, because they inhabit the same areas, need the same food, and face the same dangers. Among varieties of the same species, the struggle will generally be almost equally severe, and we sometimes see the contest decided quickly. For instance, if several varieties of wheat are sown together and the mixed seed is resown, some of the varieties that best suit the soil or climate, or are naturally the most fertile, will beat the others and produce more seed, and within a few years will replace the other varieties. To maintain a mixed stock of even such extremely similar varieties as differently colored sweet peas, they must be harvested separately each year and the seed then mixed in the right proportions; otherwise the weaker kinds will steadily decrease and disappear. The same is true of sheep varieties: it has been claimed that certain mountain varieties will starve out other mountain varieties, so they cannot be kept together. The same result has followed from keeping different varieties of the medicinal leech together. It may even be doubted whether the varieties of any of our domestic plants or animals have so exactly the same strength, habits, and constitution that the original proportions of a mixed stock -- with crossing prevented -- could be maintained for even half a dozen generations if they were allowed to struggle together in the same way as organisms in the wild, and if the seed or young were not annually preserved in the right proportions.
Since species of the same genus usually -- though by no means always -- have much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them, when they come into competition, than between species of different genera. We see this in the recent spread of one species of swallow across parts of the United States causing the decline of another species. The recent increase of the mistle thrush in parts of Scotland has caused the decline of the song thrush. How often we hear of one species of rat replacing another under the most different climates! In Russia the small Asian cockroach has everywhere driven out its larger relative. In Australia the imported honeybee is rapidly wiping out the small, stingless native bee. One species of charlock has been known to replace another species -- and so in other cases. We can dimly see why competition should be most severe between closely related forms that fill nearly the same role in nature. But probably in no single case could we say precisely why one species has been victorious over another in the great battle of life.
An extremely important conclusion can be drawn from the foregoing remarks: the structure of every organism is related, in the most essential yet often hidden way, to that of all other organisms with which it competes for food or living space, from which it has to escape, or on which it preys. This is obvious in the teeth and claws of the tiger, and in the legs and claws of the parasite clinging to the hair on the tiger's body. But in the beautifully plumed seed of the dandelion and the flattened, fringed legs of the water beetle, the relationship seems at first limited to the elements of air and water. Yet the advantage of plumed seeds no doubt depends closely on the land being already thickly covered with other plants, so that the seeds may be widely scattered and fall on unoccupied ground. For the water beetle, its legs, so well adapted for diving, allow it to compete with other aquatic insects, to hunt its own prey, and to escape being prey to other animals.
The store of nutrients packed within the seeds of many plants seems at first sight to have no connection to other plants. But from the strong growth of young plants produced from such seeds -- like peas and beans -- when sown in the midst of tall grass, it may be suspected that the main purpose of the nutrients in the seed is to fuel the growth of the seedling while it struggles with other plants growing vigorously all around.
Look at a plant in the middle of its range! Why does it not double or quadruple its numbers? We know that it can perfectly well withstand a little more heat or cold, dampness or dryness, because elsewhere it ranges into slightly hotter or colder, wetter or drier areas. In this case we can clearly see that if we wanted to give the plant the power to increase in numbers, we would have to give it some advantage over its competitors, or over the animals that prey on it. At the edges of its geographic range, a change of constitution with respect to climate would clearly be an advantage. But we have reason to believe that only a few plants or animals range so far that they are destroyed solely by the severity of the climate. Not until we reach the extreme limits of life -- in the Arctic regions or on the borders of an utter desert -- does competition cease. The land may be extremely cold or dry, yet there will still be competition between a few species, or between individuals of the same species, for the warmest or dampest spots.
So we can see that when a plant or animal is placed in a new country among new competitors, the conditions of its life will generally be changed in an essential way, even if the climate is exactly the same as in its former home. If its average numbers are to increase in its new home, we would have to modify it in a different way than we would have in its native country, because we would have to give it some advantage over a different set of competitors or enemies.
It is a useful exercise to try in imagination to give any one species an advantage over another. Probably in no single case would we know what to do. This ought to convince us of our ignorance about the mutual relationships of all living things -- a conviction as necessary as it is hard to acquire. All we can do is to keep steadily in mind that each organism is striving to increase at a geometrical ratio; that each, at some period of its life, during some season of the year, during each generation or at intervals, has to struggle for life and suffer great destruction. When we reflect on this struggle, we may console ourselves with the full belief that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.
How will the struggle for existence, briefly discussed in the last chapter, act on variation? Can the principle of selection, which we've seen is so powerful in human hands, apply in nature? I think we'll see that it can act with tremendous efficiency. Keep in mind the endless number of slight variations and individual differences that occur in our domesticated organisms, and to a lesser degree in those in the wild -- along with the strength of heredity. Under domestication, you could truly say that the whole organism becomes somewhat plastic. But the variability we almost universally find in our domestic organisms is not directly produced by humans, as the botanist Joseph Hooker and the botanist Asa Gray have rightly pointed out. We can neither create varieties nor prevent them from appearing. We can only preserve and accumulate those that do occur. Unintentionally, we expose organisms to new and changing conditions of life, and variability follows -- but similar changes of conditions can and do occur in nature. Keep in mind, too, how infinitely complex and tightly interlocking are the relationships of all organisms to each other and to their physical environment -- and consequently, what an infinitely varied range of structural differences might be useful to each organism under changing conditions of life. Can it really be thought improbable, seeing that variations useful to humans have undoubtedly occurred, that other variations useful in some way to each organism in the great and complex battle of life should occur over the course of many successive generations? If such variations do occur, can we doubt -- remembering that many more individuals are born than can possibly survive -- that individuals having any advantage, however slight, over others would have the best chance of surviving and reproducing? On the other hand, we can be sure that any variation even slightly harmful would be ruthlessly eliminated. This preservation of favorable individual differences and variations, and the destruction of harmful ones, I have called natural selection, or the survival of the fittest. Variations that are neither useful nor harmful would not be affected by natural selection, and would be left as a fluctuating element, as we perhaps see in certain polymorphic species, or would eventually become fixed, depending on the nature of the organism and the nature of the conditions.
Several writers have misunderstood or objected to the term natural selection. Some have even imagined that natural selection induces variability, when in fact it only implies the preservation of variations that arise and are beneficial to an organism under its conditions of life. No one objects to farmers speaking of the powerful effects of human selection -- and in that case, the individual differences provided by nature, which the farmer selects for some purpose, must of course first occur. Others have objected that the term selection implies conscious choice in the animals that become modified. It has even been argued that since plants have no will, natural selection doesn't apply to them! Taken literally, no doubt, natural selection is an imperfect term -- but who ever objected to chemists speaking of the "elective affinities" of various elements? An acid cannot strictly be said to choose the base it combines with. It has been said that I speak of natural selection as an active power or deity -- but who objects to an author speaking of the attraction of gravity as ruling the movements of the planets? Everyone knows what is meant by such metaphorical expressions, and they're almost necessary for the sake of brevity. In the same way, it's hard to avoid personifying the word "Nature" -- but by nature I mean only the combined action and result of many natural laws, and by laws I mean the sequence of events as we've determined them. With a little familiarity, these superficial objections will be forgotten.
We'll best understand the probable course of natural selection by taking the case of a country undergoing some slight physical change -- a shift in climate, for instance. The proportional numbers of its inhabitants will almost immediately change, and some species will probably go extinct. From what we've seen of the intricate and complex way the inhabitants of any country are bound together, any change in the numerical proportions of the inhabitants -- quite apart from the climate change itself -- would seriously affect the others. If the country were open on its borders, new forms would certainly immigrate, and this too would seriously disturb the relationships among the former inhabitants. But in the case of an island, or of a country partly surrounded by barriers, where new and better-adapted forms couldn't freely enter, there would be openings in the natural order that would certainly be better filled if some of the original inhabitants were modified in some way. Had the area been open to immigration, those same openings would have been seized by intruders. In such cases, slight modifications that in any way helped the individuals of any species by better adapting them to their altered conditions would tend to be preserved -- and natural selection would have free scope for its work of improvement.
We have good reason to believe, as shown in the first chapter, that changes in conditions of life tend to increase variability. In the cases just described, the conditions have changed, and this would clearly favor natural selection by providing a better chance for beneficial variations to appear. Without such variations, natural selection can do nothing. Under the term "variations," it must never be forgotten that mere individual differences are included. Just as a human breeder can produce great results with domestic animals and plants by accumulating individual differences in any given direction, so could natural selection -- but far more easily, since it has incomparably more time to work with. Nor do I believe that any great physical change, such as a shift in climate, or any unusual degree of isolation to block immigration, is actually necessary to leave new and unoccupied openings for natural selection to fill by improving some of the varying inhabitants. Since all the inhabitants of each country are struggling together with finely balanced forces, extremely slight modifications in the structure or habits of one species would often give it an advantage over others. And still further modifications of the same kind would often further increase the advantage, as long as the species continued under the same conditions of life and benefited from similar means of subsistence and defense. No country can be named where all the native inhabitants are now so perfectly adapted to each other and to the physical conditions that none of them could be still better adapted or improved. In all countries, the natives have been so far outcompeted by introduced species that they've allowed some foreigners to take firm possession of the land. And since foreigners have beaten some of the natives everywhere, we can safely conclude that the natives might have been modified to their advantage, so as to have better resisted the intruders.
Human breeders can produce, and certainly have produced, great results by their methodical and unconscious means of selection -- so what might natural selection not accomplish? Humans can act only on external and visible traits. Nature -- if I may be allowed to personify the natural preservation or survival of the fittest -- cares nothing for appearances, except insofar as they're useful to any organism. She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Humans select only for their own benefit; Nature only for the benefit of the organism she tends. Every selected trait is fully exercised by her, as is implied by the fact of its selection. Humans keep organisms from many climates in the same country. They seldom exercise each selected trait in some especially fitting manner. They feed a long-beaked pigeon and a short-beaked pigeon on the same food. They don't exercise a long-backed or long-legged mammal in any special way. They expose sheep with long wool and short wool to the same climate. They don't allow the most vigorous males to compete for the females. They don't rigorously destroy all inferior animals, but protect all their stock during each varying season, as far as lies in their power. They often begin their selection with some half-monstrous form, or at least with some modification prominent enough to catch the eye or to be plainly useful to them. Under nature, the slightest differences of structure or constitution may well tip the finely balanced scale in the struggle for life, and so be preserved. How fleeting are the wishes and efforts of humans! How short their time, and consequently how poor their results, compared with those accumulated by Nature during whole geological periods! Can we wonder, then, that Nature's products should be far "truer" in character than ours -- that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship?
It may metaphorically be said that natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations -- rejecting those that are bad, preserving and adding up all that are good -- silently and imperceptibly working, whenever and wherever opportunity offers, at the improvement of each organism in relation to its living and non-living conditions. We see nothing of these slow changes in progress, until the hand of time has marked the long passage of ages, and then so imperfect is our view into long-past geological ages that we see only that the forms of life are now different from what they formerly were.
For any great amount of modification to take place in a species, a variety, once formed, must again -- perhaps after a long interval of time -- vary or present individual differences of the same favorable nature as before. And these must again be preserved, and so onward, step by step. Seeing that individual differences of the same kind continually recur, this can hardly be considered an unreasonable assumption. But whether it's true, we can judge only by seeing how far the hypothesis accords with and explains the general phenomena of nature. On the other hand, the common belief that the amount of possible variation is a strictly limited quantity is likewise a simple assumption.
Although natural selection can act only through and for the good of each organism, traits and structures that we might consider utterly trivial may be acted on. When we see leaf-eating insects green, and bark-feeders mottled grey; the alpine ptarmigan white in winter, the red grouse the color of heather -- we must believe that these colors serve these birds and insects by protecting them from danger. Grouse, if not killed at some point in their lives, would increase in countless numbers. They're known to suffer heavily from birds of prey, and hawks are guided by eyesight to their prey -- so much so that in parts of the continent, people are warned not to keep white pigeons, as they're the most likely to be killed. So natural selection could be effective in giving each kind of grouse its proper color, and in keeping that color, once acquired, true and constant. And we shouldn't think that the occasional killing of an animal of a particular color would have little effect. Remember how essential it is, in a flock of white sheep, to destroy a lamb with the faintest trace of black. We've seen how the color of hogs that feed on the "paint-root" in Virginia determines whether they live or die. In plants, the down on the fruit and the color of the flesh are considered by botanists to be traits of the most trivial importance. Yet we hear from an excellent horticulturist, Downing, that in the United States smooth-skinned fruits suffer far more from a beetle, a curculio, than those with down; that purple plums suffer far more from a certain disease than yellow plums; while another disease attacks yellow-fleshed peaches far more than those with other colored flesh. If, with all the aids of cultivation, these slight differences make a great difference in growing the various varieties, then surely in nature, where the trees would have to struggle with other trees and with a host of enemies, such differences would effectively settle which variety -- whether smooth or downy, yellow-fleshed or purple-fleshed -- should succeed.
When we look at the many small differences between species, which as far as our ignorance allows us to judge seem quite unimportant, we must not forget that climate, food, and so on have no doubt produced some direct effect. It's also necessary to keep in mind that, due to correlated development, when one part varies and those variations are accumulated through natural selection, other modifications, often of the most unexpected nature, will follow.
Just as we see that variations appearing at any particular period of life under domestication tend to reappear in the offspring at the same period -- for instance, in the shape, size, and flavor of the seeds of our many varieties of crop plants; in the caterpillar and cocoon stages of silkworm varieties; in the eggs of poultry and in the color of their chicks' down; in the horns of our sheep and cattle when nearly adult -- so in nature, natural selection will be able to act on and modify organisms at any age, by accumulating variations profitable at that age and inherited at a corresponding age. If it profits a plant to have its seeds more and more widely spread by the wind, I can see no greater difficulty in this being brought about through natural selection than in the cotton grower increasing and improving by selection the down in the pods of his cotton plants. Natural selection may modify and adapt the larva of an insect to a score of circumstances wholly different from those affecting the mature insect -- and these modifications may affect, through correlated development, the structure of the adult. Conversely, modifications in the adult may affect the structure of the larva. But in all cases natural selection will ensure that such changes are not harmful -- for if they were, the species would go extinct.
Natural selection will modify the structure of the young in relation to the parent, and of the parent in relation to the young. In social animals, it will adapt the structure of each individual for the benefit of the whole community, if the community benefits from the selected change. What natural selection cannot do is modify the structure of one species, without giving it any advantage, for the good of another species. Although statements to this effect can be found in works of natural history, I cannot find a single case that holds up under investigation. A structure used only once in an animal's life, if of great importance, might be modified to any extent by natural selection -- for instance, the great jaws of certain insects, used exclusively for opening the cocoon, or the hard tip on the beak of unhatched birds, used for breaking the egg. It has been claimed that of the best short-beaked tumbler pigeons, more die in the egg than manage to get out of it, so that fanciers assist in the hatching. Now, if nature had to make the beak of a full-grown pigeon very short for the bird's own advantage, the process of modification would be very slow, and there would simultaneously be the most rigorous selection of all the young birds within the egg that had the most powerful and hardest beaks, since all with weak beaks would inevitably die. Or, more delicate and more easily broken shells might be selected, since the thickness of the shell is known to vary like every other structure.
It's worth noting here that with all organisms there must be much random destruction, which can have little or no influence on the course of natural selection. For instance, a vast number of eggs or seeds are devoured annually, and these could be modified through natural selection only if they varied in some way that protected them from their enemies. Yet many of these eggs or seeds would perhaps, if not destroyed, have produced individuals better adapted to their conditions of life than any of those that happened to survive. Similarly, a vast number of mature animals and plants, whether or not they are the best adapted to their conditions, must be destroyed annually by accidental causes -- causes that would not be lessened in the slightest by certain changes of structure or constitution that would in other ways be beneficial to the species. But even if the destruction of adults is very heavy, as long as the number that can exist in any area is not wholly kept down by such causes -- or again, even if the destruction of eggs or seeds is so great that only a hundredth or a thousandth part develop -- yet among those that do survive, the best-adapted individuals, assuming there is any variability in a favorable direction, will tend to reproduce in larger numbers than the less well adapted. If the numbers are wholly kept down by the causes just mentioned, as will often be the case, natural selection will be powerless in certain beneficial directions. But this is no valid objection to its effectiveness at other times and in other ways -- for we're far from having any reason to suppose that many species ever undergo modification and improvement at the same time in the same area.
Since peculiarities often appear under domestication in one sex and become hereditably attached to that sex, the same will no doubt happen in nature. This makes it possible for the two sexes to be modified through natural selection in relation to different habits of life, as sometimes occurs, or for one sex to be modified in relation to the other, as commonly occurs. This leads me to say a few words on what I've called sexual selection. This form of selection depends not on a struggle for existence against other organisms or against external conditions, but on a struggle between individuals of one sex -- generally the males -- for possession of the other sex. The result is not death for the unsuccessful competitor, but few or no offspring. Sexual selection is therefore less rigorous than natural selection. Generally, the most vigorous males -- those best fitted for their places in nature -- will leave the most offspring. But in many cases, victory depends not so much on general vigor as on having special weapons confined to the male sex. A hornless stag or spurless rooster would have a poor chance of leaving numerous offspring. Sexual selection, by always allowing the victor to breed, could surely give indomitable courage, length of spur, and strength to the wing to strike with the spurred leg, in nearly the same way that the brutal cockfighter does through careful selection of his best birds. How low in the scale of nature the law of battle extends, I don't know. Male alligators have been described as fighting, bellowing, and whirling around, like warriors in a war dance, for possession of the females. Male salmon have been observed fighting all day long. Male stag beetles sometimes bear wounds from the huge mandibles of other males. The males of certain wasps and related insects have been frequently seen by that incomparable observer M. Fabre fighting for a particular female, who sits by as an apparently unconcerned spectator of the struggle and then retires with the conqueror. The war is perhaps most severe between the males of polygamous animals, and these seem most often equipped with special weapons. The males of carnivorous animals are already well armed, though even to them and to others, special means of defense may be given through sexual selection -- like the mane of the lion and the hooked jaw of the male salmon -- for the shield may be as important for victory as the sword or spear.
Among birds, the contest is often more peaceful in character. Everyone who has studied the subject believes that there is the fiercest rivalry among the males of many species to attract females by singing. The rock-thrush of Guiana, birds of paradise, and some others congregate, and successive males display with the most elaborate care, showing off their gorgeous plumage to best advantage. They likewise perform strange dances before the females, who stand by as spectators and at last choose the most attractive partner. Those who have closely studied birds in captivity know well that they often develop individual preferences and dislikes. Sir R. Heron described how a pied peacock was strongly attractive to all his hen birds. I can't go into the necessary details here, but if humans can in a short time give beauty and an elegant carriage to their bantam chickens, according to their standard of beauty, I see no good reason to doubt that female birds, by selecting over thousands of generations the most melodious or beautiful males according to their standard of beauty, could produce a marked effect. Some well-known patterns in the plumage of male and female birds, compared with the plumage of the young, can partly be explained through sexual selection acting on variations that occur at different ages and are transmitted to males alone or to both sexes at corresponding ages. But I don't have space here to go into this subject.
So it is, as I believe, that when the males and females of any animal have the same general habits of life but differ in structure, color, or ornament, such differences have been mainly caused by sexual selection -- that is, by individual males having had, in successive generations, some slight advantage over other males in their weapons, means of defense, or attractiveness, which they've transmitted to their male offspring alone. Yet I wouldn't wish to attribute all sexual differences to this agency. We see in our domestic animals peculiarities arising and becoming attached to the male sex that apparently have not been enhanced through selection by humans. The tuft of hair on the breast of the wild turkey cock cannot be of any use, and it's doubtful whether it's ornamental in the eyes of the female bird. Indeed, had the tuft appeared under domestication, it would have been called a monstrosity.
In order to make it clear how, as I believe, natural selection acts, I must ask permission to give one or two imaginary illustrations. Let's take the case of a wolf that preys on various animals, securing some by craft, some by strength, and some by speed. Suppose that the fastest prey -- a deer, for instance -- had increased in numbers due to some change in the country, or that other prey had decreased in numbers, during the season when the wolf was hardest pressed for food. Under such circumstances, the swiftest and slimmest wolves would have the best chance of surviving, and so would be preserved or selected -- provided always that they kept enough strength to overpower their prey at this or some other time of year, when they were forced to hunt other animals. I can see no more reason to doubt this would be the result than to doubt that humans can improve the speed of their greyhounds by careful and methodical selection, or by that kind of unconscious selection that follows from each person simply trying to keep the best dogs without any thought of modifying the breed.
Even without any change in the proportional numbers of the animals the wolf preyed on, a cub might be born with an innate tendency to pursue certain kinds of prey. This shouldn't be thought very improbable, since we often observe great differences in the natural tendencies of our domestic animals. One cat, for instance, takes to catching rats, another mice. One cat, according to Mr. St. John, brings home winged game, another hares or rabbits, and another hunts on marshy ground and almost nightly catches woodcocks or snipes. The tendency to catch rats rather than mice is known to be inherited. Now, if any slight innate change of habit or of structure benefited an individual wolf, it would have the best chance of surviving and leaving offspring. Some of its young would probably inherit the same habits or structure, and by repeating this process, a new variety might be formed that would either replace or coexist with the parent form of wolf. Or again, the wolves inhabiting a mountainous district, and those frequenting the lowlands, would naturally be forced to hunt different prey. Through the continued preservation of the individuals best fitted for each habitat, two varieties might slowly be formed. These varieties would crossbreed and blend where they met -- but to this subject of crossbreeding we'll soon have to return. I may add that, according to Mr. Pierce, there are two varieties of wolf inhabiting the Catskill Mountains in the United States -- one with a light, greyhound-like form that pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd's flocks.
It should be noted that in the above illustration, I'm speaking of the slimmest individual wolves, and not of any single strongly marked variation being preserved. In former editions of this work I sometimes spoke as if the latter had frequently occurred. I recognized the great importance of individual differences, and this led me to fully discuss the results of unconscious selection by humans, which depends on the preservation of all the more or less valuable individuals and on the destruction of the worst. I also recognized that the preservation in nature of any occasional deviation of structure, such as a monstrosity, would be a rare event -- and that, if initially preserved, it would generally be lost by subsequent crossbreeding with ordinary individuals. Nevertheless, until reading an able and valuable article in the North British Review (1867), I didn't appreciate how rarely single variations, whether slight or strongly marked, could be perpetuated. The author takes the case of a pair of animals producing during their lifetime two hundred offspring, of which, from various causes of destruction, only two on average survive to reproduce. This is rather an extreme estimate for most of the higher animals, but by no means so for many of the lower organisms. He then shows that if a single individual were born that varied in some way giving it twice as good a chance of life as the others, the chances would still be strongly against its survival. Supposing it survived and bred, and that half its young inherited the favorable variation -- still, as the reviewer goes on to show, the young would have only a slightly better chance of surviving and breeding, and this chance would go on decreasing in succeeding generations. The justice of these remarks cannot, I think, be disputed. If, for instance, a bird of some kind could get its food more easily by having a curved beak, and if one were born with a strongly curved beak and consequently flourished, there would nonetheless be a very poor chance of this one individual perpetuating its kind to the exclusion of the common form. But there can hardly be a doubt, judging by what we see under domestication, that this result would follow from the preservation during many generations of a large number of individuals with more or less strongly curved beaks, and from the destruction of a still larger number with the straightest beaks.
It should not, however, be overlooked that certain rather strongly marked variations, which no one would rank as mere individual differences, frequently recur because a similar organism is similarly acted on -- of which fact numerous instances could be given from our domestic organisms. In such cases, if the varying individual did not actually transmit its newly acquired trait to its offspring, it would undoubtedly transmit to them, as long as the existing conditions remained the same, a still stronger tendency to vary in the same way. There can also be little doubt that the tendency to vary in the same way has often been so strong that all the individuals of the same species have been similarly modified without the aid of any form of selection. Or only a third, fifth, or tenth part of the individuals may have been affected, of which fact several instances could be given. Thus Graba estimates that about one-fifth of the guillemots in the Faroe Islands consist of a variety so well marked that it was formerly ranked as a distinct species under the name Uria lacrymans. In cases of this kind, if the variation were beneficial, the original form would soon be replaced by the modified form, through the survival of the fittest.
On the effects of crossbreeding in eliminating variations of all kinds, I'll have to return to this topic later. But it may be noted here that most animals and plants keep to their own territories and don't needlessly wander about -- we see this even with migratory birds, which almost always return to the same spot. So each newly formed variety would generally be local at first, as seems to be the common rule with varieties in nature. Similarly modified individuals would soon exist in a small group together and would often breed together. If the new variety were successful in its battle for life, it would slowly spread from a central district, competing with and conquering the unchanged individuals on the margins of an ever-increasing circle.
It may be worth giving another and more complex illustration of how natural selection works. Certain plants excrete sweet juice, apparently for the sake of eliminating something harmful from the sap. This is done, for instance, by glands at the base of the stipules in some legumes, and at the backs of the leaves of the common laurel. This juice, though small in quantity, is eagerly sought by insects -- but their visits in no way benefit the plant. Now let's suppose that the juice or nectar was excreted from inside the flowers of a certain number of plants of any species. Insects seeking the nectar would get dusted with pollen and would often carry it from one flower to another. The flowers of two distinct individuals of the same species would thus get crossed -- and the act of crossing, as can be fully proved, gives rise to vigorous seedlings, which would consequently have the best chance of flourishing and surviving. The plants that produced flowers with the largest glands or nectaries, excreting the most nectar, would most often be visited by insects and would most often be crossed. So in the long run they would gain the upper hand and form a local variety. The flowers that also had their stamens and pistils positioned -- in relation to the size and habits of the particular insect visiting them -- so as to favor in any degree the transport of pollen, would likewise be favored. We might have taken the case of insects visiting flowers for the sake of collecting pollen instead of nectar. Since pollen is formed solely for the purpose of fertilization, its destruction appears to be a simple loss to the plant. Yet if a little pollen were carried, at first occasionally and then habitually, by the pollen-eating insects from flower to flower, and a cross thus achieved, then although nine-tenths of the pollen were destroyed, it might still be a great gain to the plant. And the individuals that produced more and more pollen, and had larger anthers, would be selected.
When our plant, by this long-continued process, had been made highly attractive to insects, they would unintentionally carry pollen regularly from flower to flower. That they do this effectively I could easily show by many striking facts. I'll give only one, as it also illustrates one step in the separation of the sexes of plants. Some holly trees bear only male flowers, which have four stamens producing a rather small quantity of pollen and a rudimentary pistil. Other holly trees bear only female flowers; these have a full-sized pistil and four stamens with shriveled anthers in which not a grain of pollen can be detected. Having found a female tree exactly sixty yards from a male tree, I put the stigmas of twenty flowers, taken from different branches, under the microscope, and on all of them without exception there were a few pollen grains, and on some a profusion. Since the wind had been blowing for several days from the female toward the male tree, the pollen couldn't have been carried that way. The weather had been cold and stormy, and therefore not favorable to bees. Nevertheless every female flower I examined had been effectively fertilized by bees that had flown from tree to tree in search of nectar. But to return to our imaginary case: as soon as the plant had been made so highly attractive to insects that pollen was regularly carried from flower to flower, another process might begin. No naturalist doubts the advantage of what has been called the "physiological division of labor." So we may believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole plant, and pistils alone in another flower or on another plant. In plants under cultivation and placed in new conditions of life, sometimes the male organs and sometimes the female organs become more or less impotent. Now, if we suppose this to occur even to the slightest degree in nature, then since pollen is already carried regularly from flower to flower, and since a more complete separation of the sexes would be advantageous on the principle of the division of labor, individuals with this tendency increasingly developed would be continually favored or selected, until at last a complete separation of the sexes might be achieved. It would take up too much space to show the various steps through which the separation of the sexes in plants of various kinds is apparently now in progress. But I may add that some species of holly in North America are, according to Asa Gray, in an exactly intermediate condition, or as he puts it, are more or less dioeciously polygamous.
Now let's turn to the nectar-feeding insects. We may suppose the plant whose nectar we've been slowly increasing by continued selection to be a common plant, and that certain insects depended largely on its nectar for food. I could give many facts showing how eager bees are to save time -- for instance, their habit of cutting holes and sucking the nectar at the bases of certain flowers, which with very little more trouble they could enter by the mouth. With such facts in mind, it's believable that under certain circumstances, individual differences in the curvature or length of the proboscis, too slight for us to notice, might benefit a bee or other insect, so that certain individuals would be able to get their food more quickly than others. The communities to which they belonged would then flourish and throw off many swarms inheriting the same traits. The tubes of the corolla of the common red and incarnate clovers (Trifolium pratense and incarnatum) do not at a quick glance appear to differ in length. Yet the honeybee can easily suck the nectar from the incarnate clover, but not from the common red clover, which is visited by bumblebees alone. So whole fields of red clover offer in vain an abundant supply of precious nectar to the honeybee. That this nectar is much liked by the honeybee is certain, for I have repeatedly seen -- but only in autumn -- many honeybees sucking the flowers through holes bitten in the base of the tube by bumblebees. The difference in the length of the corolla in the two kinds of clover, which determines whether the honeybee can visit, must be very small. I've been told that when red clover has been mown, the flowers of the second crop are somewhat smaller, and that these are visited by many honeybees. I don't know whether this statement is accurate, nor whether another published claim can be trusted -- namely, that the Ligurian bee, which is generally considered a mere variety of the common honeybee and freely crosses with it, is able to reach and suck the nectar of the red clover. Thus, in a country where this kind of clover was abundant, it might be a great advantage to the honeybee to have a slightly longer or differently constructed proboscis. On the other hand, since the fertility of this clover absolutely depends on bees visiting the flowers, if bumblebees were to become rare in any country, it might be a great advantage to the plant to have a shorter or more deeply divided corolla, so that honeybees could suck its flowers. In this way I can understand how a flower and a bee might slowly become -- either simultaneously or one after the other -- modified and adapted to each other in the most perfect manner, through the continued preservation of all the individuals that showed slight variations of structure mutually favorable to each other.
I'm well aware that this theory of natural selection, illustrated in the above imaginary examples, is open to the same objections that were first raised against Sir Charles Lyell's noble views on "the modern changes of the earth, as illustrative of geology." But we now seldom hear the forces we see still at work dismissed as trivial and insignificant when used to explain the excavation of the deepest valleys or the formation of long lines of inland cliffs. Natural selection acts only by the preservation and accumulation of small inherited modifications, each beneficial to the organism that's preserved. And just as modern geology has almost banished the idea that a great valley was excavated by a single massive flood, so will natural selection banish the belief in the continual creation of new organisms, or in any great and sudden modification of their structure.
I must here introduce a short digression. In the case of animals and plants with separate sexes, it's obvious that two individuals must always unite for each birth (with the exception of the curious and not well-understood cases of parthenogenesis). But in the case of hermaphrodites, this is far from obvious. Nevertheless, there is reason to believe that with all hermaphrodites, two individuals either occasionally or habitually come together for reproduction. This view was suggested long ago, though tentatively, by Sprengel, Knight, and Kolreuter. We'll soon see its importance. But I must treat the subject here with extreme brevity, though I have the materials prepared for a full discussion. All vertebrate animals, all insects, and some other large groups of animals pair for each birth. Modern research has greatly reduced the number of supposed hermaphrodites, and of the real hermaphrodites, a large number do pair -- that is, two individuals regularly unite for reproduction, which is all that concerns us. But there are still many hermaphroditic animals that certainly don't habitually pair, and the vast majority of plants are hermaphrodites. What reason, it may be asked, is there for supposing that in these cases two individuals ever come together for reproduction? Since it's impossible here to go into details, I must rely on some general considerations alone.
In the first place, I've collected a large body of facts and conducted many experiments showing -- in line with the almost universal belief of breeders -- that with animals and plants, a cross between different varieties, or between individuals of the same variety but of a different strain, gives vigor and fertility to the offspring. On the other hand, inbreeding diminishes vigor and fertility. These facts alone incline me to believe that it is a general law of nature that no organism fertilizes itself for an eternity of generations, but that a cross with another individual is occasionally -- perhaps at long intervals of time -- indispensable.
On the belief that this is a law of nature, we can, I think, understand several large classes of facts that are otherwise inexplicable. Every plant breeder knows how unfavorable exposure to wet is for the fertilization of a flower, yet what a multitude of flowers have their anthers and stigmas fully exposed to the weather! If an occasional cross is indispensable, despite the plant's own anthers and pistil standing so near each other as almost to ensure self-fertilization, the fullest freedom for the entrance of pollen from another individual would explain this exposure of the organs. Many flowers, on the other hand, have their reproductive organs closely enclosed, as in the great pea family (the Papilionaceae). But these almost invariably show beautiful and curious adaptations related to the visits of insects. So necessary are the visits of bees to many of these flowers that their fertility is greatly diminished if these visits are prevented. Now, it's scarcely possible for insects to fly from flower to flower without carrying pollen from one to another, greatly benefiting the plant. Insects act like a camel-hair paintbrush -- it's enough to touch the anthers of one flower and then the stigma of another to ensure fertilization. But it must not be supposed that bees would thus produce a multitude of hybrids between distinct species, for if a plant's own pollen and that from another species are placed on the same stigma, the plant's own pollen is so dominant that it invariably and completely destroys, as the botanist Gartner has shown, the influence of the foreign pollen.
When the stamens of a flower suddenly spring toward the pistil, or slowly move one after another toward it, the mechanism seems designed solely to ensure self-fertilization -- and no doubt it's useful for that purpose. But the action of insects is often required to cause the stamens to spring forward, as Kolreuter showed to be the case with the barberry. And in this very genus, which seems to have a special mechanism for self-fertilization, it's well known that if closely related forms or varieties are planted near each other, it's nearly impossible to raise pure seedlings, so freely do they naturally cross. In numerous other cases, far from self-fertilization being favored, there are special mechanisms that effectively prevent the stigma from receiving pollen from its own flower, as I could show from the works of Sprengel and others, as well as from my own observations. For instance, in Lobelia fulgens, there is a truly beautiful and elaborate mechanism by which all the infinitely numerous pollen grains are swept out of the joined anthers of each flower before the stigma of that individual flower is ready to receive them. Since this flower is never visited, at least in my garden, by insects, it never sets a seed -- though by placing pollen from one flower on the stigma of another, I raise plenty of seedlings. Another species of Lobelia, which is visited by bees, seeds freely in my garden. In very many other cases, though there's no special mechanical device to prevent the stigma from receiving pollen from the same flower, either the anthers burst before the stigma is ready for fertilization, or the stigma is ready before the pollen of that flower is ready, as Sprengel and more recently Hildebrand and others have shown, and as I can confirm. So these so-called dichogamous plants have in fact separated sexes and must habitually be crossed. The same is true of the reciprocally dimorphic and trimorphic plants mentioned earlier. How strange these facts are! How strange that the pollen and stigmatic surface of the same flower, though placed so close together as if for the very purpose of self-fertilization, should in so many cases be mutually useless to each other! How simply these facts are explained by the view that an occasional cross with a distinct individual is advantageous or indispensable!
If several varieties of cabbage, radish, onion, and some other plants are allowed to set seed near each other, a large majority of the seedlings turn out, as I found, to be hybrids. For instance, I raised 233 seedling cabbages from some plants of different varieties growing near each other, and of these only 78 were true to their kind, and some even of these were not perfectly true. Yet the pistil of each cabbage flower is surrounded not only by its own six stamens but by those of the many other flowers on the same plant, and the pollen of each flower readily gets on its stigma without insect assistance -- for I've found that plants carefully protected from insects produce the full number of pods. How, then, does it happen that such a vast number of the seedlings are hybrids? It must be because the pollen of a distinct variety has a dominant effect over the flower's own pollen -- and this is part of the general law that good comes from the crossbreeding of distinct individuals of the same species. When distinct species are crossed the case is reversed, for a plant's own pollen is always dominant over foreign pollen. But to this subject we'll return in a future chapter.
In the case of a large tree covered with innumerable flowers, it may be objected that pollen could seldom be carried from tree to tree, and at most only from flower to flower on the same tree -- and flowers on the same tree can be considered as distinct individuals only in a limited sense. I believe this objection is valid, but that nature has largely guarded against it by giving trees a strong tendency to bear flowers with separate sexes. When the sexes are separated, even though the male and female flowers may be produced on the same tree, pollen must be regularly carried from flower to flower -- and this gives a better chance of pollen being occasionally carried from tree to tree. That trees of all groups have their sexes separated more often than other plants, I find to be the case in this country. At my request, the botanist Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the United States, and the result was as I had anticipated. On the other hand, Dr. Hooker tells me the rule doesn't hold in Australia. But if most Australian trees are dichogamous -- having their male and female parts maturing at different times -- the same result would follow as if they bore flowers with separate sexes. I've made these few remarks on trees simply to draw attention to the subject.
Turning briefly to animals: various terrestrial species are hermaphrodites, such as the land snails and earthworms, but these all pair. As yet I haven't found a single terrestrial animal that can fertilize itself. This remarkable fact, which offers so strong a contrast with terrestrial plants, makes sense if an occasional cross is indispensable. Because of the nature of the fertilizing element in terrestrial animals, there are no means analogous to the action of insects and wind with plants by which an occasional cross could be achieved without two individuals coming together. Among aquatic animals, there are many self-fertilizing hermaphrodites -- but here currents of water offer an obvious means for an occasional cross. As with flowers, I have been unable, after consulting one of the highest authorities -- namely, Professor Huxley -- to find a single hermaphrodite animal with the reproductive organs so perfectly enclosed that access from outside, and the occasional influence of a distinct individual, can be shown to be physically impossible. Barnacles (Cirripedes) long appeared to me to present a case of great difficulty on this point. But I've been able, by a fortunate chance, to prove that two individuals, though both are self-fertilizing hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange oddity that, in both animals and plants, some species of the same family and even of the same genus -- though agreeing closely with each other in their whole organization -- are hermaphrodites, while some have separate sexes. But if, in fact, all hermaphrodites do occasionally crossbreed, the difference between them and species with separate sexes is, as far as function is concerned, very small.
From these several considerations, and from the many specific facts I've collected but cannot give here, it appears that with animals and plants an occasional cross between distinct individuals is a very general, if not universal, law of nature.
This is an extremely intricate subject. A great amount of variability -- under which term individual differences are always included -- will obviously be favorable. A large number of individuals, by giving a better chance within any given period for beneficial variations to appear, will compensate for a lesser amount of variability in each individual, and is, I believe, a highly important element of success. Though nature grants long periods of time for the work of natural selection, she does not grant an indefinite period. Since all organisms are striving to seize every place in the natural order, if any one species does not become modified and improved at a corresponding rate with its competitors, it will be driven to extinction. Unless favorable variations are inherited by at least some of the offspring, nothing can be accomplished by natural selection. The tendency to reversion may often check or prevent the work -- but since this tendency hasn't prevented humans from forming numerous domestic breeds by selection, why should it prevail against natural selection?
In the case of methodical selection, a breeder selects for some definite purpose, and if the individuals are allowed to freely crossbreed, the work will completely fail. But when many people, without intending to alter the breed, share a nearly common standard of perfection and all try to obtain and breed from the best animals, improvement surely but slowly follows from this unconscious process of selection, even though there is no separation of selected individuals. So it will be in nature. Within a confined area, with some place in the natural order not perfectly occupied, all the individuals varying in the right direction, though in different degrees, will tend to be preserved. But if the area is large, its several districts will almost certainly present different conditions of life. Then, if the same species undergoes modification in different districts, the newly formed varieties will crossbreed on the borders of each. But we'll see in the sixth chapter that intermediate varieties, inhabiting intermediate districts, will in the long run generally be replaced by one of the adjoining varieties. Crossbreeding will chiefly affect those animals that mate for each birth, wander widely, and don't breed at a very rapid rate. So with animals of this nature -- birds, for instance -- varieties will generally be confined to separate regions, and this I find to be the case. With hermaphroditic organisms that cross only occasionally, and likewise with animals that mate for each birth but wander little and can increase rapidly, a new and improved variety might be quickly formed in any one spot. It might maintain itself as a group there and afterwards spread, so that the individuals of the new variety would chiefly cross with each other. On this principle, nursery growers always prefer saving seed from a large body of plants, as the chance of crossbreeding is thus lessened.
Even with animals that mate for each birth and don't reproduce rapidly, we must not assume that free crossbreeding would always eliminate the effects of natural selection. For I can bring forward a considerable body of facts showing that within the same area, two varieties of the same animal may long remain distinct -- from frequenting different habitats, from breeding at slightly different seasons, or from the individuals of each variety preferring to pair together.
Crossbreeding plays a very important part in nature by keeping the individuals of the same species, or of the same variety, true and uniform in character. It will obviously be far more effective with those animals that mate for each birth. But as already stated, we have reason to believe that occasional crossbreeding takes place with all animals and plants. Even if these occur only at long intervals of time, the young thus produced will gain so much in vigor and fertility over the offspring from long-continued self-fertilization that they'll have a better chance of surviving and reproducing. And so in the long run, the influence of crosses, even at rare intervals, will be great. With organisms extremely low in the scale that don't reproduce sexually, can't conjugate, and can't possibly crossbreed, uniformity of character can be retained only through the principle of inheritance and through natural selection destroying any individuals departing from the proper type. If conditions change and the form undergoes modification, uniformity of character can be given to the modified offspring solely by natural selection preserving similar favorable variations.
Isolation is also an important element in the modification of species through natural selection. In a confined or isolated area, if not very large, the living and non-living conditions of life will generally be almost uniform, so that natural selection will tend to modify all the varying individuals of the same species in the same way. Crossbreeding with the inhabitants of surrounding areas will also be prevented. Moritz Wagner has published an interesting essay on this subject, showing that the role of isolation in preventing crosses between newly formed varieties is probably even greater than I supposed. But for reasons already given, I can by no means agree with this naturalist that migration and isolation are necessary elements for the formation of new species. The importance of isolation is likewise great in preventing, after any physical change in conditions such as climate or elevation of the land, the immigration of better-adapted organisms. Thus new openings in the natural order of the district will be left to be filled by the modification of the old inhabitants. Finally, isolation will give time for a new variety to be improved at a slow rate, and this may sometimes be of much importance. If, however, an isolated area is very small -- either from being surrounded by barriers or from having very unusual physical conditions -- the total number of inhabitants will be small, and this will slow the production of new species through natural selection by decreasing the chances of favorable variations arising.
The mere passage of time by itself does nothing, either for or against natural selection. I state this because it has been wrongly asserted that I assume time plays an all-important part in modifying species, as if all forms of life were necessarily undergoing change through some innate law. The passage of time is important only insofar as it gives a better chance of beneficial variations arising and of their being selected, accumulated, and fixed. It also tends to increase the direct action of the physical conditions of life on each organism's constitution.
If we turn to nature to test the truth of these remarks, and look at any small isolated area such as an oceanic island, although the number of species inhabiting it is small, as we'll see in our chapter on geographical distribution, yet a very large proportion of these species are endemic -- that is, they've been produced there and nowhere else in the world. So an oceanic island at first sight seems to have been highly favorable for the production of new species. But we may deceive ourselves, for to determine whether a small isolated area or a large open area like a continent has been more favorable for the production of new forms, we ought to make the comparison within equal time periods -- and this we're unable to do.
Although isolation is of great importance in the production of new species, on the whole I'm inclined to believe that largeness of area is still more important, especially for the production of species that will prove capable of enduring for a long period and of spreading widely. Throughout a great and open area, not only will there be a better chance of favorable variations arising from the large number of individuals of the same species living there, but the conditions of life are much more complex because of the large number of already existing species. If some of these many species become modified and improved, others will have to be improved at a corresponding rate, or they'll be driven to extinction. Each new form, also, as soon as it has been much improved, will be able to spread over the open and continuous area and will thus come into competition with many other forms. Moreover, great areas, though now continuous, will often -- owing to former changes in sea level -- have existed in a broken condition, so that the good effects of isolation will generally, to some extent, have played their part as well. Finally, I conclude that although small isolated areas have been in some respects highly favorable for the production of new species, the course of modification will generally have been more rapid on large areas. And what is more important, the new forms produced on large areas -- which have already been victorious over many competitors -- will be those that spread most widely and give rise to the greatest number of new varieties and species. They will thus play a more important part in the changing history of the living world.
In line with this view, we can perhaps understand some facts that will come up again in our chapter on geographical distribution -- for instance, the fact that the organisms of the smaller continent of Australia are now giving way before those of the larger Eurasian area. In the same way, continental organisms have everywhere become widely established on islands. On a small island, the race for life will have been less severe, and there will have been less modification and less extinction. This is why the flora of Madeira, according to Oswald Heer, resembles to some extent the extinct tertiary flora of Europe. All freshwater basins, taken together, make up a small area compared with that of the sea or the land. Consequently, the competition between freshwater organisms will have been less severe than elsewhere. New forms will have been more slowly produced, and old forms more slowly driven to extinction. And it is in freshwater basins that we find seven genera of ganoid fishes -- remnants of a once dominant order. And in fresh water we find some of the most unusual forms now known in the world, such as the platypus (Ornithorhynchus) and the lungfish (Lepidosiren), which, like fossils, connect to a certain extent orders that are now widely separated in the natural scale. These unusual forms may be called living fossils: they've endured to the present day from having inhabited a confined area and from having been exposed to less varied, and therefore less severe, competition.
To sum up, as far as the extreme intricacy of the subject allows: the circumstances favorable and unfavorable for the production of new species through natural selection. I conclude that for land-dwelling organisms, a large continental area that has undergone many changes in elevation will have been the most favorable for the production of many new forms of life, fitted to endure for a long time and to spread widely. While the area existed as a continent, the inhabitants will have been numerous in individuals and kinds, and will have been subjected to severe competition. When converted by subsidence into large separate islands, many individuals of the same species will still have existed on each island. Crossbreeding on the borders of each new species' range will have been prevented. After physical changes of any kind, immigration will have been blocked, so that new openings in the natural order of each island will have had to be filled by the modification of the old inhabitants. And time will have been allowed for the varieties on each island to become well modified and perfected. When, by renewed elevation, the islands were converted back into a continental area, there will again have been very severe competition. The most favored or improved varieties will have been able to spread. There will have been much extinction of the less improved forms, and the relative proportional numbers of the various inhabitants of the reunited continent will again have been changed. And again there will have been a fair field for natural selection to further improve the inhabitants, and thus to produce new species.
That natural selection generally acts with extreme slowness I fully admit. It can act only when there are openings in the natural order of a district that can be better occupied by the modification of some of its existing inhabitants. The occurrence of such openings will often depend on physical changes, which generally take place very slowly, and on the immigration of better-adapted forms being prevented. As some of the old inhabitants become modified, the mutual relationships of others will often be disturbed, and this will create new openings, ready to be filled by better-adapted forms. But all this will take place very slowly. Although all the individuals of the same species differ in some slight degree from each other, it would often be a long time before differences of the right nature in various parts of the body might occur. The result would often be greatly delayed by free crossbreeding. Many will exclaim that these several causes are more than sufficient to neutralize the power of natural selection. I don't believe so. But I do believe that natural selection will generally act very slowly, only at long intervals of time, and only on a few of the inhabitants of the same region. I further believe that these slow, intermittent results accord well with what geology tells us of the rate and manner at which the inhabitants of the world have changed.
Slow though the process of selection may be, if feeble humans can do so much by artificial selection, I can see no limit to the amount of change, to the beauty and complexity of the mutual adaptations between all organisms, with one another and with their physical conditions of life, that may have been brought about in the long course of time through nature's power of selection -- that is, by the survival of the fittest.
This subject will be more fully discussed in our chapter on geology, but it must be touched on here because it is intimately connected with natural selection. Natural selection acts solely through the preservation of variations that are in some way advantageous, which consequently endure. Because of the high geometrical rate of increase of all organisms, each area is already fully stocked with inhabitants, and it follows that as the favored forms increase in number, the less favored will generally decrease and become rare. Rarity, as geology tells us, is the precursor to extinction. We can see that any form represented by few individuals will run a good chance of total extinction during great fluctuations in the seasons or from a temporary increase in the number of its enemies. But we may go further than this. Unless we admit that the number of species can go on indefinitely increasing, many old forms must become extinct as new ones are produced. That the number of species has not indefinitely increased, geology plainly tells us, and we'll soon attempt to show why the number of species throughout the world has not become immeasurably great.
We've seen that the species most numerous in individuals have the best chance of producing favorable variations within any given period. We have evidence of this in the facts stated in the second chapter, showing that it is the common, widespread, and dominant species that offer the greatest number of recorded varieties. Therefore, rare species will be less quickly modified or improved within any given period, and they'll consequently be beaten in the race for life by the modified and improved descendants of the commoner species.
From these several considerations, I think it inevitably follows that as new species are formed through natural selection over the course of time, others will become rarer and rarer, and finally extinct. The forms that stand in closest competition with those undergoing modification and improvement will naturally suffer most. And we've seen in the chapter on the struggle for existence that it is the most closely allied forms -- varieties of the same species, and species of the same genus or related genera -- which, from having nearly the same structure, constitution, and habits, generally come into the severest competition with each other. Consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest relatives, and tend to exterminate them. We see the same process of extermination among our domesticated organisms, through the selection of improved forms by humans. Many curious examples could be given showing how quickly new breeds of cattle, sheep, and other animals, and varieties of flowers, take the place of older and inferior kinds. In Yorkshire, it is historically known that the ancient black cattle were displaced by the longhorns, and that these "were swept away by the shorthorns" (I quote the words of an agricultural writer) "as if by some murderous pestilence."
The principle I've designated by this term is of high importance, and explains, as I believe, several important facts. In the first place, varieties, even strongly marked ones, though having somewhat of the character of species -- as is shown by the hopeless doubts in many cases about how to rank them -- yet certainly differ far less from each other than do good and distinct species. Nevertheless, according to my view, varieties are species in the process of formation, or are, as I've called them, incipient species. How, then, does the lesser difference between varieties become amplified into the greater difference between species? That this does habitually happen, we must infer from the fact that most of the innumerable species throughout nature show well-marked differences, whereas varieties, the supposed prototypes and parents of future well-marked species, show slight and poorly defined differences. Mere chance, as we may call it, might cause one variety to differ in some trait from its parents, and the offspring of this variety again to differ from its parent in the same trait and to a greater degree. But this alone would never account for so habitual and large a degree of difference as that between species of the same genus.
As has always been my practice, I've sought light on this question from our domestic organisms. We find something analogous here. It will be admitted that the production of breeds so different as Shorthorn and Hereford cattle, racehorses and draft horses, the several breeds of pigeons, and so on, could never have been accomplished by the mere chance accumulation of similar variations over many successive generations. In practice, one fancier is struck by a pigeon having a slightly shorter beak; another fancier is struck by a pigeon having a somewhat longer beak; and on the acknowledged principle that "fanciers do not and will not admire a medium standard, but like extremes," they both go on -- as has actually occurred with the sub-breeds of the tumbler pigeon -- choosing and breeding from birds with longer and longer beaks, or with shorter and shorter beaks. Again, we may suppose that at an early period of history, the people of one nation or district required swifter horses, while those of another required stronger and bulkier horses. The early differences would be very slight. But in the course of time, from the continued selection of swifter horses in the one case and of stronger ones in the other, the differences would become greater and would be noted as forming two sub-breeds. Ultimately, after the passage of centuries, these sub-breeds would become two well-established and distinct breeds. As the differences became greater, the inferior animals with intermediate traits -- being neither very swift nor very strong -- would not have been used for breeding, and would thus have tended to disappear. Here, then, we see in human production the action of what may be called the principle of divergence, causing differences that at first were barely noticeable to steadily increase, and the breeds to diverge in character both from each other and from their common parent.
But how, it may be asked, can any analogous principle apply in nature? I believe it can and does apply most efficiently -- though it was a long time before I saw how -- from the simple fact that the more diversified the descendants from any one species become in structure, constitution, and habits, the better they'll be able to seize on many and widely diversified places in the natural order, and so increase in numbers.
We can clearly see this in the case of animals with simple habits. Take the case of a carnivorous mammal whose population in any country has long ago reached its full average. If its natural power of increase is allowed to act, it can succeed in increasing -- the country not undergoing any change in conditions -- only by its varying descendants seizing on places currently occupied by other animals. Some of them, for instance, might become able to feed on new kinds of prey, either dead or alive; some might inhabit new habitats, climbing trees, going into the water, and some might perhaps become less carnivorous. The more diversified in habits and structure the descendants of our carnivorous animal become, the more niches they'll be able to occupy. What applies to one animal applies throughout all time to all animals -- that is, if they vary -- for otherwise natural selection can do nothing. So it will be with plants. It has been experimentally proven that if a plot of ground is sown with one species of grass, and a similar plot is sown with several distinct genera of grasses, a greater number of plants and a greater weight of dry herbage can be raised in the latter than in the former case. The same has been found to hold when one variety and several mixed varieties of wheat have been sown on equal areas of ground. So if any one species of grass were to keep on varying, and the varieties were continually selected that differed from each other in the same way, though to a very slight degree, as do the distinct species and genera of grasses, a greater number of individual plants of this species, including its modified descendants, would succeed in living on the same piece of ground. And we know that each species and each variety of grass is annually sowing almost countless seeds, and is thus striving, as it were, to the utmost to increase in number. Consequently, over the course of many thousand generations, the most distinct varieties of any one species of grass would have the best chance of succeeding and of increasing in numbers, and thus of replacing the less distinct varieties. And varieties, when rendered very distinct from each other, take the rank of species.
The truth of the principle that the greatest amount of life can be supported by great diversification of structure is seen under many natural circumstances. In an extremely small area, especially if freely open to immigration and where the contest between individual and individual must be very severe, we always find great diversity in its inhabitants. For instance, I found that a piece of turf, three feet by four in size, which had been exposed for many years to exactly the same conditions, supported twenty species of plants belonging to eighteen genera and eight orders -- which shows how much these plants differed from each other. So it is with the plants and insects on small and uniform islands, and also in small ponds of fresh water. Farmers find that they can raise more food by a rotation of plants belonging to the most different groups. Nature follows what may be called a simultaneous rotation. Most of the animals and plants that live close around any small piece of ground could live on it (supposing its nature not to be in any way unusual), and may be said to be striving to the utmost to live there. But it turns out that where they come into the closest competition, the advantages of diversification of structure, with the accompanying differences of habit and constitution, determine that the inhabitants jostling each other most closely shall, as a general rule, belong to what we call different genera and orders.
The same principle is seen in the establishment of plants through human agency in foreign lands. It might have been expected that the plants succeeding in becoming established in any land would generally be closely allied to the native plants -- for these are commonly regarded as specially created and adapted for their own country. It might also have been expected that established plants would belong to a few groups especially adapted to certain habitats in their new homes. But the case is very different. Alphonse de Candolle has well remarked, in his great and admirable work, that floras gain through the establishment of foreign plants, proportionally with the number of native genera and species, far more in new genera than in new species. To give a single instance: in the last edition of Dr. Asa Gray's Manual of the Flora of the Northern United States, 260 established plants are listed, and these belong to 162 genera. We thus see that these established plants are of a highly diversified nature. They also differ to a large extent from the natives, for out of the 162 genera of established plants, no less than 100 genera are not native there, and thus a large proportional addition is made to the genera now living in the United States.
By considering the nature of the plants or animals that have successfully competed with the natives in any country and become established there, we may gain some rough idea of how some of the natives would have had to be modified in order to gain an advantage over their countrymen. We may at least infer that diversification of structure, amounting to new differences at the genus level, would be profitable to them.
The advantage of diversification of structure in the inhabitants of the same region is, in fact, the same as that of the physiological division of labor in the organs of the same individual body -- a subject so well explained by Milne Edwards. No physiologist doubts that a stomach adapted to digest vegetable matter alone, or flesh alone, draws more nutrition from these substances. So in the overall economy of any land, the more widely and perfectly the animals and plants are diversified for different ways of life, the greater the number of individuals capable of supporting themselves there. A set of animals with their organization only slightly diversified could hardly compete with a set more perfectly diversified in structure. It may be doubted, for instance, whether the Australian marsupials, which are divided into groups differing but little from each other and feebly representing, as Mr. Waterhouse and others have remarked, our carnivorous, grazing, and rodent mammals, could successfully compete with these well-developed groups. In the Australian mammals, we see the process of diversification in an early and incomplete stage of development.
The Probable Effects of Natural Selection Through Divergence of Character and Extinction, on the Descendants of a Common Ancestor
After the foregoing discussion, which has been much compressed, we may assume that the modified descendants of any one species will succeed better the more diversified they become in structure, and are thus enabled to encroach on places occupied by other organisms. Now let's see how this principle of benefit derived from divergence of character, combined with the principles of natural selection and of extinction, tends to act.
The accompanying diagram will help us understand this rather complex subject. Let A to L represent the species of a genus that is large in its own country. These species are supposed to resemble each other in unequal degrees, as is generally the case in nature, and as is represented in the diagram by the letters standing at unequal distances. I've said a large genus because, as we saw in the second chapter, on average more species vary in large genera than in small genera, and the varying species of the large genera present a greater number of varieties. We've also seen that the species that are the commonest and most widely spread vary more than the rare and restricted species. Let (A) be a common, widely spread, and varying species belonging to a genus large in its own country. The branching and diverging dotted lines of unequal lengths proceeding from (A) may represent its varying offspring. The variations are supposed to be extremely slight, but of the most diversified nature. They're not supposed to all appear simultaneously, but often after long intervals of time, nor are they all supposed to last for equal periods. Only those variations that are in some way beneficial will be preserved or naturally selected. And here the importance of the principle of benefit derived from divergence of character comes in -- for this will generally lead to the most different or divergent variations (represented by the outer dotted lines) being preserved and accumulated by natural selection. When a dotted line reaches one of the horizontal lines and is there marked by a small numbered letter, a sufficient amount of variation is supposed to have been accumulated to form it into a fairly well-marked variety, such as would be thought worthy of record in a systematic work.
The intervals between the horizontal lines in the diagram may represent each a thousand or more generations. After a thousand generations, species (A) is supposed to have produced two fairly well-marked varieties, namely a1 and m1. These two varieties will generally still be exposed to the same conditions that made their parents variable, and the tendency to variability is itself hereditary. Consequently they will also tend to vary, and commonly in nearly the same manner as their parents. Moreover, these two varieties, being only slightly modified forms, will tend to inherit those advantages that made their parent (A) more numerous than most of the other inhabitants of the same country. They will also share in those more general advantages that made the genus to which the parent species belonged a large genus in its own country. All these circumstances are favorable to the production of new varieties.
If, then, these two varieties are variable, the most divergent of their variations will generally be preserved during the next thousand generations. After this interval, variety a1 is supposed in the diagram to have produced variety a2, which will, owing to the principle of divergence, differ more from (A) than did variety a1. Variety m1 is supposed to have produced two varieties, m2 and s2, differing from each other and more considerably from their common parent (A). We may continue the process by similar steps for any length of time: some of the varieties, after each thousand generations, producing only a single variety but in a more and more modified condition; some producing two or three varieties; and some failing to produce any. Thus the varieties or modified descendants of the common parent (A) will generally go on increasing in number and diverging in character. In the diagram the process is represented up to the ten-thousandth generation, and in a condensed and simplified form up to the fourteen-thousandth generation.
But I must remark here that I don't suppose the process ever goes on so regularly as represented in the diagram (though in itself made somewhat irregular), nor that it goes on continuously. It's far more probable that each form remains unchanged for long periods and then again undergoes modification. Nor do I suppose that the most divergent varieties are invariably preserved: a medium form may often long endure, and may or may not produce more than one modified descendant. For natural selection will always act according to the nature of the places that are either unoccupied or not perfectly occupied by other organisms, and this will depend on infinitely complex relationships. But as a general rule, the more diversified in structure the descendants from any one species can be made, the more places they'll be able to seize on, and the more their modified offspring will increase. In our diagram the line of succession is broken at regular intervals by small numbered letters marking the successive forms that have become sufficiently distinct to be recorded as varieties. But these breaks are imaginary, and might have been inserted anywhere, after intervals long enough to allow the accumulation of a considerable amount of divergent variation.
Since all the modified descendants from a common and widely spread species belonging to a large genus will tend to share the same advantages that made their parent successful in life, they will generally go on multiplying in number as well as diverging in character. This is represented in the diagram by the several divergent branches proceeding from (A). The modified offspring from the later and more highly improved branches in the lines of descent will probably often take the place of, and so destroy, the earlier and less improved branches. This is represented in the diagram by some of the lower branches not reaching to the upper horizontal lines. In some cases, no doubt, the process of modification will be confined to a single line of descent, and the number of modified descendants will not be increased, although the amount of divergent modification may have been amplified. This case would be represented in the diagram if all the lines proceeding from (A) were removed except that from a1 to a10. In the same way, the English racehorse and English pointer have apparently both gone on slowly diverging in character from their original stocks, without either having given off any fresh branches or breeds.
After ten thousand generations, species (A) is supposed to have produced three forms -- a10, f10, and m10 -- which, from having diverged in character during the successive generations, will have come to differ greatly, but perhaps unequally, from each other and from their common parent. If we suppose the amount of change between each horizontal line in our diagram to be very small, these three forms may still be only well-marked varieties. But we have only to suppose the steps in the process of modification to be more numerous or greater in amount to convert these three forms into doubtful or at least into well-defined species. Thus the diagram illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences distinguishing species. By continuing the same process for a greater number of generations (as shown in the diagram in a condensed and simplified manner), we get eight species, marked by the letters between a14 and m14, all descended from (A). Thus, as I believe, species are multiplied and genera are formed.
In a large genus it's probable that more than one species would vary. In the diagram I've assumed that a second species (I) has produced, by analogous steps, after ten thousand generations, either two well-marked varieties (w10 and z10) or two species, according to the amount of change supposed to be represented between the horizontal lines. After fourteen thousand generations, six new species, marked by the letters n14 to z14, are supposed to have been produced. In any genus, the species that are already very different in character from each other will generally tend to produce the greatest number of modified descendants -- for these will have the best chance of seizing on new and widely different places in the natural order. So in the diagram I've chosen the extreme species (A), and the nearly extreme species (I), as those that have varied most and have given rise to new varieties and species. The other nine species (marked by capital letters) of our original genus may for long but unequal periods continue to transmit unaltered descendants, and this is shown in the diagram by the dotted lines prolonged upward at unequal lengths.
But during the process of modification represented in the diagram, another of our principles -- namely, that of extinction -- will have played an important part. Since in each fully stocked country natural selection necessarily acts by the selected form having some advantage in the struggle for life over other forms, there will be a constant tendency in the improved descendants of any one species to replace and exterminate at each stage of descent their predecessors and their original ancestor. For it should be remembered that the competition will generally be most severe between those forms that are most nearly related to each other in habits, constitution, and structure. So all the intermediate forms between the earlier and later states -- that is, between the less and more improved states of the same species -- as well as the original parent species itself, will generally tend to become extinct. The same will probably be true of many whole collateral lines of descent, which will be conquered by later and improved lines. If, however, the modified offspring of a species gets into some distinct country, or becomes quickly adapted to some quite new habitat where offspring and ancestor don't come into competition, both may continue to exist.
If, then, our diagram is assumed to represent a considerable amount of modification, species (A) and all the earlier varieties will have become extinct, being replaced by eight new species (a14 to m14), and species (I) will be replaced by six new species (n14 to z14).
But we may go further than this. The original species of our genus were supposed to resemble each other in unequal degrees, as is generally the case in nature. Species (A) was more nearly related to B, C, and D than to the other species, and species (I) more to G, H, K, and L than to the others. These two species (A and I) were also supposed to be very common and widely spread, so that they must originally have had some advantage over most of the other species of the genus. Their modified descendants, fourteen in number at the fourteen-thousandth generation, will probably have inherited some of these same advantages. They've also been modified and improved in a diversified manner at each stage of descent, so as to have become adapted to many related places in the natural economy of their country. It seems therefore extremely probable that they will have taken the places of, and thus exterminated, not only their parents (A) and (I), but also some of the original species most closely related to their parents. So very few of the original species will have transmitted offspring to the fourteen-thousandth generation. We may suppose that only one (F) of the two species (E and F) that were least closely related to the other nine original species has transmitted descendants to this late stage of descent.
The new species in our diagram, descended from the original eleven species, will now number fifteen. Owing to the divergent tendency of natural selection, the extreme amount of difference in character between species a14 and z14 will be much greater than that between the most distinct of the original eleven species. The new species, moreover, will be related to each other in a widely different manner. Of the eight descendants from (A), the three marked a14, q14, and p14 will be closely related from having recently branched off from a10. The two marked b14 and f14, from having diverged at an earlier period from a5, will be somewhat distinct from the first three. And finally, o14, e14, and m14 will be closely related to one another but, from having diverged at the very beginning of the process of modification, will be widely different from the other five species. They may constitute a subgenus or a distinct genus.
The six descendants from (I) will form two subgenera or genera. But since the original species (I) differed greatly from (A), standing nearly at the extreme end of the original genus, the six descendants from (I) will, owing to inheritance alone, differ considerably from the eight descendants from (A). The two groups, moreover, are supposed to have gone on diverging in different directions. The intermediate species (and this is a very important point) that connected the original species (A) and (I) have all become extinct except (F), and have left no descendants. Therefore the six new species descended from (I) and the eight descendants from (A) will have to be ranked as very distinct genera, or even as distinct subfamilies.
Thus it is, as I believe, that two or more genera are produced by descent with modification from two or more species of the same genus. And the two or more parent species are supposed to be descended from some one species of an earlier genus. In our diagram this is indicated by the broken lines beneath the capital letters, converging in sub-branches downward toward a single point -- this point represents a species, the supposed ancestor of our several new subgenera and genera.
It's worth pausing to reflect on the character of the new species F14, which is supposed not to have diverged much in character but to have retained the form of (F), either unaltered or altered only slightly. In this case, its relationships to the other fourteen new species will be of a curious and roundabout nature. Being descended from a form that stood between the parent species (A) and (I), now supposed to be extinct and unknown, it will be somewhat intermediate in character between the two groups descended from these two species. But since these two groups have gone on diverging in character from the type of their parents, the new species (F14) will not be directly intermediate between them, but rather between the types of the two groups -- and every naturalist will be able to call such cases to mind.
In the diagram, each horizontal line has so far been supposed to represent a thousand generations, but each may represent a million or more generations. It may also represent a section of the successive layers of the earth's crust including fossil remains. We'll have to refer to this subject again when we come to our chapter on geology, and I think we'll see then that the diagram throws light on the relationships of extinct species. These, though generally belonging to the same orders, families, or genera as those now living, are often to some degree intermediate in character between existing groups. And we can understand this, for the extinct species lived at various remote epochs when the branching lines of descent had diverged less.
I see no reason to limit the process of modification, as now explained, to the formation of genera alone. If in the diagram we suppose the amount of change represented by each successive group of diverging dotted lines to be great, the forms marked a14 to p14, those marked b14 and f14, and those marked o14 to m14, will form three very distinct genera. We'll also have two very distinct genera descended from (I), differing widely from the descendants of (A). These two groups of genera will thus form two distinct families, or orders, according to the amount of divergent modification supposed to be represented in the diagram. And the two new families, or orders, descend from two species of the original genus, and these are supposed to be descended from some still more ancient and unknown form.
We've seen that in each country it is the species belonging to the larger genera that most often present varieties or incipient species. This is indeed to be expected, for since natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those that already have some advantage. The largeness of any group shows that its species have inherited from a common ancestor some advantage in common. So the struggle for the production of new and modified descendants will mainly be between the larger groups, which are all trying to increase in number. One large group will slowly conquer another large group, reduce its numbers, and thus lessen its chance of further variation and improvement. Within the same large group, the later and more highly perfected subgroups, from branching out and seizing on many new places in the natural order, will constantly tend to replace and destroy the earlier and less improved subgroups. Small and broken groups and subgroups will finally disappear. Looking to the future, we can predict that the groups of organisms that are now large and triumphant, and which are least broken up -- that is, which have as yet suffered least extinction -- will for a long period continue to increase. But which groups will ultimately prevail, no one can predict, for we know that many groups formerly most extensively developed have now become extinct. Looking still more remotely to the future, we may predict that, owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct and leave no modified descendants. Consequently, of the species living at any one period, extremely few will transmit descendants to a remote futurity. I'll have to return to this subject in the chapter on classification, but I may add that since, according to this view, extremely few of the more ancient species have transmitted descendants to the present day, and since all the descendants of the same species form a class, we can understand how it is that there exist so few classes in each main division of the animal and vegetable kingdoms. Although few of the most ancient species have left modified descendants, at remote geological periods the earth may have been almost as well populated with species of many genera, families, orders, and classes as it is today.
Natural selection acts exclusively by the preservation and accumulation of variations that are beneficial under the living and non-living conditions to which each creature is exposed at all periods of life. The ultimate result is that each creature tends to become more and more improved in relation to its conditions. This improvement inevitably leads to the gradual advancement of the organization of the greater number of living beings throughout the world. But here we enter on a very intricate subject, for naturalists have not defined to each other's satisfaction what is meant by an advance in organization. Among the vertebrates, the degree of intellect and an approach in structure to humans clearly come into play. It might be thought that the amount of change the various parts and organs pass through in their development from embryo to maturity would serve as a standard of comparison. But there are cases, as with certain parasitic crustaceans, in which several parts of the structure become less developed, so that the mature animal cannot be called higher than its larva. Von Baer's standard seems the most widely applicable and the best -- namely, the amount of differentiation of the parts of the same organism in the adult state (as I should be inclined to add), and their specialization for different functions; or, as Milne Edwards would express it, the completeness of the division of physiological labor. But we'll see how unclear this subject is if we look, for instance, at fishes. Some naturalists rank as highest those that, like the sharks, come closest to amphibians, while other naturalists rank the common bony or teleostean fishes as highest, since they are most strictly fish-like and differ most from the other vertebrate classes. We see the obscurity of the subject still more plainly by turning to plants, among which the standard of intellect is of course completely excluded. Here some botanists rank as highest those plants that have every organ -- sepals, petals, stamens, and pistils -- fully developed in each flower, whereas other botanists, probably with more truth, look at plants that have their several organs much modified and reduced in number as the highest.
If we take as the standard of high organization the amount of differentiation and specialization of the several organs in each adult organism (and this will include the advancement of the brain for intellectual purposes), natural selection clearly leads toward this standard. All physiologists admit that the specialization of organs, insofar as they perform their functions better in this state, is an advantage to each organism. So the accumulation of variations tending toward specialization is within the scope of natural selection. On the other hand, keeping in mind that all organisms are striving to increase at a high rate and to seize on every unoccupied or poorly occupied place in the natural order, it's quite possible for natural selection to gradually fit an organism to a situation in which several organs would be unnecessary or useless. In such cases there would be a regression in the scale of organization. Whether organization on the whole has actually advanced from the remotest geological periods to the present day will be more conveniently discussed in our chapter on geological succession.
But someone might object: if all organisms thus tend to rise in the scale, how is it that throughout the world a multitude of the lowest forms still exist? And how is it that in each great class, some forms are far more highly developed than others? Why haven't the more highly developed forms everywhere replaced and exterminated the lower? Lamarck, who believed in an innate and inevitable tendency toward perfection in all organisms, seems to have felt this difficulty so strongly that he was led to suppose that new and simple forms are continually being produced by spontaneous generation. Science has not yet proved the truth of this belief, whatever the future may reveal. On our theory, the continued existence of lowly organisms offers no difficulty -- for natural selection, or the survival of the fittest, does not necessarily include progressive development. It only takes advantage of such variations as arise and are beneficial to each creature under its complex conditions of life. And it may be asked: what advantage, as far as we can see, would it be to a single-celled organism, to an intestinal worm, or even to an earthworm, to be highly organized? If it were no advantage, these forms would be left by natural selection unimproved, or only slightly improved, and might remain for indefinite ages in their present lowly condition. And geology tells us that some of the lowest forms, such as single-celled organisms and rhizopods, have remained for an enormous period in nearly their present state. But to suppose that most of the many now existing low forms have not advanced at all since the first dawn of life would be extremely rash. Any naturalist who has dissected some of the organisms now ranked as very low in the scale must have been struck by their truly wondrous and beautiful organization.
Nearly the same remarks apply if we look at the different grades of organization within the same great group -- for instance, in the vertebrates, to the coexistence of mammals and fish; among mammals, to the coexistence of humans and the platypus; among fishes, to the coexistence of the shark and the lancelet (Amphioxus), which in the extreme simplicity of its structure approaches the invertebrate classes. But mammals and fish hardly come into competition with each other. The advancement of the whole class of mammals, or of certain members in this class, to the highest grade would not lead to their taking the place of fishes. Physiologists believe that the brain must be bathed by warm blood to be highly active, and this requires breathing air -- so warm-blooded mammals living in the water are at a disadvantage in having to come continually to the surface to breathe. Among fishes, members of the shark family would not tend to replace the lancelet, for the lancelet, as I hear from Fritz Muller, has as its sole companion and competitor on the barren sandy shore of South Brazil an unusual annelid worm. The three lowest orders of mammals -- namely, marsupials, edentates, and rodents -- coexist in South America in the same region with numerous monkeys, and probably interfere little with each other. Although organization on the whole may have advanced and may still be advancing throughout the world, the scale will always present many degrees of perfection. The high advancement of certain whole classes, or of certain members of each class, does not at all necessarily lead to the extinction of those groups with which they don't enter into close competition. In some cases, as we'll see later, lowly organized forms appear to have been preserved to the present day from inhabiting confined or unusual habitats, where they've been subjected to less severe competition and where their small numbers have reduced the chance of favorable variations arising.
Finally, I believe that many lowly organized forms now exist throughout the world from various causes. In some cases, variations of a favorable nature may never have arisen for natural selection to act on and accumulate. In no case, probably, has time been sufficient for the utmost possible amount of development. In some few cases there has been what we must call regression of organization. But the main cause lies in the fact that under very simple conditions of life, a high organization would be of no service -- and might possibly be an actual disadvantage, as being of a more delicate nature and more liable to be disrupted and injured.
Looking to the first dawn of life, when all organisms, as we may believe, had the simplest structure -- how, it has been asked, could the first step in the advancement or differentiation of parts have arisen? Herbert Spencer would probably answer that as soon as simple single-celled organisms came by growth or division to be composed of several cells, or became attached to any supporting surface, his law that "homologous units of any order become differentiated in proportion as their relations to incident forces become different" would come into action. But as we have no facts to guide us, speculation on the subject is almost useless. It is, however, an error to suppose that there would be no struggle for existence, and consequently no natural selection, until many forms had been produced. Variations in a single species inhabiting an isolated location might be beneficial, and thus the whole body of individuals might be modified, or two distinct forms might arise. But, as I remarked toward the close of the introduction, no one ought to feel surprised at much remaining unexplained about the origin of species, if we make due allowance for our profound ignorance of the relationships among the inhabitants of the world at the present time, and still more so during past ages.
Mr. H. C. Watson thinks that I've overrated the importance of divergence of character (in which, however, he apparently believes), and that convergence, as it may be called, has likewise played a part. If two species belonging to two distinct though related genera had both produced a large number of new and divergent forms, it's conceivable that these might approach each other so closely that they would all have to be classed under the same genus -- and thus the descendants of two distinct genera would converge into one. But it would in most cases be extremely rash to attribute to convergence a close and general similarity of structure in the modified descendants of widely distinct forms. The shape of a crystal is determined solely by molecular forces, and it's not surprising that dissimilar substances should sometimes assume the same form. But with living organisms we should bear in mind that the form of each depends on an infinity of complex relationships: on the variations that have arisen (these being due to causes far too intricate to trace); on the nature of the variations that have been preserved or selected (and this depends on the surrounding physical conditions, and in a still higher degree on the surrounding organisms with which each being has come into competition); and finally, on inheritance -- in itself a fluctuating element -- from innumerable ancestors, all of which have had their forms determined through equally complex relationships. It's incredible that the descendants of two organisms that originally differed in a marked manner should ever afterward converge so closely as to approach identity throughout their whole organization. If this had occurred, we should find the same form, unrelated by descent, recurring in widely separated geological formations -- and the balance of evidence is opposed to any such idea.
Mr. Watson has also objected that the continued action of natural selection, together with divergence of character, would tend to make an indefinite number of species. As far as merely non-living conditions are concerned, it seems probable that a sufficient number of species would soon become adapted to all considerable diversities of heat, moisture, and so on. But I fully admit that the relationships between organisms are more important, and as the number of species in any country goes on increasing, the living conditions must become more and more complex. Consequently, there seems at first no limit to the amount of beneficial diversification of structure, and therefore no limit to the number of species that might be produced. We don't know that even the most species-rich area is fully stocked with species: at the Cape of Good Hope and in Australia, which support an astonishing number of species, many European plants have become established. But geology shows us that from an early part of the tertiary period, the number of species of shells, and from the middle part of this same period, the number of mammals, has not greatly or at all increased. What then checks an indefinite increase in the number of species? The amount of life -- I don't mean the number of species -- supported on an area must have a limit, depending so largely as it does on physical conditions. Therefore, if an area is inhabited by very many species, each or nearly each species will be represented by few individuals, and such species will be liable to extermination from accidental fluctuations in the seasons or in the number of their enemies. The process of extermination in such cases would be rapid, whereas the production of new species must always be slow. Imagine the extreme case of as many species as individuals in England -- the first severe winter or very dry summer would exterminate thousands upon thousands of species. Rare species -- and each species will become rare if the number of species in any country becomes indefinitely increased -- will, on the principle often explained, present within a given period few favorable variations. Consequently, the process of giving birth to new species would thus be slowed. When any species becomes very rare, inbreeding will help to exterminate it. Authors have thought that this comes into play in accounting for the decline of the aurochs in Lithuania, of red deer in Scotland, and of bears in Norway, among others. Finally, and this I'm inclined to think is the most important factor, a dominant species that has already beaten many competitors in its own home will tend to spread and replace many others. Alphonse de Candolle has shown that those species which spread widely tend generally to spread very widely. Consequently they will tend to replace and exterminate several species in several areas, and thus check the runaway increase of species throughout the world. The botanist Dr. Hooker has recently shown that in the southeast corner of Australia, where apparently there are many invaders from different quarters of the globe, the endemic Australian species have been greatly reduced in number. How much weight to attribute to these several considerations I won't pretend to say. But together they must limit in each country the tendency toward an indefinite increase of species.
If under changing conditions of life organisms present individual differences in almost every part of their structure -- and this cannot be disputed; if there is, owing to their geometrical rate of increase, a severe struggle for life at some age, season, or year -- and this certainly cannot be disputed; then, considering the infinite complexity of the relationships of all organisms to each other and to their conditions of life, causing an infinite diversity in structure, constitution, and habits to be advantageous to them, it would be a most extraordinary fact if no variations had ever occurred useful to each organism's own welfare, in the same manner as so many variations have occurred useful to humans. But if variations useful to any organism ever do occur, individuals so equipped will certainly have the best chance of being preserved in the struggle for life -- and from the strong principle of inheritance, these will tend to produce offspring similarly equipped. This principle of preservation, or the survival of the fittest, I have called natural selection. It leads to the improvement of each creature in relation to its living and non-living conditions of life, and consequently, in most cases, to what must be regarded as an advance in organization. Nevertheless, low and simple forms will long endure if well fitted for their simple conditions of life.
Natural selection, on the principle of traits being inherited at corresponding ages, can modify the egg, seed, or young as easily as the adult. Among many animals, sexual selection will have given its aid to ordinary selection by assuring to the most vigorous and best-adapted males the greatest number of offspring. Sexual selection will also give characters useful to the males alone in their struggles or rivalry with other males, and these traits will be transmitted to one sex or to both sexes, according to the form of inheritance that prevails.
Whether natural selection has really acted in this way to adapt the various forms of life to their several conditions and stations must be judged by the general weight and balance of evidence given in the following chapters. But we've already seen how it leads to extinction -- and how largely extinction has acted in the world's history, geology plainly declares. Natural selection also leads to divergence of character, for the more organisms diverge in structure, habits, and constitution, the more of them can be supported on the same area -- as we see proof of by looking at the inhabitants of any small spot and at the organisms established in foreign lands. Therefore, during the modification of the descendants of any one species, and during the incessant struggle of all species to increase in numbers, the more diversified the descendants become, the better their chance of success in the battle for life. Thus the small differences distinguishing varieties of the same species steadily tend to increase, till they equal the greater differences between species of the same genus, or even of distinct genera.
We've seen that it is the common, the widely spread, and the widely ranging species, belonging to the larger genera within each class, that vary most -- and these tend to transmit to their modified offspring the superiority that now makes them dominant in their own countries. Natural selection, as has just been remarked, leads to divergence of character and to much extinction of the less improved and intermediate forms of life. On these principles, the nature of the relationships and the generally well-defined distinctions between the innumerable organisms in each class throughout the world may be explained. It is a truly wonderful fact -- the wonder of which we tend to overlook from familiarity -- that all animals and all plants throughout all time and space should be related to each other in groups subordinate to groups, in the manner we everywhere see: namely, varieties of the same species most closely related, species of the same genus less closely and unequally related, forming sections and subgenera, species of distinct genera much less closely related, and genera related in different degrees forming subfamilies, families, orders, subclasses, and classes. The several subordinate groups in any class cannot be ranked in a single line, but seem clustered around points, and these around other points, and so on in almost endless cycles. If species had been independently created, no explanation would have been possible for this kind of classification. But it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we've seen illustrated in the diagram.
The relationships of all the organisms of the same class have sometimes been represented by a great tree. I believe this comparison largely speaks the truth. The green and budding twigs may represent existing species, and those produced during former years may represent the long succession of extinct species. At each period of growth, all the growing twigs have tried to branch out on all sides and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have at all times overmastered other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was young, budding twigs -- and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. Of the many twigs that flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear the other branches. So with the species that lived during long-past geological periods: very few have left living and modified descendants. From the first growth of the tree, many a limb and branch has decayed and dropped off, and these fallen branches of various sizes may represent those whole orders, families, and genera that have now no living representatives and are known to us only as fossils. As we here and there see a thin, straggling branch springing from a fork low down in a tree, and which by some chance has been favored and is still alive on its summit, so we occasionally see an animal like the platypus or the lungfish, which in some small degree connects by its relationships two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected habitat. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications.
CHAPTER V Laws of Variation
Up to this point, I've sometimes spoken as if the variations -- so common and diverse in domesticated organisms, and to a lesser degree in wild ones -- were due to chance. This is, of course, a completely incorrect way to put it, but it serves to honestly acknowledge our ignorance about what causes each particular variation. Some authors believe that producing individual differences, or slight deviations in structure, is just as much a function of the reproductive system as making offspring resemble their parents. But the fact that variations and monstrosities occur far more often under domestication than in nature, and that species with wide ranges are more variable than those with restricted ranges, leads to the conclusion that variability is generally related to the conditions of life each species has been exposed to over many successive generations. In the first chapter, I tried to show that changed conditions act in two ways: directly on the whole body or on certain parts alone, and indirectly through the reproductive system. In all cases, two factors are at work: the nature of the organism, which is by far the more important, and the nature of the conditions. The direct action of changed conditions leads to either definite or indefinite results. In the latter case, the organism seems to become plastic, and we see a great deal of fluctuating variability. In the former case, the organism is the sort that yields readily when subjected to certain conditions, and all, or nearly all, the individuals become modified in the same way.
It's very difficult to determine how far changed conditions -- climate, food, and so on -- have acted in a definite manner. There's reason to believe that over time the effects have been greater than we can prove with clear evidence. But we can safely conclude that the countless complex mutual adaptations of structure we see throughout nature between various organisms cannot be attributed simply to such direct action. In the following cases, the conditions do seem to have produced some slight definite effect: E. Forbes asserts that shells at their southern limit, and when living in shallow water, are more brightly colored than those of the same species from further north or from greater depths -- though this certainly doesn't always hold true. Mr. Gould believes that birds of the same species are more brightly colored under a clear atmosphere than when living near the coast or on islands. And Wollaston is convinced that living near the sea affects the colors of insects. Moquin-Tandon gives a list of plants that, when growing near the seashore, develop somewhat fleshy leaves, though they aren't fleshy elsewhere. These slightly varying organisms are interesting insofar as they display characteristics analogous to those possessed by species that are confined to similar conditions.
When a variation is of even the slightest use to any organism, we can't tell how much to attribute to the accumulated action of natural selection, and how much to the direct action of the conditions of life. For example, it's well known to furriers that animals of the same species have thicker and better fur the further north they live. But who can tell how much of this difference is due to the warmest-clad individuals having been favored and preserved over many generations, and how much to the direct action of the harsh climate? For it does appear that climate has some direct effect on the hair of our domestic mammals.
Examples could be given of similar varieties being produced from the same species under conditions of life as different as you could possibly imagine -- and, on the other hand, of dissimilar varieties being produced under apparently the same conditions. Again, countless examples are known to every naturalist of species staying true to form, not varying at all, despite living under the most opposite climates. Considerations like these lead me to put less weight on the direct action of surrounding conditions, and more on a tendency to vary due to causes we're entirely ignorant of.
In one sense, the conditions of life may be said not only to cause variability, either directly or indirectly, but also to include natural selection, since the conditions determine whether this or that variety will survive. But when humans are the selecting agents, we can clearly see that the two elements of change are distinct: variability is somehow stimulated, but it's the human will that accumulates variations in a certain direction. And it's this latter force that corresponds to the survival of the fittest in nature.
From the facts discussed in the first chapter, I think there can be no doubt that use in our domestic animals has strengthened and enlarged certain parts, and disuse has diminished them -- and that such modifications are inherited. In wild nature, we have no standard of comparison for judging the effects of long-continued use or disuse, since we don't know the ancestral forms. But many animals possess structures that are best explained by the effects of disuse. As Professor Owen has remarked, there's no greater oddity in nature than a bird that cannot fly -- yet there are several in this state. The loggerhead duck of South America can only flap along the water's surface, and has its wings in nearly the same condition as the domestic Aylesbury duck. It's a remarkable fact that the young birds, according to Mr. Cunningham, can fly, while the adults have lost this power. Since the larger ground-feeding birds seldom take flight except to escape danger, it's likely that the nearly wingless condition of several birds now inhabiting (or which recently inhabited) various oceanic islands -- islands with no predatory mammals -- has been caused by disuse. The ostrich does inhabit continents and is exposed to danger from which it cannot escape by flight, but it can defend itself by kicking its enemies as effectively as many mammals. We can reasonably believe that the ancestor of the ostrich genus had habits like those of the bustard, and that as its body grew larger and heavier over successive generations, its legs were used more and its wings less, until they became incapable of flight.
Kirby has remarked (and I've observed the same thing) that the front feet of many male dung-feeding beetles are often broken off. He examined seventeen specimens in his own collection, and not one had even a remnant left. In Onites apelles, the front feet are so habitually lost that the insect has been described as not having them. In some other genera they're present, but in a rudimentary condition. In the Ateuchus, or sacred beetle of the Egyptians, they're completely absent. The evidence that accidental mutilations can be inherited is currently not decisive, but the remarkable cases observed by Brown-Sequard in guinea pigs -- involving inherited effects of operations -- should make us cautious about denying this tendency. So it's probably safest to view the complete absence of the front feet in Ateuchus, and their rudimentary condition in some other genera, not as cases of inherited mutilations, but as due to the effects of long-continued disuse. Since many dung-feeding beetles are generally found with their front feet lost, this must happen early in life, which means the feet can't be very important or much used by these insects.
In some cases, we might easily attribute to disuse modifications that are wholly or mainly due to natural selection. Mr. Wollaston discovered the remarkable fact that 200 beetles out of the 550 species (though more are now known) inhabiting Madeira are so deficient in wings that they cannot fly -- and that of the twenty-nine endemic genera, no less than twenty-three have all their species in this condition! Several facts point to natural selection as the main cause: beetles in many parts of the world are very frequently blown to sea and perish; the beetles in Madeira, as Wollaston observed, stay hidden until the wind dies down and the sun comes out; the proportion of wingless beetles is larger on the exposed Desertas than on Madeira itself; and especially the extraordinary fact, which Wollaston strongly emphasizes, that certain large groups of beetles -- elsewhere enormously abundant -- which absolutely require the use of their wings, are here almost entirely absent. All these considerations lead me to believe that the wingless condition of so many Madeira beetles is mainly due to the action of natural selection, probably combined with disuse. Over many successive generations, each individual beetle that flew least -- either because its wings were even slightly less well-developed, or because of lazier habits -- would have had the best chance of surviving by not being blown out to sea. On the other hand, those beetles that most readily took to flight would most often have been blown to sea and destroyed.
The insects in Madeira that are not ground-feeders, and which -- like certain flower-feeding beetles and moths -- must regularly use their wings to find food, have, as Wollaston suspects, their wings not reduced at all, but actually enlarged. This is perfectly compatible with the action of natural selection. When a new insect first arrived on the island, the tendency of natural selection to enlarge or reduce the wings would depend on whether more individuals were saved by successfully battling the winds, or by giving up the attempt and rarely or never flying. It's like mariners shipwrecked near a coast: it would have been better for the good swimmers if they could swim even further, while it would have been better for the bad swimmers if they couldn't swim at all and had clung to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are completely covered by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, perhaps aided by natural selection. In South America, a burrowing rodent called the tuco-tuco, or Ctenomys, is even more subterranean in its habits than the mole. I was told by a Spaniard who had often caught them that they were frequently blind. One that I kept alive was certainly in this condition; the cause, as appeared on dissection, was inflammation of the nictitating membrane. Since frequent inflammation of the eyes must be harmful to any animal, and since eyes are certainly not necessary for animals with subterranean habits, a reduction in their size -- with the eyelids fusing shut and fur growing over them -- might in such cases be an advantage. And if so, natural selection would aid the effects of disuse.
It's well known that several animals belonging to very different classes, which inhabit the caves of Carniola and Kentucky, are blind. In some of the crabs, the eye-stalk remains, though the eye is gone -- the stand for the telescope is there, but the telescope with its lenses has been lost. Since it's hard to imagine that eyes, though useless, could be in any way harmful to animals living in darkness, their loss can be attributed to disuse. In one of the blind animals -- the cave rat (Neotoma) -- two specimens were captured by Professor Silliman more than half a mile from the mouth of the cave, and therefore not in the deepest depths. Their eyes were shiny and large. And these animals, as Professor Silliman informed me, after being exposed to gradually increasing light for about a month, developed a dim ability to perceive objects.
It's hard to imagine conditions more similar than deep limestone caverns under a nearly identical climate. So under the old view that blind cave animals were separately created for the American and European caves, you'd expect very close similarity in their organization and relationships. This is certainly not the case if we look at the two whole faunas. With respect to the insects alone, Schiodte has remarked: "We are accordingly prevented from considering the entire phenomenon in any other light than something purely local, and the similarity which is exhibited in a few forms between the Mammoth Cave (in Kentucky) and the caves in Carniola, otherwise than as a very plain expression of that analogy which exists generally between the fauna of Europe and of North America." On my view, we must suppose that American animals, having in most cases ordinary powers of vision, slowly migrated over successive generations from the outer world into the deeper and deeper recesses of the Kentucky caves, just as European animals did into the caves of Europe. We have some evidence of this gradual transition. As Schiodte remarks: "We accordingly look upon the subterranean faunas as small branches which have penetrated into the earth from the geographically limited faunas of the adjacent areas, and which, as they extended themselves into darkness, have become adapted to their surroundings. Animals not far removed from ordinary forms prepare the transition from light to darkness. Next follow those that are built for twilight; and last of all, those suited for total darkness, whose structure is quite peculiar." It should be understood that Schiodte's remarks apply not to the same species, but to distinct species. By the time an animal has reached, after countless generations, the deepest recesses, disuse will on this view have more or less completely obliterated its eyes. Natural selection will often have brought about other changes, such as an increase in the length of the antennae or feelers, as compensation for blindness. Despite such modifications, we'd still expect to see in the cave animals of America relationships to the other inhabitants of that continent, and in those of Europe, relationships to the inhabitants of Europe. And this is the case with some of the American cave animals, as I hear from Professor Dana, and some of the European cave insects are very closely related to those of the surrounding country. It would be difficult to give any rational explanation for the relationships of blind cave animals to the other inhabitants of the two continents under the standard view of their independent creation. That several cave inhabitants of the Old and New Worlds should be closely related is exactly what we'd expect from the well-known relationship of most of their other organisms. Since a blind species of Bathyscia is found in abundance on shady rocks far from caves, the loss of vision in the cave species of this genus probably had nothing to do with its dark habitat -- it's natural that an insect already deprived of vision would readily become adapted to dark caverns. Another blind genus, Anophthalmus, offers a remarkable peculiarity: as Mr. Murray observes, its species have never been found anywhere except in caves, yet those inhabiting the various caves of Europe and America are distinct. But it's possible that the ancestors of these several species, back when they had functioning eyes, may formerly have ranged over both continents and then gone extinct everywhere except in their present secluded homes. Far from feeling surprised that some cave animals should be very unusual -- as Agassiz has remarked regarding the blind fish Amblyopsis, and as is the case with the blind Proteus among European reptiles -- I'm only surprised that more relics of ancient life have not been preserved, given the less intense competition that the sparse inhabitants of these dark homes must have faced.
Habit is hereditary in plants -- in the timing of flowering, in the time of dormancy, in the amount of rain required for seeds to germinate, and so on -- and this leads me to say a few words about acclimatization. Since it's extremely common for distinct species of the same genus to inhabit hot and cold countries, if it's true that all species of the same genus descended from a single ancestor, then acclimatization must be readily achieved during a long course of descent. It's well known that each species is adapted to the climate of its own home: species from arctic or even temperate regions can't endure a tropical climate, or vice versa. Similarly, many succulent plants can't endure a damp climate. But the degree to which species are adapted to the climates where they live is often overrated. We can infer this from our frequent inability to predict whether an imported plant will survive our climate, and from the number of plants and animals brought from different countries that are perfectly healthy here. We have reason to believe that species in nature are limited in their ranges by competition with other organisms quite as much as, or more than, by adaptation to particular climates. But whether or not this adaptation is in most cases very close, we do have evidence with some plants of their becoming naturally habituated to different temperatures -- that is, they become acclimatized. For example, pines and rhododendrons raised from seed collected by Dr. Hooker from the same species growing at different heights on the Himalayas were found to possess different constitutional powers of resisting cold when grown in this country. Mr. Thwaites informs me that he has observed similar facts in Ceylon. Analogous observations have been made by Mr. H. C. Watson on European plant species brought from the Azores to England, and I could give other cases. Regarding animals, several well-documented instances could be given of species having greatly extended their range from warmer to colder latitudes, or vice versa, within historical times. But we don't know for certain that these animals were strictly adapted to their native climate, though in ordinary cases we assume so. Nor do we know that they've subsequently become specially acclimatized to their new homes, so as to be better fitted for them than they were at first.
Since we can reasonably infer that our domestic animals were originally chosen by early humans because they were useful and because they bred readily in captivity -- not because they were later found capable of far-flung transportation -- the common and extraordinary ability of our domestic animals to not only withstand the most different climates but to remain perfectly fertile under them (a far more demanding test) can be used as an argument that a large proportion of other animals currently in a wild state could easily be brought to tolerate widely different climates. We shouldn't push this argument too far, however, because some of our domestic animals probably descend from several wild stocks: the blood of a tropical and an arctic wolf may perhaps be mingled in our domestic breeds. The rat and mouse can't be considered domestic animals, but they've been transported by humans to many parts of the world and now have a far wider range than any other rodent -- they live under the cold climate of the Faroes in the north and the Falklands in the south, and on many an island in the tropics. So adaptation to any special climate may be seen as a quality readily grafted onto an innate wide flexibility of constitution, common to most animals. On this view, the ability of humans and our domestic animals to endure the most different climates, and the fact that the extinct elephant and rhinoceros formerly endured a glacial climate while the living species are now all tropical or subtropical in their habits, shouldn't be seen as oddities, but as examples of a very common flexibility of constitution brought into action under special circumstances.
How much acclimatization of species to any particular climate is due to mere habit, how much to natural selection of varieties with different innate constitutions, and how much to both combined, is an unclear question. I believe that habit or custom does have some influence, both from analogy and from the constant advice given in agricultural works -- even in the ancient encyclopedias of China -- to be very cautious when transporting animals from one district to another. And since it's unlikely that humans have succeeded in selecting so many breeds and sub-breeds with constitutions specially suited to their own regions, the result must, I think, be due to habit. On the other hand, natural selection would inevitably tend to preserve those individuals born with constitutions best adapted to whatever country they inhabited. In books on many kinds of cultivated plants, certain varieties are said to withstand certain climates better than others. This is strikingly shown in works on fruit trees published in the United States, where certain varieties are routinely recommended for the northern states and others for the southern states. And since most of these varieties are of recent origin, they can't owe their constitutional differences to habit. The case of the Jerusalem artichoke -- which is never grown from seed in England, meaning new varieties haven't been produced -- has even been put forward as proof that acclimatization can't be achieved, since it's now as tender as it ever was! The case of the kidney bean has also been often cited for a similar purpose, and with much greater force. But until someone actually sows, for twenty or more generations, his kidney beans so early that a very large proportion are destroyed by frost, then collects seed from the few survivors with care to prevent accidental crosses, and then again gets seed from these seedlings with the same precautions, the experiment can't be said to have even been tried. Nor should we suppose that differences in the constitution of seedling kidney beans never appear -- an account has been published showing how much hardier some seedlings are than others, and I've personally observed striking instances of this.
On the whole, we can conclude that habit, or use and disuse, have in some cases played a considerable part in modifying constitution and structure. But their effects have often been largely combined with, and sometimes overruled by, the natural selection of innate variations.
By this expression I mean that the whole body is so tied together during its growth and development that when slight variations in any one part occur and are accumulated through natural selection, other parts become modified too. This is a very important subject, most imperfectly understood, and no doubt entirely different kinds of facts can easily be confused here. We'll see shortly that simple inheritance often gives the false appearance of correlation. One of the most obvious real cases is that variations arising in the young or larval stage naturally tend to affect the structure of the mature animal. The parts that are homologous -- identical in structure at an early embryonic stage, and necessarily exposed to similar conditions -- seem especially liable to vary in a similar way. We see this in the right and left sides of the body varying in the same manner, in the front and hind legs varying together, and even in the jaws and limbs varying together, for the lower jaw is believed by some anatomists to be homologous with the limbs. I have no doubt that these tendencies can be more or less completely overridden by natural selection. For example, a family of stags once existed with an antler on only one side, and if this had been of any great use to the breed, natural selection could probably have made it permanent.
Homologous parts, as some authors have remarked, tend to fuse together. This is often seen in abnormal plants, and nothing is more common than the union of homologous parts in normal structures -- such as the union of petals into a tube. Hard parts seem to affect the shape of adjoining soft parts. Some authors believe that in birds, the diversity in the shape of the pelvis causes the remarkable diversity in the shape of the kidneys. Others believe that the shape of the pelvis in the human mother influences the shape of the child's head through pressure. In snakes, according to Schlegel, the shape of the body and the manner of swallowing determine the position and shape of several of the most important internal organs.
The nature of the bond between correlated parts is frequently quite obscure. Geoffroy St. Hilaire has forcefully pointed out that certain malformations frequently coexist, while others rarely do, without our being able to explain why. What could be more singular than the relationship in cats between complete whiteness, blue eyes, and deafness? Or between the tortoiseshell color and the female sex? Or in pigeons, between feathered feet and skin between the outer toes, or between the presence of more or less down on newly hatched chicks and the future color of their plumage? Or again, the relationship between hair and teeth in the naked Turkish dog, though here homology no doubt plays a role? On this last case, I think it can hardly be accidental that the two orders of mammals most abnormal in their skin covering -- Cetacea (whales) and Edentata (armadillos, scaly anteaters, and so on) -- are also, on the whole, the most abnormal in their teeth. But there are so many exceptions to this rule, as Mr. Mivart has pointed out, that it has little value.
I know of no case better suited to show the importance of the laws of correlation and variation -- independently of utility, and therefore of natural selection -- than the difference between the outer and inner flowers in some Composite and Umbelliferous plants. Everyone is familiar with the difference between the ray florets and central florets of, for instance, the daisy, and this difference is often accompanied by the partial or complete abortion of the reproductive organs. But in some of these plants, the seeds also differ in shape and texture. These differences have sometimes been attributed to the pressure of the surrounding bracts on the florets, or to their mutual pressure, and the shape of the seeds in the ray florets of some Composites supports this idea. But with the Umbellifers, it's by no means the case, as Dr. Hooker informs me, that the species with the densest heads are those which most frequently differ in their inner and outer flowers. It might have been thought that the development of the ray petals, by drawing nourishment from the reproductive organs, causes their abortion. But this can hardly be the only explanation, because in some Composites the seeds of the outer and inner florets differ without any difference in the petals. Possibly these various differences may be connected with the different flow of nutrients toward the central and outer flowers. We know, at least, that with irregular flowers, those nearest the center axis are most subject to becoming abnormally symmetrical -- a condition called peloria. I can add, as an example of this and as a striking case of correlation, that in many pelargoniums the two upper petals in the central flower of the cluster often lose their patches of darker color. And when this occurs, the attached nectar tube is completely aborted -- the central flower thus becoming regular. When the color is absent from only one of the two upper petals, the nectar tube isn't completely aborted but is much shortened.
Regarding the development of the petals, Sprengel's idea that the ray florets serve to attract insects -- whose role is highly advantageous or necessary for the fertilization of these plants -- is very probably correct. And if so, natural selection may have come into play. But when it comes to the seeds, it seems impossible that their differences in shape -- which aren't always correlated with any difference in the petals -- can be beneficial in any way. Yet in the Umbellifers, these differences are of such apparent importance -- the seeds being sometimes straight-ribbed in the outer flowers and curved in the central flowers -- that the elder De Candolle founded his main divisions in the order on such characteristics. So modifications of structure that taxonomists consider highly significant may be entirely due to the laws of variation and correlation, without being, as far as we can judge, of the slightest use to the species.
We may often falsely attribute to correlated variation structures that are common to whole groups of species, and which in truth are simply due to inheritance. An ancient ancestor may have acquired through natural selection some one modification in structure, and after thousands of generations, some other independent modification. These two modifications, having been transmitted to a whole group of descendants with diverse habits, would naturally be thought to be correlated in some necessary way. Some other correlations are apparently due to the manner in which natural selection can act. For example, Alphonse de Candolle has remarked that winged seeds are never found in fruits that don't open. I'd explain this rule by the impossibility of seeds gradually becoming winged through natural selection unless the seed cases were open. Only in that case could seeds that were slightly better adapted for wind dispersal gain an advantage over those less well-suited for wide distribution.
The elder Geoffroy and Goethe proposed, at about the same time, their law of compensation or balance of growth. Or as Goethe expressed it, "in order to spend on one side, nature is forced to economize on the other side." I think this holds true to some extent with our domestic animals: if nourishment flows to one part or organ in excess, it rarely flows in excess to another part as well. It's hard to get a cow to give lots of milk and fatten up easily at the same time. The same varieties of cabbage don't yield both abundant, nutritious foliage and a plentiful supply of oil-bearing seeds. When the seeds in our fruits become shrunken, the fruit itself gains considerably in size and quality. In our poultry, a large tuft of feathers on the head is generally accompanied by a smaller comb, and a large beard by smaller wattles. For species in the wild, it can hardly be claimed that the law applies universally. But many good observers, especially botanists, believe it to be true. I won't give any examples here, though, because I see hardly any way to distinguish between, on one hand, a part being greatly developed through natural selection while an adjoining part is reduced by the same process or by disuse, and on the other hand, the actual diversion of nutrients from one part due to excessive growth in another adjoining part.
I also suspect that some of the cases of compensation that have been put forward, along with some other facts, may fall under a more general principle: namely, that natural selection is continually trying to economize in every part of the body. If under changed conditions a structure that was previously useful becomes less useful, its reduction will be favored, because it benefits the individual not to have its nutrients wasted in building up a useless structure. This is the only way I can understand a fact that struck me greatly when I was examining barnacles, and of which many other examples could be given: when a barnacle is parasitic within another barnacle and thus protected, it loses its own shell or carapace more or less completely. This is the case with the male Ibla, and in a truly extraordinary way with Proteolepas. In all other barnacles, the carapace consists of three highly important anterior segments of the head, enormously developed and furnished with large nerves and muscles. But in the parasitic and protected Proteolepas, the whole front part of the head is reduced to the merest rudiment attached to the bases of the grasping antennae. Now, the saving of a large and complex structure, when it's been made unnecessary, would be a real advantage to each successive individual of the species. In the struggle for life to which every animal is exposed, each would have a better chance of supporting itself by wasting less nutriment.
Thus, as I believe, natural selection will tend in the long run to reduce any part of the body as soon as it becomes unnecessary through changed habits -- without necessarily causing some other part to be developed to a corresponding degree. And conversely, natural selection may perfectly well succeed in greatly developing an organ without requiring, as a necessary trade-off, the reduction of some adjoining part.
It seems to be a rule, as noted by Geoffroy St. Hilaire, both with varieties and species, that when any part or organ is repeated many times in the same individual -- such as the vertebrae in snakes, or the stamens in flowers with many stamens -- the number is variable. But when the same part or organ occurs in smaller numbers, the count is constant. The same author, along with some botanists, has further remarked that multiple parts are extremely prone to vary in structure. Since "vegetative repetition," to use Professor Owen's expression, is a sign of low organization, these observations agree with the common view among naturalists that organisms lower on the scale of life are more variable than those higher up. I believe this is because, in low organisms, the various parts haven't been highly specialized for particular functions. And as long as the same part has to perform diverse tasks, we can perhaps see why it should remain variable -- why natural selection shouldn't have so carefully preserved or rejected each little deviation of form as when the part serves a single specific purpose. In the same way, a knife that has to cut all sorts of things can be almost any shape, while a tool for a particular job must be a particular shape. Natural selection, we should never forget, can act only through and for the advantage of each organism.
Rudimentary parts, as is generally recognized, tend to be highly variable. We'll have to return to this subject later. I'll only add here that their variability seems to result from their uselessness, and consequently from natural selection having had no power to keep deviations in their structure in check.
A Part Developed in Any Species in an Extraordinary Degree or Manner, in Comparison with the Same Part in Allied Species, Tends to Be Highly Variable
Several years ago, I was much struck by a remark to this effect made by Mr. Waterhouse. Professor Owen also seems to have reached a nearly similar conclusion. It's hopeless to try to convince anyone of the truth of this proposition without laying out the long array of facts I've collected, which can't possibly be presented here. I can only state my conviction that it's a rule of high generality. I'm aware of several sources of error, but I hope I've made proper allowances for them. It should be understood that the rule doesn't apply to any part, however unusually developed, unless it's unusually developed in one species or a few species compared to the same part in many closely related species. For instance, the wing of the bat is a highly abnormal structure in the class of mammals, but the rule wouldn't apply here because the entire group of bats possesses wings. It would apply only if some one species had wings developed in a remarkable manner compared to the other species of the same genus. The rule applies very strongly to secondary sexual characters when displayed in any unusual manner. The term "secondary sexual characters," coined by Hunter, refers to characters attached to one sex but not directly connected with the act of reproduction. The rule applies to males and females, but more rarely to females, since they seldom display remarkable secondary sexual characters. That the rule applies so clearly to secondary sexual characters may be because these characters are highly variable in general, whether or not they're displayed in any unusual way -- and I think there can be little doubt about this. But that our rule isn't confined to secondary sexual characters is clearly shown in the case of hermaphrodite barnacles. I paid particular attention to Mr. Waterhouse's remark while investigating this order, and I'm fully convinced that the rule almost always holds. I will, in a future work, give a list of all the more remarkable cases. I'll give just one here, as it illustrates the rule in its broadest application. The opercular valves of sessile barnacles (rock barnacles) are, in every sense of the word, very important structures, and they differ extremely little even between distinct genera. But in the several species of one genus, Pyrgoma, these valves show a marvelous amount of diversification. The homologous valves in the different species are sometimes completely unlike in shape. And the amount of variation among individuals of the same species is so great that it's no exaggeration to say that varieties of the same species differ from each other in the characters of these important structures more than do species belonging to other distinct genera.
Since birds of the same species inhabiting the same country vary extremely little, I've paid particular attention to them, and the rule certainly seems to hold in this class. I can't establish that it applies to plants, and this would have seriously shaken my belief in its truth, had not the great variability of plants made it particularly difficult to compare their relative degrees of variability.
When we see any part or organ developed to a remarkable degree in a species, the fair assumption is that it's of high importance to that species. Nevertheless, it's in this case especially prone to variation. Why should this be so? Under the view that each species has been independently created, with all its parts as we now see them, I can see no explanation. But under the view that groups of species descended from some other species and have been modified through natural selection, I think we can shed some light on this. First, let me make some preliminary observations. If, in our domestic animals, any part or the whole animal is neglected and no selection is applied, that part (for instance, the comb in the Dorking fowl) or the whole breed will cease to have a uniform character. The breed may be said to be degenerating. In rudimentary organs, and in those that have been only slightly specialized for any particular purpose, and perhaps in polymorphic groups, we see a nearly parallel case. In such situations, natural selection either hasn't come or can't come into full play, and the organism is left in a fluctuating condition. But what concerns us more here is that those features in our domestic animals currently undergoing rapid change by continued selection are also especially prone to variation. Look at individual birds of the same breed of pigeon, and see what a tremendous amount of difference there is in the beak of tumblers, the beak and wattle of carriers, the posture and tail of fantails, and so on -- these being the features now mainly focused on by English breeders. Even in the same sub-breed, as in that of the short-faced tumbler, it's notoriously difficult to breed nearly perfect birds, since many depart widely from the standard. There may truly be said to be a constant struggle going on between, on one hand, the tendency to revert to a less refined state combined with an innate tendency toward new variations, and on the other hand, the power of steady selection to keep the breed true. In the long run, selection wins the day, and we wouldn't expect to fail so completely as to breed a bird as rough as a common tumbler from a good short-faced strain. But as long as selection is actively going on, much variability in the parts undergoing modification can always be expected.
Now let's turn to nature. When a part has been developed in an extraordinary manner in any one species compared to the other species of the same genus, we can conclude that this part has undergone an extraordinary amount of modification since the period when the several species branched off from the common ancestor of the genus. This period will seldom be extremely remote, as species rarely endure for more than one geological period. An extraordinary amount of modification implies an unusually large and long-continued amount of variability, which has been continually accumulated by natural selection for the benefit of the species. But since the variability of the extraordinarily developed part has been so great and long-continued within a period not excessively remote, we might, as a general rule, still expect to find more variability in such parts than in other parts of the body that have remained nearly constant for a much longer period. And this, I'm convinced, is the case. I see no reason to doubt that the struggle between natural selection on one hand, and the tendency to reversion and variability on the other, will in time cease, and that the most abnormally developed organs may be made constant. So when an organ, however abnormal, has been transmitted in approximately the same condition to many modified descendants -- as with the wing of the bat -- it must have existed, according to our theory, for an immense period in nearly the same state. And thus it has come to be no more variable than any other structure. It's only in those cases where the modification has been comparatively recent and extraordinarily great that we should expect to find what might be called generative variability still present to a high degree. In this case, the variability won't yet have been fixed by the continued selection of individuals varying in the required manner and degree, and by the continued rejection of those tending to revert to a former, less modified condition.
The principle just discussed can be applied to our present subject. It's well known that specific characters are more variable than generic ones. To explain by a simple example: if in a large genus of plants some species had blue flowers and some had red, the color would be only a specific character, and no one would be surprised at one of the blue species varying to red, or vice versa. But if all the species had blue flowers, the color would become a generic character, and its variation would be a more unusual event. I've chosen this example because the explanation most naturalists would offer doesn't apply here -- namely, that specific characters are more variable than generic ones because they come from parts of less physiological importance than those commonly used for classifying genera. I believe this explanation is partly, yet only indirectly, true. I'll have to return to this point in the chapter on classification. It would be almost unnecessary to provide evidence for the statement that ordinary specific characters are more variable than generic ones. But with respect to important characters, I've repeatedly noticed in works on natural history that when an author remarks with surprise that some important organ or part, generally very constant throughout a large group of species, differs considerably in closely related species, it's often variable among individuals of the same species too. And this fact shows that a character which is generally of generic value, when it drops in rank and becomes only of specific value, often becomes variable -- even though its physiological importance may remain the same. Something of the same kind applies to monstrosities: Geoffroy St. Hilaire apparently has no doubt that the more an organ normally differs across species of the same group, the more subject it is to abnormalities in individuals.
Under the standard view of each species having been independently created, why should the part of the structure that differs from the same part in other independently created species of the same genus be more variable than those parts that are closely alike in the several species? I don't see that any explanation can be given. But under the view that species are only strongly marked and fixed varieties, we might often expect to find them still continuing to vary in those parts of their structure that have varied within a moderately recent period and have thus come to differ. Or to state the case another way: the features in which all the species of a genus resemble each other, and in which they differ from related genera, are called generic characters. These can be attributed to inheritance from a common ancestor, since it can rarely have happened that natural selection would have modified several distinct species, adapted to more or less different habits, in exactly the same way. And since these so-called generic characters have been inherited from before the period when the several species first branched off from their common ancestor, and have subsequently not varied or come to differ in any degree, or only slightly, it's not likely that they should vary today. On the other hand, the features in which species differ from other species of the same genus are called specific characters. Since these specific characters have varied and come to differ since the period when the species branched off from a common ancestor, it's likely that they should still often be somewhat variable -- at least more variable than those parts of the body that have remained constant for a very long period.
I think naturalists would agree, without my going into detail, that secondary sexual characters are highly variable. It will also be agreed that species of the same group differ from each other more widely in their secondary sexual characters than in other parts of their organization. Compare, for instance, the amount of difference between the males of game birds, in which secondary sexual characters are strongly displayed, with the amount of difference between the females. The cause of the original variability of these characters isn't clear. But we can see why they wouldn't have been rendered as constant and uniform as other traits: they're accumulated by sexual selection, which is less rigid in its action than ordinary selection, since it doesn't entail death, but only gives fewer offspring to the less favored males. Whatever the cause of the variability of secondary sexual characters, since they are highly variable, sexual selection has had a wide scope for action. And it may thus have succeeded in giving to the species of the same group a greater amount of difference in these traits than in other respects.
It's a remarkable fact that the secondary differences between the two sexes of the same species are generally displayed in the very same parts of the body where species of the same genus differ from each other. I'll give in illustration the first two instances that happen to be on my list, and since the differences in these cases are of a very unusual nature, the relationship can hardly be accidental. The same number of joints in the feet is a character common to very large groups of beetles, but in the Engidae, as Westwood has remarked, the number varies greatly -- and the number likewise differs between the two sexes of the same species. Again, in the digging wasps (fossorial Hymenoptera), the wing venation is a character of the highest importance, because it's common to large groups. But in certain genera, the venation differs between different species, and likewise between the two sexes of the same species. Sir J. Lubbock has recently remarked that several tiny crustaceans offer excellent illustrations of this law: "In Pontella, for instance, the sexual characters are afforded mainly by the anterior antennae and by the fifth pair of legs: the specific differences also are principally given by these organs." This relationship has a clear meaning on my view: I look at all the species of the same genus as having as certainly descended from the same ancestor as have the two sexes of any one species. Consequently, whatever part of the structure of the common ancestor, or of its early descendants, became variable, variations of this part would very likely be taken advantage of by natural and sexual selection -- in order to fit the several species to their various places in the natural order, and likewise to fit the two sexes of the same species to each other, or to fit the males to compete with other males for the possession of females.
Finally, then, I conclude that the greater variability of specific characters -- those that distinguish species from species -- compared to generic characters, which are shared by all the species; that the frequent extreme variability of any part developed to an extraordinary degree in one species compared to its relatives; and the slight variability of a part, however extraordinarily developed, if it's common to a whole group of species; that the great variability of secondary sexual characters and their great difference in closely related species; and that secondary sexual and ordinary specific differences are generally displayed in the same parts of the body -- all these principles are closely connected together. All are mainly due to the species of the same group being the descendants of a common ancestor, from whom they have inherited much in common. Parts that have recently and extensively varied are more likely still to go on varying than parts that have long been inherited and have not varied. Natural selection has, depending on the lapse of time, more or less completely overcome the tendency to reversion and to further variability. Sexual selection is less rigid than ordinary selection. And variations in the same parts have been accumulated by natural and sexual selection, thus becoming adapted for both secondary sexual and for ordinary purposes.
Distinct Species Present Analogous Variations, So That a Variety of One Species Often Assumes a Character Proper to an Allied Species, or Reverts to Some of the Characters of an Early Progenitor
These propositions will be most easily understood by looking at our domestic breeds. The most distinct breeds of pigeon, in countries widely apart, have sub-varieties with reversed feathers on the head and feathers on the feet -- characters not possessed by the ancestral rock pigeon. These are analogous variations in two or more distinct breeds. The frequent presence of fourteen or even sixteen tail feathers in the pouter may be considered a variation mimicking the normal structure of another breed, the fantail. I believe no one would doubt that all such analogous variations are due to the several breeds of pigeon having inherited from a common parent the same constitution and tendency to variation, when acted upon by similar unknown influences. In the plant kingdom, we have a case of analogous variation in the enlarged stems -- or, as they're commonly called, roots -- of the Swedish turnip and rutabaga, plants which several botanists consider varieties produced by cultivation from a common parent. If this isn't the case, it would then be one of analogous variation in two so-called distinct species, and a third could be added: namely, the common turnip. Under the standard view of each species having been independently created, we'd have to attribute this similarity in the enlarged stems of these three plants not to the true cause of shared descent and a resulting tendency to vary in a similar way, but to three separate yet closely related acts of creation. Many similar cases of analogous variation have been observed by Naudin in the great gourd family, and by various authors in our cereals. Similar cases occurring with insects in the wild have lately been discussed with great skill by Mr. Walsh, who has grouped them under his "law of equable variability."
With pigeons, however, we have another case: the occasional appearance in all the breeds of slaty-blue birds with two black bars on the wings, white loins, a bar at the end of the tail, and the outer feathers edged near their bases with white. Since all these marks are characteristic of the parent rock pigeon, I believe no one would doubt that this is a case of reversion, not of a new yet analogous variation appearing in the several breeds. We can, I think, confidently reach this conclusion, because, as we've seen, these colored marks are especially likely to appear in the crossed offspring of two distinct and differently colored breeds. In this case, there's nothing in the external conditions of life to cause the reappearance of the slaty-blue, with all its markings, beyond the influence of the mere act of crossing on the laws of inheritance.
It's certainly a very surprising fact that characters should reappear after being lost for many -- probably for hundreds of -- generations. But when a breed has been crossed only once with some other breed, the offspring occasionally show for many generations a tendency to revert to the foreign breed -- some say, for a dozen or even twenty generations. After twelve generations, the proportion of blood (to use a common expression) from one ancestor is only 1 in 2,048. And yet, as we see, it's generally believed that a tendency to reversion is retained by this tiny remnant of foreign blood. In a breed that hasn't been crossed, but in which both parents have lost some character their ancestor possessed, the tendency to reproduce the lost character -- whether strong or weak -- might, as I previously remarked, be transmitted for almost any number of generations, for all we know. When a character that has been lost in a breed reappears after a great number of generations, the most probable explanation isn't that one individual suddenly takes after an ancestor removed by hundreds of generations. Rather, in each successive generation, the character in question has been lying dormant, and at last, under unknown favorable conditions, it is expressed. With the barb pigeon, for instance, which very rarely produces a blue bird, it's probable that there's a dormant tendency in each generation to produce blue plumage. The abstract improbability of such a tendency being transmitted through a vast number of generations is no greater than the improbability of quite useless or rudimentary organs being similarly transmitted. A mere tendency to produce a rudiment is indeed sometimes inherited this way.
Since all the species of the same genus are supposed to have descended from a common ancestor, it might be expected that they would occasionally vary in an analogous manner -- so that the varieties of two or more species would resemble each other, or that a variety of one species would resemble another distinct species in certain characters (this other species being, on our view, only a well-marked and permanent variety). But characters due exclusively to analogous variation would probably be unimportant, because the preservation of all functionally important characters will have been determined through natural selection in accordance with the different habits of the species. It might further be expected that species of the same genus would occasionally show reversions to long-lost characters. However, since we don't know the common ancestor of any natural group, we can't distinguish between reversionary and analogous characters. If, for instance, we didn't know that the parent rock pigeon was neither feather-footed nor crest-crowned, we couldn't have told whether such characters in our domestic breeds were reversions or only analogous variations. But we might have inferred that the blue color was a case of reversion from the number of markings correlated with this tint, which wouldn't all likely have appeared together from simple variation. We especially might have inferred this from the blue color and its associated marks so often appearing when differently colored breeds are crossed. So, although in nature it must generally be left doubtful which cases are reversions to formerly existing characters and which are new but analogous variations, we ought, on our theory, sometimes to find the varying offspring of a species taking on characters already present in other members of the same group. And this is undoubtedly the case.
The difficulty in distinguishing variable species is largely due to the varieties mimicking, as it were, other species of the same genus. A considerable catalog could also be given of forms intermediate between two other forms, which themselves can only doubtfully be ranked as species. This shows, unless all these closely related forms are considered independently created species, that they have in varying assumed some of the characters of the others. But the best evidence of analogous variations comes from parts or organs that are generally constant in character but occasionally vary so as to resemble, in some degree, the same part or organ in a related species. I've collected a long list of such cases, but here, as before, I'm at the great disadvantage of not being able to present them. I can only repeat that such cases certainly occur and seem to me very remarkable.
I will, however, give one curious and complex case -- not because it affects any important character, but because it occurs in several species of the same genus, partly under domestication and partly in the wild. It's a case almost certainly of reversion. The donkey sometimes has very distinct transverse bars on its legs, like those on the legs of a zebra. It has been said that these are most visible in the foal, and from inquiries I've made, I believe this to be true. The stripe on the shoulder is sometimes double, and very variable in length and outline. A white donkey (but not an albino) has been described without either spinal or shoulder stripe, and these stripes are sometimes very faint or actually completely absent in dark-colored donkeys. The koulan of Pallas is said to have been seen with a double shoulder stripe. Mr. Blyth has seen a specimen of the hemionus with a distinct shoulder stripe, though it properly has none. And I've been informed by Colonel Poole that foals of this species are generally striped on the legs and faintly on the shoulder. The quagga, though so plainly barred like a zebra over the body, has no bars on the legs -- but Dr. Gray has illustrated one specimen with very distinct zebra-like bars on the hocks.
Regarding the horse, I've collected cases in England of the spinal stripe in horses of the most distinct breeds and of all colors. Transverse bars on the legs are not rare in duns and mouse-duns, and in one instance in a chestnut. A faint shoulder stripe can sometimes be seen in duns, and I've seen a trace in a bay horse. My son made a careful examination and sketch for me of a dun Belgian carthorse with a double stripe on each shoulder and with leg stripes. I've personally seen a dun Devonshire pony, and a small dun Welsh pony has been carefully described to me, both with three parallel stripes on each shoulder.
In the northwest part of India, the Kattywar breed of horses is so generally striped that, as I hear from Colonel Poole, who examined this breed for the Indian government, a horse without stripes isn't considered purely bred. The spine is always striped; the legs are generally barred; and the shoulder stripe, which is sometimes double and sometimes triple, is common. The side of the face is moreover sometimes striped. The stripes are often most visible in the foal and sometimes quite disappear in old horses. Colonel Poole has seen both gray and bay Kattywar horses striped when first foaled. I also have reason to suspect, from information given me by Mr. W. W. Edwards, that in the English racehorse the spinal stripe is much more common in the foal than in the full-grown animal. I recently bred a foal myself from a bay mare (the offspring of a Turkoman horse and a Flemish mare) by a bay English racehorse. This foal, when a week old, was marked on its hindquarters and on its forehead with numerous very narrow, dark, zebra-like bars, and its legs were faintly striped. All the stripes soon disappeared completely. Without going into further details, I can state that I've collected cases of leg and shoulder stripes in horses of very different breeds in various countries, from Britain to eastern China and from Norway in the north to the Malay Archipelago in the south. In all parts of the world, these stripes occur far most often in duns and mouse-duns -- and by the term "dun," a large range of colors is included, from one between brown and black to a close approach to cream.
I'm aware that Colonel Hamilton Smith, who has written on this subject, believes that the several breeds of the horse descended from several ancestral species, one of which, the dun, was striped -- and that the appearances described above are all due to ancient crosses with the dun stock. But this view can safely be rejected, for it's highly improbable that the heavy Belgian carthorse, Welsh ponies, Norwegian cobs, the lanky Kattywar breed, and so on -- inhabiting the most distant parts of the world -- should all have been crossed with one supposed ancestral stock.
Now let's turn to the effects of crossing the several species of the horse genus. Rollin asserts that the common mule from the donkey and horse is particularly apt to have bars on its legs. According to Mr. Gosse, in certain parts of the United States, about nine out of ten mules have striped legs. I once saw a mule with its legs so heavily striped that anyone might have thought it was a hybrid zebra. And Mr. W. C. Martin, in his excellent treatise on the horse, has illustrated a similar mule. In four colored drawings I've seen of hybrids between the donkey and zebra, the legs were much more plainly barred than the rest of the body, and in one of them there was a double shoulder stripe. In Lord Morton's famous hybrid from a chestnut mare and male quagga, the hybrid and even the pure offspring subsequently produced from the same mare by a black Arabian sire were much more plainly barred across the legs than even the pure quagga. Lastly, and this is another most remarkable case, a hybrid has been illustrated by Dr. Gray (and he tells me he knows of a second case) from the donkey and the hemionus. This hybrid -- though the donkey only occasionally has stripes on its legs, and the hemionus has none and doesn't even have a shoulder stripe -- nevertheless had all four legs barred and had three short shoulder stripes, like those on the dun Devonshire and Welsh ponies, and even had some zebra-like stripes on the sides of its face. Regarding this last detail, I was so convinced that not even a stripe of color appears by what is commonly called chance, that I was led solely by the occurrence of the face stripes on this hybrid from the donkey and hemionus to ask Colonel Poole whether such face stripes ever occurred in the eminently striped Kattywar breed of horses -- and was, as we've seen, answered in the affirmative.
What are we to make of all these facts? We see several distinct species of the horse genus becoming, by simple variation, striped on the legs like a zebra, or striped on the shoulders like a donkey. In the horse, we see this tendency strongest whenever a dun tint appears -- a tint approaching the general coloring of the other species of the genus. The appearance of the stripes isn't accompanied by any change of form or by any other new character. We see this tendency to become striped most strongly displayed in hybrids between several of the most distinct species. Now consider the case of the several breeds of pigeons: they descended from a pigeon (including two or three subspecies or geographical races) of a bluish color with certain bars and other marks. When any breed takes on a bluish tint by simple variation, these bars and other marks invariably reappear -- but without any other change of form or character. When the oldest and truest breeds of various colors are crossed, we see a strong tendency for the blue tint and bars and marks to reappear in the hybrids. I've stated that the most probable explanation for the reappearance of very ancient characters is that there's a tendency in the young of each successive generation to produce the long-lost character, and that this tendency, from unknown causes, sometimes prevails. And we've just seen that in several species of the horse genus the stripes are either plainer or appear more commonly in the young than in the old. Call the breeds of pigeons -- some of which have bred true for centuries -- species, and how exactly parallel is the case with that of the species of the horse genus! For myself, I venture confidently to look back thousands upon thousands of generations, and I see an animal striped like a zebra, but perhaps otherwise very differently built -- the common parent of our domestic horse (whether or not it descended from one or more wild stocks), of the donkey, the hemionus, the quagga, and the zebra.
Anyone who believes that each equine species was independently created will, I presume, assert that each species was created with a tendency to vary, both in nature and under domestication, in this particular manner, so as often to become striped like the other species of the genus. And that each was created with a strong tendency, when crossed with species inhabiting distant parts of the world, to produce hybrids resembling in their stripes not their own parents but other species of the genus. To accept this view is, it seems to me, to reject a real cause for an unreal, or at least unknown, one. It makes the works of God a mere mockery and deception. I would almost as soon believe, with the old and ignorant cosmogonists, that fossil shells had never lived but had been created in stone to mock the shells now living on the seashore.
Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we claim to assign any reason why this or that part has varied. But whenever we have the means of making a comparison, the same laws appear to have acted in producing the lesser differences between varieties of the same species and the greater differences between species of the same genus. Changed conditions generally cause mere fluctuating variability, but sometimes they produce direct and definite effects -- and these may become strongly marked over time, though we don't have enough evidence on this point. Habit in producing constitutional peculiarities, and use in strengthening and disuse in weakening and diminishing organs, appear in many cases to have been powerful in their effects. Homologous parts tend to vary in the same manner, and homologous parts tend to fuse together. Modifications in hard parts and in external parts sometimes affect softer and internal parts. When one part is greatly developed, it perhaps tends to draw nourishment from adjoining parts. And every part of the structure that can be saved without harm will be saved. Changes of structure at an early age may affect parts that develop later. And many cases of correlated variation, whose nature we're unable to understand, undoubtedly occur. Multiple parts are variable in number and in structure, perhaps because such parts haven't been closely specialized for any particular function, so that their modifications haven't been closely checked by natural selection. It probably follows from this same cause that organisms low on the scale of life are more variable than those standing higher, which have their whole body more specialized. Rudimentary organs, being useless, are not regulated by natural selection, and are therefore variable. Specific characters -- that is, the characters that have come to differ since the several species of the same genus branched off from a common parent -- are more variable than generic characters, or those that have long been inherited and haven't differed within this same period. In these remarks, we've been discussing special parts or organs being still variable because they have recently varied and thus come to differ. But we also saw in the second chapter that the same principle applies to the whole individual: in a district where many species of a genus are found -- where there has been much former variation and differentiation, or where the factory of new specific forms has been actively at work -- in that district and among these species, we now find, on average, the most varieties. Secondary sexual characters are highly variable, and such characters differ greatly across species of the same group. Variability in the same parts of the body has generally been taken advantage of in giving secondary sexual differences to the two sexes of the same species, and specific differences to the several species of the same genus. Any part or organ developed to an extraordinary size or in an extraordinary manner, compared with the same part or organ in related species, must have gone through an extraordinary amount of modification since the genus arose. And so we can understand why it should often still be variable to a much higher degree than other parts, for variation is a long-continued and slow process, and natural selection will in such cases not yet have had time to overcome the tendency to further variability and to reversion to a less modified state. But when a species with an extraordinarily developed organ has become the parent of many modified descendants -- which on our view must be a very slow process, requiring a long lapse of time -- in this case, natural selection has succeeded in giving a fixed character to the organ, however extraordinarily it may have been developed. Species inheriting nearly the same constitution from a common parent, and exposed to similar influences, naturally tend to present analogous variations. Or these same species may occasionally revert to some of the characters of their ancient ancestors. Although new and important modifications may not arise from reversion and analogous variation, such modifications will add to the beautiful and harmonious diversity of nature.
Whatever the cause may be of each slight difference between offspring and their parents -- and a cause for each must exist -- we have reason to believe that it is the steady accumulation of beneficial differences which has given rise to all the more important modifications of structure in relation to the habits of each species.
Long before reaching this part of my book, the reader will have thought of a whole crowd of difficulties with the theory. Some of them are so serious that to this day I can hardly reflect on them without being staggered. But to the best of my judgment, most of these difficulties are only apparent, and those that are real are not, I think, fatal to the theory.
These difficulties and objections can be grouped under the following headings. First, if species have descended from other species by tiny gradual steps, why don't we see countless transitional forms everywhere? Why isn't all of nature a confused mess, instead of species being, as we actually observe, well defined?
Second, is it possible that an animal with the structure and habits of a bat could have been formed by modifying some other animal with very different habits and structure? Can we believe that natural selection could produce, on the one hand, something as trivial as the tail of a giraffe, which serves as a fly swatter, and on the other hand, something as wonderful as the eye?
Third, can instincts be acquired and modified through natural selection? What do we make of the instinct that leads bees to build their cells -- an instinct that has practically anticipated the discoveries of advanced mathematicians?
Fourth, how can we explain that when species are crossed they produce sterile offspring, while when varieties are crossed their fertility is unaffected?
The first two of these headings will be discussed here. Some miscellaneous objections will be addressed in the following chapter, and instinct and hybridism in the two chapters after that.
Since natural selection works solely by preserving beneficial changes, each new form will tend, in a fully occupied environment, to take the place of and eventually wipe out its own less improved parent form and other less favored forms it competes with. Extinction and natural selection go hand in hand. So if we view each species as descended from some unknown earlier form, both the parent and all the transitional varieties will generally have been driven extinct by the very process that created and perfected the new form.
But if my theory is right and countless transitional forms must have existed, why don't we find them buried in enormous numbers in the earth's crust? It will be more convenient to discuss this question in the chapter on the imperfection of the geological record. I'll simply say here that I believe the answer mainly lies in the fossil record being incomparably less complete than is generally supposed. The earth's crust is a vast museum, but the natural collections have been poorly assembled, and only at long intervals of time.
But someone might argue that when several closely related species live in the same territory, we surely ought to find many transitional forms alive today. Let's take a simple case: traveling from north to south across a continent, we typically encounter closely related or representative species at successive intervals, each clearly filling nearly the same role in the natural order of the land. These representative species often meet and overlap. As one becomes rarer and rarer, the other becomes more and more common, until one replaces the other. But if we compare these species where they intermingle, they are generally as absolutely distinct from each other in every detail of structure as specimens taken from the heartland of each species. According to my theory, these related species descended from a common parent. During the process of modification, each became adapted to the conditions of life in its own region, and supplanted and wiped out both its original parent form and all the transitional varieties between its past and present states. So we shouldn't expect to find numerous transitional varieties living in each region today, even though they must have existed there and may be preserved as fossils. But in the intermediate region, with its intermediate conditions of life, why don't we now find closely connecting intermediate varieties? This difficulty confused me for a long time. But I think it can be largely explained.
First, we should be extremely cautious about assuming that because an area is now continuous, it has been continuous for a long period. Geology suggests that most continents have been broken up into islands even during the later Tertiary periods. On such islands, distinct species could have formed separately, with no possibility of intermediate varieties existing in the zones between them. Due to changes in landforms and climate, marine areas that are now continuous must often have existed in a far less continuous and uniform state within recent times. But I'll pass over this way of escaping the difficulty, because I believe that many perfectly defined species have formed on strictly continuous areas. Still, I don't doubt that the formerly broken-up condition of areas now continuous has played an important part in the formation of new species, especially for animals that crossbreed freely and wander widely.
When we look at species as they are now distributed over a wide area, we generally find them fairly numerous across a large territory, then becoming somewhat abruptly rarer and rarer at the edges, and finally disappearing. The neutral territory between two representative species is therefore generally narrow compared to the territory proper to each. We see the same thing going up mountains, and sometimes the cutoff is quite remarkable -- as Alphonse de Candolle has observed, a common alpine species can disappear quite abruptly. The naturalist E. Forbes noticed the same fact when sounding the depths of the sea with a dredge. For those who see climate and physical conditions as the all-important factors in distribution, these facts should be surprising, since climate, altitude, and depth change gradually and imperceptibly. But when we keep in mind that almost every species, even in its heartland, would increase enormously in numbers if not for other competing species; that nearly all either prey on or serve as prey for others; in short, that every organism is related, either directly or indirectly, in the most important way to other organisms -- we can see that the range of any country's inhabitants doesn't depend solely on gradually changing physical conditions. It depends in large part on the presence of other species: what it feeds on, what destroys it, and what it competes with. And since these other species are already well-defined entities that don't blend into one another by imperceptible gradations, the range of any one species, depending as it does on the ranges of others, will tend to be sharply defined. Furthermore, each species at the edges of its range, where it exists in reduced numbers, will be extremely vulnerable to total extinction during fluctuations in its enemies, its prey, or the seasons. And so its geographical range will become even more sharply defined.
Since related or representative species living in a continuous area are generally distributed so that each has a wide range with a comparatively narrow neutral territory between them -- a territory where they become rather suddenly rarer -- the same rule will probably apply to varieties, since varieties don't fundamentally differ from species. If we take a varying species inhabiting a very large area, we'll need to picture two varieties adapted to two large areas, and a third variety adapted to a narrow intermediate zone. The intermediate variety will consequently exist in smaller numbers because it inhabits a narrower, smaller area. As far as I can tell, this rule holds true for varieties in nature. I've found striking examples in varieties intermediate between well-marked varieties in the barnacle genus Balanus. And from information given to me by the botanist H. C. Watson, the botanist Asa Gray, and the entomologist Wollaston, it appears that when varieties intermediate between two other forms occur, they are generally much rarer numerically than the forms they connect. Now, if we can trust these facts and inferences, and conclude that varieties linking two other varieties together have generally existed in smaller numbers than the forms they connect, we can understand why intermediate varieties shouldn't endure for very long periods -- why, as a general rule, they should be wiped out and disappear sooner than the forms they originally linked.
For any form existing in smaller numbers would, as I've already noted, run a greater chance of being wiped out than one existing in large numbers. And in this particular case, the intermediate form would be especially vulnerable to encroachment from closely related forms on both sides of it. But there's a far more important consideration. During the process of further modification -- by which two varieties are converted and perfected into two distinct species -- the two that exist in larger numbers, because they inhabit larger areas, will have a great advantage over the intermediate variety existing in smaller numbers in its narrow intermediate zone. Forms existing in larger numbers will have a better chance, in any given period, of producing further favorable variations for natural selection to act on than will the rarer forms. So the more common forms, in the race for life, will tend to beat and replace the less common forms, because the common forms will be more quickly modified and improved. This is the same principle that, as I showed in the second chapter, accounts for common species in each country presenting on average a greater number of well-marked varieties than do rare species. I can illustrate what I mean with an analogy: suppose three varieties of sheep are kept, one adapted to an extensive mountainous region, a second to a comparatively narrow hilly tract, and a third to the wide plains at the base. If the farmers in all three areas are trying with equal dedication and skill to improve their flocks by selection, the chances will strongly favor the large-scale mountain or plains breeders improving their flocks more quickly than the small-scale breeders on the narrow, hilly tract. As a result, the improved mountain or plain breed will soon take the place of the less improved hill breed. The two breeds that originally existed in greater numbers will come into close contact with each other, without the now-replaced intermediate hill variety between them.
To sum up, I believe that species come to be fairly well-defined entities, and don't at any one time present a hopeless tangle of varying and intermediate links. First, because new varieties are very slowly formed -- variation is a slow process, and natural selection can do nothing until favorable individual differences or variations occur and until a niche in the natural order of the country can be better filled by some modification of one or more of its inhabitants. Such new niches will depend on slow changes of climate, on the occasional immigration of new inhabitants, and probably -- even more importantly -- on some of the old inhabitants becoming slowly modified, with the new forms and old ones acting and reacting on each other. So in any one region at any one time, we ought to see only a few species showing slight modifications of structure that are somewhat permanent. And that is exactly what we do see.
Second, areas that are now continuous must often have existed within recent times as isolated portions. In such isolated areas, many forms -- especially among the kinds that mate for each birth and wander widely -- may have separately become distinct enough to rank as representative species. In that case, intermediate varieties between the several representative species and their common parent must formerly have existed within each isolated portion of land. But these links, through the process of natural selection, will have been replaced and wiped out, so they will no longer be found alive.
Third, when two or more varieties have formed in different parts of a strictly continuous area, intermediate varieties probably formed at first in the intermediate zones, but they will generally have been short-lived. These intermediate varieties will, for reasons already given -- namely, from what we know about the actual distribution of closely related or representative species, and also of acknowledged varieties -- exist in the intermediate zones in smaller numbers than the varieties they tend to connect. For this reason alone, the intermediate varieties will be vulnerable to accidental extinction. And during the process of further modification through natural selection, they will almost certainly be beaten and replaced by the forms they connect, since those forms, existing in greater numbers, will collectively produce more variations and thus be further improved through natural selection and gain still more advantages.
Finally, looking not at any one time but at all of time: if my theory is true, countless intermediate varieties, closely linking all the species of each group, must certainly have existed. But the very process of natural selection constantly tends, as I've so often noted, to wipe out parent forms and intermediate links. Consequently, evidence of their former existence could be found only among fossil remains, which are preserved, as I'll attempt to show in a future chapter, in an extremely imperfect and intermittent record.
Opponents of views like mine have asked: how, for instance, could a land-based carnivore have been converted into one with aquatic habits? How could the animal have survived in its transitional state? It would be easy to show that carnivorous animals exist today presenting close intermediate grades from strictly land-based to aquatic habits. And since each survives through the struggle for existence, each must clearly be well adapted to its place in nature. Look at the American mink (Mustela vison) of North America, which has webbed feet and resembles an otter in its fur, short legs, and tail shape. During summer, this animal dives for fish, but during the long winter it leaves the frozen waters and preys on mice and other land animals like other polecats. If a different example had been chosen, and someone asked how an insect-eating mammal could possibly have been converted into a flying bat, the question would have been far more difficult to answer. Yet I think such difficulties carry little weight.
Here, as on other occasions, I'm at a heavy disadvantage, because out of the many striking cases I've collected, I can give only one or two examples of transitional habits and structures in related species, and of diversified habits -- whether constant or occasional -- in the same species. And it seems to me that nothing less than a long list of such cases is sufficient to lessen the difficulty in any particular case like that of the bat.
Look at the family of squirrels. Here we have the finest gradation from animals with their tails only slightly flattened, and from others -- as the naturalist Sir J. Richardson has noted -- with the rear part of their bodies rather wide and with somewhat loose skin on their flanks, all the way to the so-called flying squirrels. Flying squirrels have their limbs and even the base of the tail connected by a broad expanse of skin, which serves as a parachute and allows them to glide through the air for astonishing distances from tree to tree. We can't doubt that each structure is useful to each kind of squirrel in its own environment, enabling it to escape predators or to collect food more quickly, or -- as there is reason to believe -- to reduce the danger from occasional falls. But it doesn't follow that the structure of each squirrel is the best conceivable under all possible conditions. Let the climate and vegetation change, let other competing rodents or new predators arrive, or old ones become modified, and all our experience would lead us to expect that at least some of the squirrels would decline in numbers or go extinct -- unless they too became modified and improved in a corresponding way. So I can see no difficulty, especially under changing conditions of life, in the continued preservation of individuals with fuller and fuller flank membranes, each modification being useful, each being passed on, until through the accumulated effects of natural selection, a perfect so-called flying squirrel was produced.
Now look at the Galeopithecus, or so-called flying lemur, which was formerly classified with bats but is now believed to belong to the insectivores. An extremely wide flank membrane stretches from the corners of the jaw to the tail and includes the limbs with their elongated fingers. This flank membrane has its own extensor muscle. Although no graduated links of structure fitted for gliding through the air now connect the Galeopithecus with the other insectivores, there's no difficulty in supposing that such links formerly existed, and that each was developed in the same way as with the less perfectly gliding squirrels -- each grade of structure being useful to its possessor. Nor do I see any insurmountable difficulty in going further and believing that the membrane-connected fingers and forearm of the Galeopithecus might have been greatly lengthened by natural selection. This, as far as the organs of flight are concerned, would have converted the animal into a bat. In certain bats whose wing membrane extends from the top of the shoulder to the tail and includes the hind legs, we may perhaps see traces of an apparatus originally adapted for gliding through the air rather than for true flight.
If about a dozen genera of birds were to go extinct, who would have dared to guess that birds might have existed which used their wings solely as flappers, like the loggerhead duck; as fins in the water and front legs on the land, like the penguin; as sails, like the ostrich; and for functionally no purpose at all, like the apteryx? Yet the structure of each of these birds works well for it under its actual conditions of life, because each has to survive through the struggle for existence. But its structure isn't necessarily the best possible under all possible conditions. We shouldn't infer from these remarks that any of the grades of wing structure I've mentioned -- which may all be the result of disuse -- show the actual steps by which birds acquired their perfect power of flight. But they do show what diversified means of transition are at least possible.
Seeing that a few members of water-breathing groups like the crustaceans and mollusks are adapted to live on land; and seeing that we have flying birds and mammals, flying insects of the most varied types, and once had flying reptiles -- it's conceivable that flying fish, which now glide far through the air while slightly rising and turning with the aid of their fluttering fins, might have been modified into perfectly winged animals. If that had happened, who would have ever imagined that in an early transitional state they had been inhabitants of the open ocean, using their developing organs of flight exclusively, as far as we know, to escape being devoured by other fish?
When we see any structure highly perfected for a particular function, like the wings of a bird for flight, we should keep in mind that animals displaying early transitional grades of that structure will seldom have survived to the present day, because they will have been replaced by their successors, which were gradually made more perfect through natural selection. Furthermore, we can conclude that transitional states between structures suited for very different ways of life will rarely have developed at an early period in great numbers and in many varied forms. So, to return to our imaginary example of the flying fish: it doesn't seem likely that fish capable of true flight would have evolved into many different forms, taking prey of many kinds in many ways on land and in water, until their organs of flight had reached a high stage of perfection, giving them a decisive advantage over other animals in the battle for life. So the chances of discovering species with transitional grades of structure as fossils will always be less, because they existed in smaller numbers, than the chances for species with fully developed structures.
I'll now give two or three examples of diversified and changed habits within the same species. In either case, it would be easy for natural selection to adapt the animal's structure to its changed habits, or exclusively to one of its several habits. It is difficult to decide, and immaterial for us, whether habits generally change first with structure following, or whether slight modifications of structure lead to changed habits. Both probably often happen almost simultaneously. For cases of changed habits, it's enough to mention the many British insects that now feed on exotic plants or exclusively on artificial substances. For diversified habits, countless examples could be given. I've often watched a tyrant flycatcher (Saurophagus sulphuratus) in South America hovering over one spot and then moving to another, like a kestrel, and at other times standing still at the edge of the water, then plunging in like a kingfisher after a fish. In our own country, the great tit (Parus major) can be seen climbing branches almost like a creeper; it sometimes, like a shrike, kills small birds by blows to the head; and I've many times seen and heard it hammering the seeds of yew on a branch, breaking them open like a nuthatch. In North America, the black bear was observed by Hearne swimming for hours with its mouth wide open, catching insects in the water almost like a whale.
Since we sometimes see individuals following habits different from those normal for their species and for other species of the same genus, we might expect that such individuals would occasionally give rise to new species with unusual habits and with their structure either slightly or considerably modified from the typical form. And such cases do occur in nature. Can you find a more striking example of adaptation than the woodpecker, built for climbing trees and seizing insects in the crevices of bark? Yet in North America, there are woodpeckers that feed mainly on fruit, and others with elongated wings that chase insects in flight. On the plains of La Plata, where hardly a tree grows, there is a woodpecker (Colaptes campestris) with two toes in front and two behind, a long pointed tongue, pointed tail feathers stiff enough to support the bird in a vertical position on a post -- but not as stiff as in typical woodpeckers -- and a straight, strong beak. The beak, however, is not quite as straight or strong as in typical woodpeckers, but it's strong enough to bore into wood. So this Colaptes, in all the essential parts of its structure, is a woodpecker. Even in such minor details as coloring, harsh voice, and undulating flight, its close blood relationship to our common woodpecker is plainly obvious. Yet, as I can confirm from my own observations and those of the careful naturalist Azara, in certain large districts it does not climb trees and it makes its nest in holes in riverbanks! In certain other districts, however, this same woodpecker, as Mr. Hudson reports, does frequent trees and bores holes in the trunk for its nest. I might mention as another illustration of the varied habits of this genus that a Mexican Colaptes has been described by De Saussure as boring holes into hard wood in order to store acorns.
Petrels are the most aerial and oceanic of birds, but in the quiet sounds of Tierra del Fuego, the Puffinuria berardi -- in its general habits, its astonishing diving ability, its manner of swimming and of flying when forced to take flight -- would be mistaken by anyone for an auk or a grebe. Nevertheless, it is essentially a petrel, but with many parts of its body profoundly modified in relation to its new way of life. The woodpecker of La Plata, by contrast, has had its structure only slightly modified. In the case of the water ouzel, the most careful observer, by examining its dead body, would never have suspected its underwater habits. Yet this bird, which is related to the thrush family, lives by diving -- using its wings underwater and gripping stones with its feet. All the members of the great order Hymenoptera are land-based, except for the genus Proctotrupes, which the naturalist Sir John Lubbock discovered to be aquatic in its habits. It often enters the water and dives about using not its legs but its wings, and remains as long as four hours beneath the surface. Yet it shows no modification in structure to match its unusual habits.
Anyone who believes that each organism was created as we now see it must occasionally have felt surprise when encountering an animal whose habits and structure don't agree. What could be plainer than that the webbed feet of ducks and geese are made for swimming? Yet there are upland geese with webbed feet that rarely go near the water, and no one except Audubon has ever seen the frigate bird -- which has all four toes webbed -- actually land on the surface of the ocean. On the other hand, grebes and coots are superb swimmers even though their toes are only bordered by membrane. What seems plainer than that the long toes of wading birds, without membrane, are formed for walking over swamps and floating plants? The water hen and landrail belong to this order, yet the first is nearly as aquatic as the coot, and the second is nearly as terrestrial as the quail or partridge. In these cases, and many others could be given, habits have changed without a corresponding change of structure. The webbed feet of the upland goose have become almost useless in function, though not in structure. In the frigate bird, the deeply scooped membrane between the toes shows that structure has begun to change.
Anyone who believes in separate and innumerable acts of creation may say that in these cases it pleased the Creator to cause a being of one type to take the place of one belonging to another type. But this seems to me like just restating the fact in dignified language. On the other hand, anyone who believes in the struggle for existence and in the principle of natural selection will recognize that every organism is constantly striving to increase in numbers, and that if any one organism varies even a little -- either in habits or structure -- and thereby gains an advantage over some other inhabitant of the same country, it will seize on the place of that inhabitant, however different that place may be from its own. So it should cause no surprise that there are geese and frigate birds with webbed feet living on dry land and rarely landing on water; that there are long-toed corncrakes living in meadows instead of swamps; that there are woodpeckers where hardly a tree grows; that there are diving thrushes, diving wasps, and petrels with the habits of auks.
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for correcting spherical and chromatic aberration, could have been formed by natural selection seems, I freely confess, absurd in the highest degree. When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false. But the old saying "the voice of the people is the voice of God," as every philosopher knows, cannot be trusted in science. Reason tells me that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor -- as is certainly the case; if further, the eye ever varies and the variations are inherited -- as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life -- then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as fatal to the theory. How a nerve came to be sensitive to light hardly concerns us more than how life itself originated. But I may note that since some of the lowest organisms, in which nerves can't be detected at all, are capable of perceiving light, it doesn't seem impossible that certain light-sensitive elements in their tissue could have become aggregated and developed into nerves endowed with this special sensitivity.
In searching for the gradations through which an organ in any species has been perfected, we ought to look exclusively to its direct ancestors. But this is almost never possible, and we're forced to look to other species and genera of the same group -- the collateral descendants from the same parent form -- to see what gradations are possible, and for the chance that some gradations have been transmitted in an unaltered or little-altered condition. The state of the same organ in distinct classes may also incidentally shed light on the steps by which it has been perfected.
The simplest organ that can be called an eye consists of an optic nerve surrounded by pigment cells and covered by translucent skin, but without any lens or other light-bending structure. We may, however, according to the work of M. Jourdain, descend even a step lower and find clusters of pigment cells apparently serving as organs of vision, without any nerves, and resting merely on simple tissue. Eyes of this basic nature are not capable of distinct vision and serve only to distinguish light from darkness. In certain starfish, small depressions in the layer of pigment surrounding the nerve are filled, as described by the same researcher, with transparent gelatinous matter that projects with a convex surface, like the cornea in higher animals. He suggests that this serves not to form an image but only to concentrate the light rays and make them easier to perceive. In this concentration of rays, we gain the first and by far the most important step toward the formation of a true, image-forming eye. For we have only to place the exposed end of the optic nerve -- which in some lower animals lies deeply buried in the body, and in some near the surface -- at the right distance from the concentrating apparatus, and an image will be formed on it.
In the great class of arthropods, we may start from an optic nerve simply coated with pigment -- the pigment sometimes forming a sort of pupil -- but without a lens or other optical device. In insects, it's now known that the numerous facets on the cornea of their large compound eyes form true lenses, and that the cones include curiously modified nerve fibers. But these organs in the arthropods are so diversified that the researcher Muller formerly divided them into three main classes with seven subdivisions, plus a fourth main class of clustered simple eyes.
When we reflect on these facts -- given here far too briefly -- about the wide, diversified, and graduated range of eye structures in lower animals, and when we keep in mind how small the number of all living forms must be compared with those that have gone extinct, the difficulty is no longer very great in believing that natural selection could have converted the simple apparatus of an optic nerve coated with pigment and enclosed in transparent membrane into an optical instrument as perfect as any possessed by a member of the arthropod class.
Anyone who will go this far should not hesitate to go one step further. If, on finishing this book, he finds that large bodies of facts -- otherwise inexplicable -- can be explained by the theory of modification through natural selection, he ought to admit that even a structure as perfect as an eagle's eye might have been formed in this way, even though he doesn't know the transitional states. It has been objected that to modify the eye while still preserving it as a working instrument, many changes would have to be made simultaneously -- something that, it is assumed, couldn't be done through natural selection. But as I've tried to show in my work on the variation of domestic animals, there's no need to suppose the modifications were all simultaneous, so long as they were extremely slight and gradual. Different kinds of modification would also serve the same general purpose. As Alfred Russel Wallace has pointed out: "If a lens has too short or too long a focus, it may be corrected either by a change of curvature or a change of density. If the curvature is irregular and the rays don't converge to a point, then any increased regularity of curvature will be an improvement. The contraction of the iris and the muscular movements of the eye are neither of them essential to vision, but only improvements that might have been added and perfected at any stage of the instrument's construction." Within the highest division of the animal kingdom, the vertebrates, we can start from an eye so simple that it consists, as in the lancelet, of a little sac of transparent skin furnished with a nerve and lined with pigment, but without any other apparatus. In fish and reptiles, as the anatomist Owen has remarked, "the range of gradation of optical structures is very great." It's a significant fact that even in humans, according to the high authority of Virchow, the beautiful crystalline lens is formed in the embryo by an accumulation of skin cells lying in a sac-like fold of skin, and the vitreous body is formed from embryonic tissue beneath the skin. To reach a fair conclusion about the formation of the eye, with all its marvelous yet not absolutely perfect features, it is essential that reason should conquer imagination. But I've felt this difficulty far too keenly to be surprised at others hesitating to extend the principle of natural selection to such a startling length.
It's almost impossible to avoid comparing the eye with a telescope. We know that the telescope has been perfected by the long-continued efforts of the highest human intellects, and we naturally infer that the eye was formed by a somewhat similar process. But may not this inference be presumptuous? Have we any right to assume that the Creator works by intellectual powers like those of humans? If we must compare the eye to an optical instrument, we should in our imagination take a thick layer of transparent tissue, with spaces filled with fluid and with a light-sensitive nerve beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. Further, we must suppose that there is a power -- represented by natural selection or the survival of the fittest -- always intently watching each slight alteration in the transparent layers and carefully preserving each one that, under varied circumstances, in any way or degree tends to produce a sharper image. We must suppose each new state of the instrument to be multiplied by the millions, each to be preserved until a better one is produced, and then the old ones to be all destroyed. In living bodies, variation will cause the slight alterations, reproduction will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions of years, and during each year on millions of individuals of many kinds -- and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man?
If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find no such case. No doubt many organs exist of which we don't know the transitional grades, especially if we look at highly isolated species, around which, according to the theory, there has been much extinction. Or again, if we take an organ common to all the members of a class -- for in that case the organ must have originally formed at a very remote period, since which all the many members of the class have developed -- and in order to discover the early transitional grades through which the organ has passed, we would have to look to very ancient ancestral forms, long since extinct.
We should be extremely cautious about concluding that an organ could not have been formed by transitional gradations of some kind. Numerous cases could be given among the lower animals of the same organ performing wholly distinct functions at the same time. In the larva of the dragonfly and in the fish Cobites, the digestive tract respires, digests, and excretes. In the Hydra, the animal may be turned inside out, and the exterior surface will then digest while the stomach respires. In such cases, natural selection might specialize -- if any advantage were gained -- the whole or part of an organ that had previously performed two functions for one function alone, and thus, by imperceptible steps, greatly change its nature. Many plants are known that regularly produce differently constructed flowers at the same time. If such plants were to produce only one kind, a great change would be brought about fairly suddenly in the character of the species. It's likely, however, that the two sorts of flowers produced by the same plant were originally differentiated by finely graduated steps, which may still be traced in a few cases.
Again, two distinct organs, or the same organ in two very different forms, may simultaneously perform the same function in the same individual, and this is an extremely important means of transition. To give one example: there are fish with gills that breathe the air dissolved in water, at the same time that they breathe free air in their swim bladders, the latter organ being divided by highly vascular partitions and having a duct for the supply of air. To give another example from the plant kingdom: plants climb by three distinct means -- by spirally twining, by clasping a support with their sensitive tendrils, and by sending out aerial rootlets. These three methods are usually found in distinct groups, but a few species use two or even all three combined in the same individual. In all such cases, one of the two organs might readily be modified and perfected to perform all the work, aided during the process of modification by the other organ. Then the other organ might be modified for some entirely different purpose, or be completely eliminated.
The example of the swim bladder in fish is a good one, because it clearly shows us the highly important fact that an organ originally built for one purpose -- namely, buoyancy -- may be converted to serve a widely different purpose -- namely, breathing. The swim bladder has also been co-opted as an accessory to the hearing organs of certain fish. All physiologists agree that the swim bladder is homologous, or structurally equivalent in position and construction, with the lungs of the higher vertebrate animals. So there's no reason to doubt that the swim bladder has actually been converted into lungs, or an organ used exclusively for breathing.
According to this view, we can infer that all vertebrate animals with true lungs are descended by ordinary reproduction from an ancient, unknown ancestor that was equipped with a floating apparatus or swim bladder. We can thus understand -- as I gather from the anatomist Professor Owen's interesting description of these parts -- the strange fact that every particle of food and drink we swallow has to pass over the opening of the windpipe, with some risk of falling into the lungs, despite the elegant mechanism by which the glottis is closed. In the higher vertebrates, the gills have completely disappeared -- but in the embryo, the slits on the sides of the neck and the loop-like course of the arteries still mark their former position. But it's conceivable that the now utterly lost gills might have been gradually co-opted by natural selection for some distinct purpose. For instance, the researcher Landois has shown that the wings of insects develop from the breathing tubes (tracheae). So it's highly probable that in this great class, organs that once served for breathing have actually been converted into organs for flight.
In considering transitions of organs, it's so important to keep in mind the possibility of conversion from one function to another that I'll give yet another example. Stalked barnacles have two tiny folds of skin, which I've called the ovigerous frena, that serve through a sticky secretion to hold the eggs until they hatch within the sac. These barnacles have no gills; the whole surface of the body and of the sac, together with the small frena, serves for breathing. The acorn barnacles (Balanidae), or sessile barnacles, on the other hand, have no ovigerous frena -- the eggs lie loose at the bottom of the sac within the well-enclosed shell. But they have, in the same relative position as the frena, large, much-folded membranes that freely communicate with the circulatory spaces of the sac and body, and which all naturalists consider to function as gills. Now I think no one will dispute that the ovigerous frena in the one family are strictly homologous with the gills of the other family -- indeed, they grade into each other. Therefore, we need not doubt that the two little folds of skin, which originally served as egg-retaining frena but also very slightly aided in breathing, have been gradually converted by natural selection into gills, simply through an increase in their size and the disappearance of their adhesive glands. If all stalked barnacles had gone extinct -- and they have suffered far more extinction than sessile barnacles have -- who would ever have imagined that the gills in the latter family had originally existed as organs for preventing the eggs from being washed out of the sac?
There is another possible mode of transition: through the acceleration or retardation of the timing of reproduction. This has recently been emphasized by Professor Cope and others in the United States. It's now known that some animals are capable of reproducing at a very early age, before they have acquired their adult features. If this capacity became thoroughly well developed in a species, it seems likely that the adult stage of development would sooner or later be lost. In that case, especially if the larva differed considerably from the mature form, the character of the species would be greatly changed and simplified. Again, quite a few animals, after reaching maturity, continue changing in character throughout nearly their whole lives. In mammals, for instance, the shape of the skull often changes considerably with age; Dr. Murie has given some striking examples with seals. Everyone knows how the antlers of stags become more and more branched, and the plumage of some birds becomes more finely developed, as they grow older. Professor Cope reports that the teeth of certain lizards change considerably in shape with advancing years. In crustaceans, not only many minor features but some important parts take on a new character after maturity, as recorded by Fritz Muller. In all such cases -- and many could be given -- if the age for reproduction were delayed, the character of the species, at least in its adult state, would be modified. It's also not unlikely that the earlier stages of development would in some cases be rushed through and eventually lost. Whether species have often or ever been modified through this relatively sudden mode of transition, I can't form an opinion. But if it has occurred, it's probable that the differences between the young and the mature, and between the mature and the old, were originally acquired by graduated steps.
Although we must be extremely cautious about concluding that any organ could not have been produced by successive, small, transitional gradations, undoubtedly serious cases of difficulty do occur.
One of the most serious is that of sterile worker insects, which are often built differently from either the males or the fertile females. But this case will be dealt with in the next chapter. The electric organs of fish present another special difficulty, because it's impossible to imagine by what steps these extraordinary organs were produced. But this isn't surprising, because we don't even know what they're used for. In the electric eel and the torpedo ray, they undoubtedly serve as powerful means of defense and perhaps for capturing prey. Yet in another ray, as the researcher Matteucci observed, an analogous organ in the tail produces so little electricity, even when the animal is highly agitated, that it can hardly serve either of those purposes. Moreover, in this same ray, besides the organ just mentioned, there is, as Dr. R. McDonnell has shown, another organ near the head, not known to be electrical, but which appears to be the true homologue of the electric battery in the torpedo ray. It's generally accepted that there exists between these organs and ordinary muscle a close resemblance in fine structure, in the distribution of nerves, and in how they respond to various chemical agents. It should also be especially noted that muscular contraction is accompanied by an electrical discharge. As Dr. Radcliffe insists, "in the electrical apparatus of the torpedo during rest, there would seem to be a charge in every respect like that which is met with in muscle and nerve during rest, and the discharge of the torpedo, instead of being peculiar, may be only another form of the discharge which attends upon the action of muscle and motor nerve." Beyond this we cannot presently go in the way of explanation. But since we know so little about the uses of these organs, and since we know nothing about the habits and structure of the ancestors of living electric fish, it would be extremely bold to maintain that no useful transitions are possible by which these organs might have been gradually developed.
These organs seem at first to present another and far more serious difficulty, because they occur in about a dozen kinds of fish, several of which are very distantly related. When the same organ is found in several members of the same class -- especially in members with very different ways of life -- we may generally attribute its presence to inheritance from a common ancestor, and its absence in some members to loss through disuse or natural selection. So if the electric organs had been inherited from a single ancient ancestor, we'd expect all electric fish to be closely related to each other. But this is far from the case. Nor does geology at all suggest that most fish formerly possessed electric organs that their modified descendants have now lost. But when we look at the subject more closely, we find that in the several fish equipped with electric organs, these organs are situated in different parts of the body. They differ in construction, as in the arrangement of the plates. According to Pacini, they differ in the process by which the electricity is generated. And finally -- perhaps most importantly -- they are supplied with nerves from different sources. So in the various fish with electric organs, these cannot be considered homologous (sharing a common structural origin), but only analogous (similar in function). Consequently, there's no reason to suppose they were inherited from a common ancestor, since if they had been, they would closely resemble each other in all respects. The difficulty of the same organ apparently arising in several distantly related species thus disappears, leaving only the lesser yet still considerable difficulty: by what graduated steps have these organs been developed in each separate group of fish?
The luminous organs that occur in a few insects belonging to widely different families, and situated in different parts of the body, present a difficulty almost exactly parallel to that of the electric organs, given our present state of ignorance. Other similar cases could be given. For instance, in plants, the very curious mechanism of a mass of pollen grains borne on a stalk with a sticky gland is apparently the same in orchids and milkweed -- genera almost as distantly related as possible among flowering plants. But here again the parts are not homologous. In all cases where organisms far removed from each other in the scale of organization are equipped with similar and peculiar organs, it will be found that although the general appearance and function of the organs may be the same, fundamental differences between them can always be detected. For instance, the eyes of cephalopods (cuttlefish and their relatives) and of vertebrate animals look wonderfully alike. Yet in such widely separated groups, no part of this resemblance can be due to inheritance from a common ancestor. The critic Mr. Mivart has put forward this case as one of special difficulty, but I'm unable to see the force of his argument. An organ for vision must be formed of transparent tissue and must include some sort of lens for throwing an image at the back of a darkened chamber. Beyond this superficial resemblance, there is hardly any real similarity between the eyes of cephalopods and vertebrates, as can be seen by consulting Hensen's excellent study of these organs in the cephalopods. I can't enter into details here, but I may mention a few of the key differences. The crystalline lens in the higher cuttlefish consists of two parts placed one behind the other like two lenses, both with very different structure and arrangement from what occurs in vertebrates. The retina is entirely different, with an actual inversion of the fundamental parts, and with a large nervous ganglion included within the membranes of the eye. The relationships of the muscles are as different as it's possible to imagine, and so on in other respects. It's therefore quite difficult to decide how far even the same terms should be used in describing the eyes of cephalopods and vertebrates. Of course, anyone is free to deny that the eye in either case could have been developed through the natural selection of successive slight variations. But if this is admitted in the one case, it's clearly possible in the other. And fundamental differences in the structure of the visual organs of these two groups might actually have been expected, given the way they were formed. Just as two inventors sometimes independently hit on the same invention, so in the several cases I've described, natural selection -- working for the good of each organism and taking advantage of all favorable variations -- has produced similar organs, as far as function is concerned, in distinct organisms that owe none of their structure in common to inheritance from a common ancestor.
The zoologist Fritz Muller, in order to test the conclusions reached in this book, followed out a very similar line of argument with great care. Several families of crustaceans include a few species that possess an air-breathing apparatus and are adapted to live out of water. In two of these families, which were more closely examined by Muller and which are closely related to each other, the species agree most closely in all important characters: their sense organs, circulatory systems, the position of the tufts of hair within their complex stomachs, and finally the whole structure of the water-breathing gills, right down to the microscopic hooks by which they are cleaned. Given all this, you might have expected that in the few species of both families that live on land, the equally important air-breathing apparatus would have been the same. After all, why should this one apparatus, serving the same purpose, have been made differently, while all the other important organs were closely similar or even identical?
Fritz Muller argues that this close similarity in so many structural features must, in line with my views, be explained by inheritance from a common ancestor. But since the vast majority of species in these two families, as well as most other crustaceans, are aquatic, it's highly improbable that their common ancestor was adapted for breathing air. Muller was thus led to carefully examine the air-breathing apparatus in the land-dwelling species. He found that it differed in each in several important ways: in the position of the openings, in the way they open and close, and in some accessory details. Now, such differences are perfectly intelligible -- and might even have been expected -- on the assumption that species belonging to distinct families had slowly become adapted to live more and more out of water and to breathe air. These species, belonging to distinct families, would already have differed to a certain extent. And in line with the principle that the nature of each variation depends on two factors -- the nature of the organism and the nature of its environment -- their variability would certainly not have been exactly the same. Consequently, natural selection would have had different raw material to work with in order to arrive at the same functional result, and the structures thus acquired would almost inevitably have differed. On the hypothesis of separate acts of creation, the whole case remains completely unintelligible. This line of argument seems to have carried great weight in leading Fritz Muller to accept the views I maintain in this book.
Another distinguished zoologist, the late Professor Claparede, argued along the same lines and reached the same conclusion. He shows that there are parasitic mites (Acaridae) belonging to distinct subfamilies and families that are equipped with hair-clasping organs. These organs must have been independently developed, since they could not have been inherited from a common ancestor. And in the several groups, they are formed by the modification of the forelegs, the hind legs, the mouthparts or lips, and appendages on the underside of the rear part of the body.
In the foregoing cases, we see the same end achieved and the same function performed by organs that appear closely similar, though not in their developmental origin, in organisms that are not at all -- or only distantly -- related. On the other hand, a common rule throughout nature is that the same end is achieved, even sometimes in the case of closely related organisms, by the most varied means. How differently constructed is the feathered wing of a bird and the membrane-covered wing of a bat! And still more so the four wings of a butterfly, the two wings of a fly, and the two wings with the hard wing-covers of a beetle. Bivalve shells are made to open and shut, but on what a number of patterns is the hinge built, from the long row of neatly interlocking teeth in a Nucula to the simple ligament of a mussel! Seeds are spread by their minuteness; by their capsule being converted into a light balloon-like envelope; by being embedded in pulp or flesh formed from the most diverse parts, made nutritious and conspicuously colored so as to attract and be eaten by birds; by having hooks and grapnels of many kinds and serrated barbs so as to cling to the fur of mammals; and by being furnished with wings and plumes, as different in shape as they are elegant in structure, so as to be carried by every breeze. I'll give one more example, for this subject of the same end being achieved by the most varied means well deserves attention. Some authors maintain that organisms have been formed in many ways for the sake of mere variety, almost like toys in a shop. But such a view of nature is incredible. In plants with separate sexes, and in those that, though hermaphrodite, don't spontaneously drop pollen on the stigma, some aid is necessary for their fertilization. In several kinds, this is accomplished simply by the pollen grains -- which are light and loose -- being blown by the wind by mere chance onto the stigma. This is the simplest method imaginable. An almost equally simple, though very different, plan occurs in many plants: a symmetrical flower secretes a few drops of nectar, and is consequently visited by insects, which carry the pollen from the anthers to the stigma.
From this simple stage, we may pass through a seemingly inexhaustible number of mechanisms, all for the same purpose and working in essentially the same manner, but involving changes in every part of the flower. The nectar may be stored in variously shaped containers, with the stamens and pistils modified in many ways -- sometimes forming trap-like devices, and sometimes capable of precisely adapted movements through irritability or elasticity. From such structures, we may advance until we come to such a case of extraordinary adaptation as that described by Dr. Cruger in the orchid Coryanthes. This orchid has part of its lower lip hollowed out into a great bucket, into which drops of almost pure water continually fall from two secreting horns that stand above it. When the bucket is half full, the water overflows through a spout on one side. The base of the lip stands over the bucket and is itself hollowed out into a sort of chamber with two side entrances. Inside this chamber are curious fleshy ridges. The most ingenious person, if they hadn't witnessed what happens, could never have imagined what purpose all these parts serve. But Dr. Cruger saw crowds of large bumblebees visiting the gigantic flowers of this orchid, not to suck nectar, but to gnaw off the ridges within the chamber above the bucket. In doing this, they frequently pushed each other into the bucket, and with their wings thus soaked, they couldn't fly away. They were forced to crawl out through the passage formed by the spout or overflow. Dr. Cruger saw a "continual procession" of bees crawling out of their involuntary bath. The passage is narrow and roofed over by the column, so that a bee, forcing its way out, first rubs its back against the sticky stigma and then against the sticky glands of the pollen masses. The pollen masses are thus glued to the back of the bee that first happens to crawl out through the passage of a newly opened flower, and are carried away. Dr. Cruger sent me a flower preserved in spirits, with a bee he had killed before it had quite crawled out, with a pollen mass still fastened to its back. When the bee, thus loaded, flies to another flower -- or to the same flower a second time -- and is pushed by its companions into the bucket and then crawls out through the passage, the pollen mass necessarily comes first into contact with the sticky stigma and adheres to it, and the flower is fertilized. Now at last we see the full purpose of every part of the flower: the water-secreting horns, the bucket half full of water that prevents the bees from flying away and forces them to crawl out through the spout, and rub against the properly placed sticky pollen masses and the sticky stigma.
The structure of the flower in another closely related orchid, the Catasetum, is very different, though it serves the same end and is equally remarkable. Bees visit these flowers, like those of the Coryanthes, to gnaw the lower lip. In doing so, they inevitably touch a long, tapering, sensitive projection -- or, as I've called it, the antenna. This antenna, when touched, transmits a sensation or vibration to a certain membrane, which instantly ruptures. This releases a spring by which the pollen mass is shot forth like an arrow, in the right direction, and sticks by its adhesive tip to the back of the bee. The pollen mass of the male plant (for the sexes are separate in this orchid) is thus carried to the flower of the female plant, where it comes into contact with the stigma, which is sticky enough to break certain elastic threads and retain the pollen, thus accomplishing fertilization.
How, it may be asked, in the foregoing and in countless other cases, can we understand the graduated scale of complexity and the many different means for achieving the same end? The answer, no doubt, is -- as I've already noted -- that when two forms vary that already differ from each other to some slight degree, the variability will not be of the exact same nature, and consequently the results obtained through natural selection for the same general purpose will not be the same. We should also keep in mind that every highly developed organism has passed through many changes, and that each modified structure tends to be inherited, so that each modification will not readily be entirely lost but may be altered again and again. The structure of each part of each species, for whatever purpose it may serve, is therefore the sum of many inherited changes through which the species has passed during its successive adaptations to changing habits and conditions of life.
Finally, then, although in many cases it's extremely difficult even to guess by what transitions organs could have arrived at their present state, considering how small the proportion of living and known forms is to the extinct and unknown, I've been astonished how rarely an organ can be named toward which no transitional grade is known to lead. It's certainly true that new organs appearing as if created for some special purpose rarely or never appear in any organism -- as indeed is shown by that old, somewhat exaggerated saying in natural history, "Nature does not make leaps." We find this acknowledged in the writings of nearly every experienced naturalist. Or as the zoologist Milne Edwards has well expressed it, "Nature is lavish in variety, but stingy with innovation." Why, on the theory of creation, should there be so much variety and so little real novelty? Why should all the parts and organs of many independent organisms, each supposedly created separately for its own proper place in nature, be so commonly linked together by graduated steps? Why should nature not take a sudden leap from structure to structure? On the theory of natural selection, we can clearly understand why she should not: natural selection acts only by taking advantage of slight successive variations. She can never take a great and sudden leap, but must advance by short and sure, though slow, steps.
Since natural selection acts by life and death -- by the survival of the fittest and the destruction of the less well-fitted individuals -- I have sometimes felt great difficulty in understanding the origin of parts of little importance. Almost as great a difficulty, though of a very different kind, as in the case of the most perfect and complex organs.
First, we are far too ignorant about the whole workings of any organism to say what slight modifications would be important or not. In an earlier chapter, I gave examples of very minor characters -- such as the fuzz on fruit and the color of its flesh, the color of skin and hair in mammals -- which, from being linked to constitutional differences or from determining the attacks of insects, might certainly be acted on by natural selection. The tail of the giraffe looks like an artificially constructed fly swatter, and it seems at first incredible that this could have been adapted for its present purpose by successive slight modifications, each better and better fitted, for so trivial a purpose as driving away flies. Yet we should pause before being too hasty even in this case, because we know that the distribution and survival of cattle and other animals in South America absolutely depend on their ability to resist the attacks of insects. Individuals that could by any means defend themselves from these small enemies would be able to range into new pastures and thus gain a great advantage. It's not that the larger mammals are actually killed by flies (except in some rare cases), but they are incessantly harassed and their strength is reduced, making them more susceptible to disease, or less able during times of scarcity to search for food, or to escape from predators.
Organs that are now of trivial importance have probably in some cases been of great importance to an early ancestor. After having been slowly perfected in the past, they have been passed down to existing species in nearly the same state, even though they now serve little purpose. But any actually harmful changes in their structure would of course have been checked by natural selection. Seeing how important the tail is as a means of locomotion in most aquatic animals, its general presence and use for many purposes in so many land animals -- which in their lungs or modified swim bladders reveal their aquatic origin -- may perhaps be explained this way. A well-developed tail, having been formed in an aquatic animal, might later come to be used for all sorts of purposes: as a fly swatter, an organ for grasping, or as an aid in turning -- as in the case of the dog, though the benefit in this last respect must be slight, since the hare, with hardly any tail at all, can double back even more quickly.
Second, we may easily err in attributing importance to characters and in believing that they have been developed through natural selection. We must by no means overlook the effects of the direct action of changed environmental conditions, of so-called spontaneous variations (which seem to depend only in a minor way on the nature of the conditions), of the tendency to revert to long-lost characters, of the complex laws of growth -- such as correlated variation, the compression of one part by another, and so on -- and finally of sexual selection, by which characters useful to one sex are often acquired and then transmitted more or less completely to the other sex, even though of no use to that sex. But structures gained indirectly in these ways, though at first of no advantage to a species, may later have been taken advantage of by its modified descendants under new conditions of life and newly acquired habits.
If green woodpeckers alone had existed and we didn't know that there were many black and pied kinds, I suspect we would have thought the green color was a beautiful adaptation to conceal this tree-dwelling bird from its enemies -- and consequently that it was an important character acquired through natural selection. As it is, the color is probably mainly due to sexual selection. A trailing palm in the Malay Archipelago climbs the tallest trees with the aid of exquisitely constructed hooks clustered around the ends of the branches, and this device is no doubt of the highest usefulness to the plant. But since we see nearly similar hooks on many trees that are not climbers -- and which, as there is reason to believe from the distribution of thorn-bearing species in Africa and South America, serve as a defense against browsing mammals -- the hooks on the palm may at first have developed for this purpose and only later been improved and put to use by the plant as it underwent further modification and became a climber. The naked skin on the head of a vulture is generally considered a direct adaptation for wallowing in rotting flesh, and so it may be. Or it may possibly be due to the direct action of decaying matter. But we should be very cautious in drawing any such conclusion, because the skin on the head of the clean-feeding male turkey is likewise naked. The sutures in the skulls of young mammals have been praised as a beautiful adaptation for aiding childbirth, and no doubt they do facilitate -- or may be essential for -- this process. But since sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we can infer that this structure arose from the laws of growth and was simply taken advantage of in the births of higher animals.
We are profoundly ignorant of the cause of each slight variation or individual difference. We are immediately made aware of this by reflecting on the differences between breeds of our domesticated animals in different countries, especially in less developed countries where there has been little systematic selective breeding. Animals kept by people in different countries often have to fend for themselves to some extent and are exposed to a degree of natural selection, with individuals of slightly different constitutions succeeding best under different climates. With cattle, susceptibility to fly attacks is linked to color, as is the tendency to be poisoned by certain plants -- so even color would be subject to natural selection. Some observers are convinced that a damp climate affects hair growth, and that hair is correlated with horns. Mountain breeds always differ from lowland breeds, and a mountainous country would probably affect the hind limbs from exercising them more, and possibly even the shape of the pelvis. Then by the law of homologous variation, the front limbs and the head would probably be affected too. The shape of the pelvis might also affect, by pressure, the shape of certain parts of the young in the womb. The laborious breathing necessary at high altitudes tends, as we have good reason to believe, to increase the size of the chest -- and again, correlated variation would come into play. The effects of reduced exercise combined with abundant food on the whole body are probably still more important, and this, as H. von Nathusius has recently shown in his excellent treatise, is apparently one of the chief causes of the great modification that pig breeds have undergone. But we are far too ignorant to speculate on the relative importance of the several known and unknown causes of variation. I have made these remarks only to show that if we can't account for the characteristic differences of our several domestic breeds -- which are generally admitted to have arisen through ordinary reproduction from one or a few parent stocks -- we ought not to place too much stress on our ignorance of the precise cause of the slight analogous differences between true species.
The foregoing remarks lead me to say a few words about the protest recently made by some naturalists against the utilitarian doctrine -- the idea that every detail of structure has been produced for the good of its possessor. They believe that many structures have been created for the sake of beauty, to delight humans or the Creator (but this latter point is beyond the scope of scientific discussion), or for the sake of mere variety, a view already discussed. Such doctrines, if true, would be absolutely fatal to my theory. I fully admit that many structures are now of no direct use to their possessors, and may never have been of any use to their ancestors. But this doesn't prove that they were formed solely for beauty or variety. The direct action of changed conditions, and the various causes of modification I've recently listed, have undoubtedly all produced an effect -- probably a great effect -- independently of any advantage gained. But a still more important consideration is that the chief part of the structure of every living creature is due to inheritance. Consequently, though each organism is certainly well fitted for its place in nature, many structures now have no very close and direct relation to present habits of life. Thus, we can hardly believe that the webbed feet of the upland goose or of the frigate bird are of special use to these birds. We can't believe that the similar bones in the arm of the monkey, in the foreleg of the horse, in the wing of the bat, and in the flipper of the seal are of special use to these animals. We may safely attribute these structures to inheritance. But webbed feet were no doubt as useful to the ancestor of the upland goose and of the frigate bird as they now are to the most aquatic of living birds. So we may believe that the ancestor of the seal did not possess a flipper but a foot with five toes fitted for walking or grasping. And we may further venture to believe that the several bones in the limbs of the monkey, horse, and bat were originally developed on the principle of utility, probably through the reduction of more numerous bones in the fin of some ancient fish-like ancestor of the whole class. It is scarcely possible to decide how much allowance should be made for such causes of change as the direct action of external conditions, so-called spontaneous variations, and the complex laws of growth. But with these important exceptions, we may conclude that the structure of every living creature either now is, or was formerly, of some direct or indirect use to its possessor.
As for the belief that organisms have been created beautiful for the delight of humans -- a belief that has been declared subversive of my whole theory -- I may first remark that the sense of beauty obviously depends on the nature of the mind, regardless of any real quality in the admired object, and that the idea of what is beautiful is neither innate nor fixed. We see this, for instance, in people of different backgrounds admiring entirely different standards of beauty. If beautiful objects had been created solely for human gratification, it should be shown that before humans appeared there was less beauty on the face of the earth than afterward. Were the beautiful volute and cone shells of the Eocene epoch, and the gracefully sculpted ammonites of the Secondary period, created so that humans might ages afterward admire them in a display case? Few objects are more beautiful than the minute silica cases of the diatoms: were these created so they might be examined and admired under the high-powered microscope? The beauty in this last case, and in many others, is apparently due entirely to the symmetry of growth. Flowers rank among the most beautiful productions of nature, but they have been made conspicuous against the green leaves -- and at the same time beautiful -- so that they may be easily noticed by insects. I've come to this conclusion from finding it an invariable rule that when a flower is pollinated by the wind, it never has a brightly colored corolla. Several plants habitually produce two kinds of flowers: one kind open and colored so as to attract insects, the other closed, uncolored, without nectar, and never visited by insects. We may therefore conclude that if insects had not evolved, our plants would not have been decked with beautiful flowers but would have produced only such drab flowers as we see on our fir, oak, nut, and ash trees, on grasses, spinach, docks, and nettles -- all of which are pollinated by wind. A similar argument holds for fruits. That a ripe strawberry or cherry is as pleasing to the eye as to the palate, that the brightly colored fruit of the spindle tree and the scarlet berries of the holly are beautiful objects -- everyone will agree. But this beauty serves merely as a signal to birds and mammals so that the fruit may be eaten and the mature seeds dispersed. I infer this from having found no exception yet to the rule that seeds are always dispersed this way when embedded within any kind of fruit (that is, within a fleshy or pulpy covering), if it is brightly colored or rendered conspicuous by being white or black.
On the other hand, I willingly admit that a great number of male animals -- all our most gorgeous birds, some fish, reptiles, and mammals, and a host of magnificently colored butterflies -- have been made beautiful for beauty's sake. But this has been achieved through sexual selection -- that is, by the more beautiful males being continually preferred by the females -- and not for the delight of humans. The same is true of the music of birds. We may infer from all this that a nearly similar taste for beautiful colors and musical sounds runs through a large part of the animal kingdom. When the female is as beautifully colored as the male -- which is not rarely the case with birds and butterflies -- the cause apparently lies in the colors acquired through sexual selection having been transmitted to both sexes rather than to the males alone. How the sense of beauty in its simplest form -- that is, the reception of a particular kind of pleasure from certain colors, forms, and sounds -- was first developed in the minds of humans and lower animals is a very obscure subject. The same sort of difficulty arises if we ask how it is that certain flavors and odors give pleasure and others displeasure. Habit seems to have played some role in all these cases, but there must be some fundamental cause in the constitution of the nervous system in each species.
Natural selection cannot possibly produce any modification in a species exclusively for the good of another species, though throughout nature one species constantly takes advantage of and profits by the structures of others. But natural selection can and does often produce structures for the direct harm of other animals, as we see in the fangs of the adder and in the egg-laying tube of the ichneumon wasp, by which its eggs are deposited in the living bodies of other insects. If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would destroy my theory, because such a thing could not have been produced through natural selection. Although many claims to this effect can be found in works on natural history, I can't find even one that seems to me to carry any weight. It's admitted that the rattlesnake has a poison fang for its own defense and for killing its prey. But some authors suppose that it is at the same time equipped with a rattle for its own harm -- namely, to warn its prey. I would almost as soon believe that the cat curls the end of its tail when preparing to spring in order to warn the doomed mouse. It's a much more likely view that the rattlesnake uses its rattle, the cobra expands its hood, and the puff adder swells while hissing so loudly and harshly in order to alarm the many birds and mammals that are known to attack even the most venomous species. Snakes act on the same principle that makes a hen ruffle her feathers and spread her wings when a dog approaches her chicks. But I don't have space here to expand on the many ways animals try to frighten away their enemies.
Natural selection will never produce in any organism a structure more harmful than beneficial to that organism, because natural selection acts solely by and for the good of each. No organ will be formed, as the theologian Paley remarked, for the purpose of causing pain or doing harm to its possessor. If a fair balance is struck between the good and harm caused by each part, each will be found on the whole to be advantageous. Over the course of time, under changing conditions of life, if any part comes to be harmful, it will be modified -- or if it is not, the organism will go extinct, as countless have.
Natural selection tends only to make each organism as perfect as, or slightly more perfect than, the other inhabitants of the same country with which it competes. And we see that this is the standard of perfection attained in nature. The native species of New Zealand, for instance, are perfect when compared with one another. But they are now rapidly giving way before the advancing legions of plants and animals introduced from Europe. Natural selection will not produce absolute perfection, nor do we always find, as far as we can judge, this high standard in nature. The correction for the aberration of light is said by the physicist Muller not to be perfect even in that most perfect organ, the human eye. The great physicist Helmholtz, whose judgment no one will dispute, after describing in the strongest terms the wonderful powers of the human eye, adds these remarkable words: "That which we have discovered in the way of inexactness and imperfection in the optical machine and in the image on the retina is as nothing in comparison with the incongruities which we have just come across in the domain of the sensations. One might say that nature has taken delight in accumulating contradictions in order to remove all foundation from the theory of a preexisting harmony between the external and internal worlds." If our reason leads us to admire with enthusiasm a multitude of inimitable contrivances in nature, this same reason tells us -- though we may easily err on both sides -- that some other contrivances are less perfect. Can we consider the sting of the bee as perfect, which, when used against many kinds of enemies, cannot be withdrawn because of its backward-pointing barbs, and thus inevitably causes the death of the insect by tearing out its internal organs?
If we look at the bee's sting as having existed in a remote ancestor as a boring and serrated instrument -- like that in so many members of the same great order -- and as having since been modified but not perfected for its present purpose, with the venom originally adapted for some other function, such as producing galls, and since intensified, we can perhaps understand how it is that using the sting so often causes the insect's own death. If on the whole the power of stinging is useful to the social community, it will satisfy all the requirements of natural selection, even though it causes the death of some individual members. If we admire the truly wonderful power of scent by which the males of many insects find their females, can we admire the production for this single purpose of thousands of drones, which are utterly useless to the community for any other purpose, and which are ultimately slaughtered by their industrious and sterile sisters? It may be difficult, but we ought to admire the savage instinctive hatred of the queen bee, which drives her to destroy the young queens, her daughters, as soon as they are born -- or to die herself in the fight. For undoubtedly this is for the good of the community, and maternal love or maternal hatred, though the latter is fortunately very rare, is all the same to the inexorable principles of natural selection. If we admire the many ingenious devices by which orchids and other plants are pollinated through insect agency, can we consider as equally perfect the production of dense clouds of pollen by our fir trees, so that a few grains may be carried by chance to the ovules?
Summary: The Law of Unity of Type and of the Conditions of Existence Embraced by the Theory of Natural Selection
We have in this chapter discussed some of the difficulties and objections that may be raised against the theory. Many of them are serious. But I think that in the discussion, light has been shed on several facts that, on the belief in independent acts of creation, are utterly obscure. We have seen that species at any one period are not indefinitely variable and are not linked together by a multitude of intermediate gradations. This is partly because the process of natural selection is always very slow and at any one time acts only on a few forms, and partly because the very process of natural selection implies the continual replacement and extinction of preceding and intermediate gradations. Closely related species now living in a continuous area must often have been formed when the area was not continuous and when the conditions of life did not grade imperceptibly from one part to another. When two varieties are formed in two districts of a continuous area, an intermediate variety will often be formed, adapted for an intermediate zone. But for reasons given, the intermediate variety will usually exist in smaller numbers than the two forms it connects. Consequently, the two latter forms -- existing in greater numbers during the course of further modification -- will have a great advantage over the less numerous intermediate variety, and will thus generally succeed in replacing and wiping it out.
We have seen in this chapter how cautious we should be in concluding that the most different habits of life could not grade into each other -- that a bat, for instance, could not have been formed by natural selection from an animal that at first only glided through the air.
We have seen that a species under new conditions of life may change its habits, or it may have diversified habits, some very unlike those of its closest relatives. So we can understand, keeping in mind that each organism is trying to live wherever it can, how there have come to be upland geese with webbed feet, ground woodpeckers, diving thrushes, and petrels with the habits of auks.
Although the belief that an organ so perfect as the eye could have been formed by natural selection is enough to stagger anyone, yet in the case of any organ, if we know of a long series of gradations in complexity -- each good for its possessor -- then under changing conditions of life, there is no logical impossibility in the acquisition of any conceivable degree of perfection through natural selection. In cases where we know of no intermediate or transitional states, we should be extremely cautious about concluding that none could have existed, because the transformations of many organs show what wonderful changes in function are at least possible. For instance, a swim bladder has apparently been converted into an air-breathing lung. The same organ having performed simultaneously very different functions, and then having been partly or wholly specialized for one function; and two distinct organs having performed the same function at the same time, one having been perfected while aided by the other -- these processes must often have greatly facilitated transitions.
We have seen that in two organisms widely remote from each other in the natural scale, organs serving the same purpose and closely similar in outward appearance may have been separately and independently formed. But when such organs are closely examined, essential differences in their structure can almost always be detected -- and this naturally follows from the principle of natural selection. On the other hand, the common rule throughout nature is infinite diversity of structure for achieving the same end -- and this again naturally follows from the same great principle.
In many cases, we are far too ignorant to be able to assert that a part or organ is so unimportant for the welfare of a species that modifications in its structure could not have been slowly accumulated by natural selection. In many other cases, modifications are probably the direct result of the laws of variation or of growth, independently of any benefit gained. But even such structures have often, as we may feel assured, been subsequently taken advantage of and further modified for the good of the species under new conditions of life. We may also believe that a part formerly of great importance has frequently been retained -- as the tail of an aquatic animal by its land-dwelling descendants -- even though it has become of such small importance that it could not, in its present state, have been acquired by natural selection.
Natural selection can produce nothing in one species for the exclusive good or injury of another, though it may well produce parts, organs, and secretions that are highly useful or even indispensable, or highly injurious, to another species -- but in all cases at the same time useful to the possessor. In each well-stocked country, natural selection acts through the competition of the inhabitants and consequently leads to success in the battle for life only in accordance with the standard of that particular country. Hence the inhabitants of one country -- generally the smaller one -- often yield to the inhabitants of another, generally larger country. For in the larger country, more individuals will have existed, and more diversified forms, and the competition will have been fiercer, and thus the standard of perfection will have been raised higher. Natural selection will not necessarily lead to absolute perfection; nor, as far as we can judge by our limited faculties, can absolute perfection be everywhere expected.
On the theory of natural selection, we can clearly understand the full meaning of that old saying in natural history, "Nature does not make leaps." This saying, if we look only at the present inhabitants of the world, is not strictly correct. But if we include all those of past times, whether known or unknown, it must on this theory be strictly true.
It is generally acknowledged that all organisms have been formed on two great laws: Unity of Type and the Conditions of Existence. By unity of type is meant the fundamental agreement in structure that we see in organisms of the same class, which is quite independent of their habits of life. On my theory, unity of type is explained by unity of descent. The expression "conditions of existence," so often insisted on by the great naturalist Cuvier, is fully embraced by the principle of natural selection. For natural selection works by either adapting the varying parts of each organism to its organic and inorganic conditions of life right now, or by having adapted them during past periods of time. These adaptations are aided in many cases by the increased use or disuse of parts, affected by the direct action of external conditions of life, and subject in all cases to the various laws of growth and variation. In fact, then, the law of the Conditions of Existence is the higher law, because it includes, through the inheritance of former variations and adaptations, that of Unity of Type.
I'll devote this chapter to addressing various objections that have been raised against my views, since doing so may help clarify some of the earlier discussions. It would be pointless to address all of them, though, because many have been made by writers who haven't bothered to understand the subject. For example, a distinguished German naturalist has claimed that the weakest part of my theory is that I consider all organisms as imperfect. What I actually said is that none are as perfectly adapted as they could be to their conditions of life -- and this is demonstrated by the fact that native species in many parts of the world have been displaced by foreign invaders. Nor could organisms, even if perfectly adapted at one point in time, remain so when their conditions changed -- unless they changed too. And no one would argue that the physical conditions of every country, along with the number and kinds of its inhabitants, haven't undergone many changes.
A critic has recently insisted, with a show of mathematical precision, that longevity is a great advantage to all species, so anyone who believes in natural selection "must arrange his genealogical tree" so that all descendants live longer than their ancestors! Can't our critics imagine that a biennial plant or a small animal might extend into a cold climate and die there every winter, yet still survive from year to year through its seeds or eggs -- thanks to advantages gained through natural selection? Mr. E. Ray Lankester has recently discussed this subject and concludes, as far as this extremely complex question allows him to judge, that longevity is generally related to the species' level of organization, as well as to how much energy it spends on reproduction and general activity. And these conditions have probably been largely determined through natural selection.
It has been argued that since none of the animals and plants of Egypt that we know about have changed in the last three or four thousand years, probably none in any part of the world have either. But as Mr. G. H. Lewes has pointed out, this line of argument proves too much: the ancient domestic breeds depicted on Egyptian monuments, or found embalmed, are closely similar or even identical to those alive today -- yet all naturalists accept that these breeds were produced through modification of their original types. The many animals that have remained unchanged since the beginning of the glacial period would actually have been an incomparably stronger case, since these have been exposed to enormous changes in climate and have migrated over vast distances. In Egypt, by contrast, the conditions of life have remained essentially uniform for several thousand years, as far as we know. The fact that little or no change has occurred since the glacial period might count against those who believe in an innate and necessary law of development, but it's powerless against the theory of natural selection -- or survival of the fittest -- which holds that when beneficial variations or individual differences happen to arise, they will be preserved. But this will only happen under certain favorable conditions.
The celebrated paleontologist Bronn, at the close of his German translation of this work, asks how, on the principle of natural selection, a variety can live side by side with the parent species. If both have become adapted to slightly different habits or conditions, they could live together. And if we set aside polymorphic species (in which variability seems to be of a peculiar nature), along with all mere temporary variations such as size, albinism, and so on, the more permanent varieties are generally found -- as far as I can discover -- inhabiting distinct areas, such as highlands versus lowlands, or dry versus wet districts. Moreover, in animals that wander a lot and crossbreed freely, their varieties seem to be generally confined to distinct regions.
Bronn also insists that distinct species never differ from each other in just a single feature, but always in many parts. He asks: how is it that many parts of an organism should have been modified at the same time through variation and natural selection? But there's no need to suppose that all parts of an organism were modified simultaneously. The most striking changes, beautifully adapted for some purpose, might -- as I've previously noted -- be acquired by successive small variations, first in one part and then in another. Since they would all be inherited together, they would appear to us as if they had developed simultaneously. The best answer, however, to this objection comes from domestic breeds that have been modified, mainly through human selection, for some special purpose. Look at the racehorse and the draft horse, or at the greyhound and the mastiff. Their entire frames, and even their temperaments, have been modified. But if we could trace each step in the history of their transformation -- and the later steps can be traced -- we wouldn't see great simultaneous changes, but first one part and then another being slightly modified and improved. Even when selection has been applied by humans to just one feature alone -- and our cultivated plants offer the best examples -- it will always be found that although this one part, whether flower, fruit, or leaves, has been greatly changed, almost all the other parts have been slightly modified too. This can be attributed partly to the principle of correlated development and partly to what's called spontaneous variation.
A much more serious objection has been raised by Bronn, and recently by the anthropologist Broca: that many traits appear to be of no use whatever to their possessors, and therefore couldn't have been influenced by natural selection. Bronn points to the length of the ears and tails in different species of hares and mice, the complex folds of enamel in the teeth of many animals, and a multitude of similar cases. With respect to plants, this subject has been discussed by the botanist Nageli in an excellent essay. He admits that natural selection has accomplished much, but he insists that plant families differ from each other mainly in structural features that appear to be quite unimportant for the welfare of the species. He therefore believes in an innate tendency toward progressive and more perfect development. He points to the arrangement of cells in tissues and of leaves on the stem as cases where natural selection couldn't have played a role. To these we could add the numerical divisions of the flower parts, the position of the ovules, the shape of the seed when it's not useful for dispersal, and so on.
There is real force in this objection. Nevertheless, we should first be extremely cautious about claiming to decide which structures are, or have formerly been, useful to each species. Second, we should always keep in mind that when one part is modified, other parts will be too, through various poorly understood causes -- such as increased or decreased nutrient flow to a part, mutual pressure, an early-developing part affecting one that develops later, and so forth -- as well as through other causes that lead to the many mysterious cases of correlated variation, which we don't understand at all. These effects can all be grouped together, for brevity, under the expression "the laws of growth." Third, we have to allow for the direct and definite effects of changed living conditions, and for so-called spontaneous variations, in which the nature of the conditions apparently plays only a minor role. Bud-variations, such as a moss-rose appearing on a common rose bush, or a nectarine on a peach tree, are good examples of spontaneous variations. But even in these cases, if we remember the power of a tiny drop of poison to produce complex galls, we shouldn't be too sure that these variations aren't the result of some local change in the sap, caused by some change in conditions. There must be some effective cause for each slight individual difference, as well as for the more strongly marked variations that occasionally arise. And if the unknown cause were to act persistently, it's almost certain that all the individuals of the species would be similarly modified.
In the earlier editions of this work, I probably underestimated the frequency and importance of modifications due to spontaneous variability. But it's impossible to attribute to this cause the countless structures so well adapted to the habits of life of each species. I can no more believe in this than in the idea that the well-adapted form of a racehorse or greyhound -- which so amazed the older naturalists before the principle of selection was well understood -- can be explained this way.
It may be worth illustrating some of these points. Regarding the supposed uselessness of various parts and organs, it hardly needs saying that even in the most familiar animals, many structures exist that are so highly developed that no one doubts their importance, yet their function has only recently been discovered -- or hasn't been discovered at all. Since Bronn offers the length of the ears and tail in different species of mice as examples (though trivial ones) of structural differences that can serve no special purpose, I should mention that, according to Dr. Schobl, the external ears of the common mouse are supplied in an extraordinary way with nerves, so that they clearly serve as touch-sensitive organs. The length of the ears, then, can hardly be unimportant. We'll also see shortly that the tail is a highly useful gripping organ in some species, and its usefulness would be very much affected by its length.
With respect to plants, to which I'll confine myself in the following remarks because of Nageli's essay, it will be admitted that the flowers of orchids present a multitude of curious structures that just a few years ago would have been considered mere structural differences without any special function. They are now known to be of the greatest importance for the fertilization of the species through the help of insects, and they were probably developed through natural selection. No one until recently would have imagined that in dimorphic and trimorphic plants, the different lengths and arrangements of the stamens and pistils could serve any purpose. But now we know they do.
In certain whole groups of plants, the ovules stand upright, and in others they are suspended. Within the same ovary of some plants, one ovule takes the former position and another the latter. These positions seem at first to be purely structural, with no physiological significance. But Dr. Hooker tells me that within the same ovary, the upper ovules alone are fertilized in some cases, and the lower ones alone in others. He suggests that this probably depends on the direction in which the pollen-tubes enter the ovary. If so, the position of the ovules -- even when one is upright and the other is suspended within the same ovary -- would result from the selection of any slight deviations in position that favored fertilization and seed production.
Several plants belonging to distinct orders regularly produce two kinds of flowers -- one open, with the ordinary structure, the other closed and imperfect. These two kinds sometimes differ remarkably in structure, yet on the same plant you can see them grading into each other. The ordinary open flowers can be crossbred, and the benefits that certainly come from this process are thus secured. The closed, imperfect flowers, however, are clearly of great importance because they produce a large stock of seed with absolute reliability, using remarkably little pollen. The two kinds of flowers often differ greatly in structure, as I just mentioned. The petals in the imperfect flowers almost always consist of mere remnants, and the pollen grains are reduced in diameter. In *Ononis columnae*, five of the alternate stamens are rudimentary, and in some species of *Viola*, three stamens are in this state, with two retaining their function but being very small. In six out of thirty closed flowers of an Indian violet (species unknown, since the plants never produced perfect flowers for me), the sepals are reduced from the normal five to three. In one section of the Malpighiaceae, the closed flowers are still further modified, according to A. de Jussieu: the five stamens opposite the sepals are all aborted, while a sixth stamen opposite a petal is alone developed -- and this stamen isn't present in the ordinary flowers of this species. The style is aborted, and the ovaries are reduced from three to two. Now, although natural selection may well have had the power to prevent some flowers from opening and to reduce the amount of pollen (which is unnecessary once the flowers are closed), hardly any of the specific modifications above can have been directly determined by natural selection. They must instead have followed from the laws of growth, including the inactivity of parts, during the progressive reduction of pollen and closure of the flowers.
It's so important to understand the effects of the laws of growth that I'll give some additional examples of another kind -- namely, differences in the same part or organ due to differences in position on the same plant. In the Spanish chestnut and certain fir trees, the angles at which the leaves diverge differ between the nearly horizontal and the upright branches, according to Schacht. In the common rue and some other plants, one flower -- usually the central or terminal one -- opens first and has five sepals, five petals, and five divisions in the ovary, while all the other flowers on the plant have four of each. In the British *Adoxa*, the uppermost flower generally has two calyx-lobes with the other organs in fours, while the surrounding flowers generally have three calyx-lobes with the other organs in fives. In many Compositae and Umbelliferae (and some other plants), the outer flowers have much more developed corollas than the central ones, and this often seems connected with the abortion of the reproductive organs. It's a more curious fact, which I've mentioned before, that the seeds of the outer and central flowers sometimes differ greatly in form, color, and other features. In *Carthamus* and some other Compositae, only the central seeds are furnished with a pappus. In *Hyoseris*, the same flower head yields seeds of three different forms. In certain Umbelliferae, the outer seeds are orthospermous and the central one is coelospermous, according to Tausch -- a distinction that De Candolle considered, in other species, to be of the highest systematic importance. Professor Braun mentions a genus of the Fumariaceae in which the flowers in the lower part of the spike bear oval, ribbed, one-seeded nutlets, while those in the upper part bear lance-shaped, two-valved, two-seeded pods. In all these cases, with the exception of the well-developed ray-florets (which serve to make the flowers conspicuous to insects), natural selection cannot, as far as we can judge, have played a role -- or only a very minor one. All these modifications follow from the relative position and interaction of the parts. And it can hardly be doubted that if all the flowers and leaves on the same plant had been subjected to the same external and internal conditions as those in certain positions, all would have been modified in the same way.
In many other cases, we find structural modifications that botanists generally consider to be of great importance, yet which affect only some flowers on the same plant, or which occur on distinct plants growing close together under the same conditions. Since these variations seem to be of no special use to the plants, they cannot have been influenced by natural selection. We're completely ignorant of their cause; we can't even attribute them, as in the last set of cases, to any direct agency like relative position. I'll give just a few examples. It's so common to see flowers on the same plant that are indifferently tetramerous (four-parted), pentamerous (five-parted), and so on, that examples are unnecessary. But since numerical variations are comparatively rare when the parts are few, I should mention that according to De Candolle, the flowers of *Papaver bracteatum* have either two sepals with four petals (which is the common type for poppies) or three sepals with six petals. The way petals are folded in the bud is usually a very constant structural feature in most groups. But Professor Asa Gray states that in some species of *Mimulus*, the bud-folding pattern is almost as frequently like that of the Rhinanthideae as that of the Antirrhinideae, to which latter group the genus actually belongs. Auguste St. Hilaire gives the following cases: the genus *Zanthoxylon* belongs to a division of the Rutaceae with a single ovary, but in some species, flowers with either one or two ovaries may be found on the same plant, even in the same flower cluster. In *Helianthemum*, the capsule has been described as having either one or three chambers. And in *H. mutabile*, "a membrane of varying width extends between the pericarp and the placenta" (originally in French). In the flowers of *Saponaria officinalis*, Dr. Masters has observed instances of both marginal and free central placentation. Finally, St. Hilaire found, toward the southern edge of the range of *Gomphia oleaeformis*, two forms that he initially had no doubt were distinct species. But he later saw them growing on the same bush, and then added: "So here in a single individual we find chambers and a style that attach sometimes to a vertical axis and sometimes to a gynobase" (originally in French).
We thus see that in plants, many structural changes can be attributed to the laws of growth and the interaction of parts, independently of natural selection. But what about Nageli's doctrine of an innate tendency toward perfection or progressive development? Can we say, in these cases of strongly pronounced variation, that the plants have been caught in the act of progressing toward a higher state? On the contrary, I would infer from the mere fact that the parts in question differ so much on the same plant that such modifications are of extremely little importance to the plants themselves -- whatever importance they may have for our classification systems. Acquiring a useless part can hardly be said to raise an organism in the natural scale. And in the case of the imperfect, closed flowers described above, if any new principle must be invoked, it would be one of regression rather than progression. The same would be true of many parasitic and degraded animals. We don't know the cause of the modifications I've described. But if the unknown cause were to act nearly uniformly over a long period, we can infer that the result would be nearly uniform, and all individuals of the species would be modified in the same way.
Because these features are unimportant for the species' welfare, any slight variations in them would not have been accumulated and enhanced through natural selection. A structure that has been developed through long-continued selection, when it ceases to be useful to a species, generally becomes variable, as we see with rudimentary organs -- because it's no longer regulated by the power of selection. But when modifications arise, due to the nature of the organism and its conditions, that are unimportant for the species' welfare, they may be -- and apparently often have been -- transmitted in nearly the same state to numerous otherwise-modified descendants. It can't have been of much importance to the majority of mammals, birds, or reptiles whether they were covered with hair, feathers, or scales. Yet hair has been transmitted to almost all mammals, feathers to all birds, and scales to all true reptiles. A structure that is common to many related forms is ranked by us as having high systematic importance, and is therefore often assumed to be of great vital importance to the species. Thus, as I'm inclined to believe, structural differences that we consider important -- such as the arrangement of leaves, the divisions of the flower or ovary, the position of the ovules, and so on -- first appeared in many cases as fluctuating variations, which sooner or later became constant through the nature of the organism and the surrounding conditions, as well as through the crossbreeding of distinct individuals, but not through natural selection. Since these structural features don't affect the species' welfare, any slight deviations in them couldn't have been governed or accumulated through natural selection. This leads us to a strange conclusion: that features of slight vital importance to the species are the most important ones to the taxonomist. But as we'll see later, when we discuss the genetic principle of classification, this isn't nearly as paradoxical as it first appears.
Although we have no good evidence for an innate tendency toward progressive development in organisms, such development does necessarily follow -- as I tried to show in the fourth chapter -- from the continued action of natural selection. The best definition ever given of a high standard of organization is the degree to which the parts have been specialized and differentiated. And natural selection tends toward this end, since specialization enables the parts to perform their functions more efficiently.
A distinguished zoologist, Mr. St. George Mivart, has recently collected all the objections that have ever been raised -- by myself and others -- against the theory of natural selection, as proposed by Mr. Wallace and myself, and has presented them with admirable skill and force. Lined up together, they make an impressive array. And since it forms no part of Mivart's plan to present the facts and considerations that oppose his conclusions, the reader who wishes to weigh the evidence on both sides is left with no small effort of reason and memory. When discussing specific cases, Mivart ignores the effects of increased use and disuse of parts, which I have always maintained to be highly important and have treated at greater length in my *Variation Under Domestication* than, I believe, any other writer. He also often assumes that I attribute nothing to variation independently of natural selection, whereas in that same work I've collected a greater number of well-established cases than can be found in any other work known to me. My judgment may not be trustworthy, but after reading Mivart's book carefully and comparing each section with what I've said on the same topic, I've never felt more strongly convinced of the general truth of the conclusions I've reached here -- subject, of course, in so complex a subject, to much partial error.
All of Mivart's objections will be, or have been, considered in this book. The one new point that appears to have struck many readers is "that natural selection is incompetent to account for the incipient stages of useful structures." This subject is closely connected to the gradation of features, often accompanied by a change of function -- for instance, the conversion of a swim bladder into lungs -- which I discussed in the last chapter under two headings. Nevertheless, I'll consider in some detail here several of the cases Mivart raises, selecting the most illustrative ones, since space prevents me from addressing all of them.
The giraffe, with its towering height, enormously long neck, forelegs, head, and tongue, has its whole body beautifully adapted for browsing on the higher branches of trees. It can reach food beyond the reach of the other hoofed animals in the same country, and this must be a great advantage during times of drought. The Niata cattle of South America show us how a small difference in structure can make a huge difference in an animal's survival during such periods. These cattle can graze on grass as well as others, but because of the projection of their lower jaw, they can't browse on tree twigs, reeds, and other vegetation that ordinary cattle and horses are driven to during the frequent droughts. So at those times, the Niatas die unless fed by their owners. Before turning to Mivart's objections, it will be helpful to explain once more how natural selection works in ordinary cases. Humans have modified some of their animals without necessarily paying attention to specific structural features, simply by preserving and breeding from the fastest individuals, as with the racehorse and greyhound, or by breeding from the victorious birds, as with the gamecock. Similarly, in nature, with the early giraffe, the individuals that could browse highest -- able during droughts to reach even an inch or two above the others -- would often have been the ones to survive, since they would have ranged over the whole country searching for food. That individuals of the same species often differ slightly in the relative lengths of all their parts can be seen in many works of natural history where careful measurements are given. These slight proportional differences, due to the laws of growth and variation, are of no use or importance to most species. But it would have been different with the early giraffe, given its probable way of life: those individuals whose bodies, or some part of them, were somewhat more elongated than usual would generally have survived. These would have interbred and left offspring that either inherited the same bodily features or tended to vary again in the same direction, while the less favorably proportioned individuals would have been the most likely to die.
Notice that there's no need to separate single pairs, as a human breeder does when methodically improving a breed: natural selection will preserve and thereby separate all the superior individuals, allowing them to interbreed freely, while destroying all the inferior ones. Through this long-continued process -- which corresponds exactly to what I've called unconscious selection by humans, combined no doubt in a very important way with the inherited effects of increased use of parts -- it seems to me almost certain that an ordinary hoofed mammal could be converted into a giraffe.
Against this conclusion, Mivart raises two objections. The first is that increased body size would obviously require more food, and he considers it "very problematical whether the disadvantages arising from this would not, in times of scarcity, more than counterbalance the advantages." But since giraffes actually exist in large numbers in Africa, and since some of the world's largest antelopes -- taller than an ox -- thrive there, why should we doubt that intermediate forms, as far as size is concerned, could formerly have existed there, subjected to the same severe droughts? Surely the ability, at each stage of increasing size, to reach a food supply untouched by the other hoofed animals of the country would have been of some advantage to the developing giraffe. And we shouldn't overlook the fact that increased bulk would serve as protection against almost all predators except the lion. And against the lion, the tall neck -- the taller the better -- would, as Mr. Chauncey Wright has pointed out, serve as a watchtower. It's for this reason, as Sir S. Baker notes, that no animal is more difficult to stalk than the giraffe. This animal also uses its long neck as a weapon, violently swinging its head with its stump-like horns. The survival of each species is rarely determined by any single advantage, but by the combination of all of them, great and small.
Mivart then asks -- and this is his second objection -- if natural selection is so powerful, and if high browsing is such a great advantage, why hasn't any other hoofed animal acquired a long neck and tall stature, besides the giraffe and, to a lesser degree, the camel, guanaco, and macrauchenia? Or again, why hasn't any member of the group developed a long trunk? With respect to South Africa, which was formerly home to enormous herds of giraffes, the answer isn't difficult, and is best given by an analogy. In every meadow in England where trees grow, we see the lower branches trimmed to an exact level by the browsing of horses or cattle. What advantage would it be to sheep, kept in the same meadow, to develop slightly longer necks? In every area, some one kind of animal will almost certainly be able to browse higher than the others. And it's almost equally certain that this one kind alone could have its neck elongated for this purpose through natural selection and increased use. In South Africa, the competition for browsing on the higher branches of acacias and other trees is between giraffe and giraffe, not between giraffes and the other hoofed animals.
Why, in other parts of the world, various animals of this same order haven't developed either a long neck or a trunk can't be clearly answered. But it's as unreasonable to expect a clear answer to such a question as to ask why some event in human history happened in one country but not in another. We don't know the conditions that determine the numbers and range of each species, and we can't even guess what structural changes would help it increase in some new area. We can, however, see in general terms that various causes might have prevented the development of a long neck or trunk. To reach foliage at a considerable height (without climbing, for which hoofed animals are remarkably ill-suited) requires greatly increased body size. And we know that some areas support remarkably few large mammals -- South America, for example, despite being so lush -- while South Africa has them in unparalleled abundance. Why this should be so, we don't know, nor why the later Tertiary periods were apparently much more favorable for large mammals than the present time. Whatever the causes, we can see that certain regions and time periods would have been much more favorable than others for the development of an animal as large as the giraffe.
For an animal to acquire some structure that is specially and substantially developed, it's almost essential that several other parts be modified and mutually adapted. Although every part of the body varies slightly, it doesn't follow that the necessary parts will always vary in the right direction and to the right degree. In different species of our domestic animals, we know that parts vary in different ways and to different degrees, and that some species are much more variable than others. Even if the right variations did arise, it doesn't follow that natural selection would be able to act on them and produce a structure that would apparently benefit the species. For instance, if a species' numbers are determined mainly by predation, or by external or internal parasites -- as often seems to be the case -- then natural selection will be able to do little, or will be greatly slowed, in modifying any particular structure for obtaining food. Finally, natural selection is a slow process, and the same favorable conditions must persist for a long time for any marked effect to be produced. Except by offering such general and vague reasons, we can't explain why hoofed mammals in many parts of the world haven't developed much longer necks or other means of browsing on high branches.
Objections of the same kind as the foregoing have been raised by many writers. In each case, various causes -- besides the general ones just mentioned -- have probably prevented the acquisition through natural selection of structures that we think would be beneficial to certain species. One writer asks why the ostrich hasn't acquired the power of flight. But a moment's reflection shows what an enormous food supply would be needed to give this bird of the desert the energy to move its huge body through the air. Oceanic islands are inhabited by bats and seals, but by no land mammals. Yet since some of these bats are unique species, they must have long inhabited their present homes. So Sir Charles Lyell asks -- and offers certain reasons in answer -- why haven't seals and bats given rise on such islands to forms fitted to live on land? But seals would first have to be converted into large land-dwelling carnivores, and bats into land-dwelling insect-eaters. For the former, there would be no prey. For the bats, ground insects would serve as food, but these would already be heavily preyed upon by the reptiles or birds that first colonized and now abound on most oceanic islands. Gradual structural changes, with each stage beneficial to the changing species, will be favored only under certain special conditions. A strictly land-dwelling animal, by occasionally hunting for food in shallow water, then in streams or lakes, might eventually be converted into an animal so thoroughly aquatic that it could brave the open ocean. But seals wouldn't find on oceanic islands the conditions favorable for their gradual reconversion into a land animal. Bats, as I previously discussed, probably acquired their wings by first gliding through the air from tree to tree, like the so-called flying squirrels, to escape enemies or avoid falls. But once the power of true flight had been acquired, it would never be converted back -- at least for these purposes -- into the less efficient power of gliding. Bats might, like many birds, have had their wings greatly reduced or completely lost through disuse. But for this to happen, they would first have needed to acquire the ability to run quickly on the ground using their hind legs alone, so as to compete with birds or other ground animals -- and for such a change, a bat seems remarkably ill-suited. These speculative remarks are made merely to show that a structural transition, with each step beneficial, is a highly complex matter, and that there's nothing strange about a particular transition not having occurred.
Finally, more than one writer has asked why some animals haven't had their mental powers more highly developed than others, since such development would be advantageous to all. Why haven't apes acquired the intellectual powers of humans? Various reasons could be suggested, but since they're speculative and their relative probability can't be weighed, it would be pointless to list them. We shouldn't expect a definitive answer to this question, since no one can solve the simpler problem of why, of two groups of human societies, one has risen higher in civilization than the other -- and this apparently implies increased brain power.
Let's return to Mivart's other objections. Insects often resemble various objects for the sake of protection -- green or decayed leaves, dead twigs, bits of lichen, flowers, spines, bird droppings, and even other living insects (though I'll return to this last point later). The resemblance is often astonishingly close and extends not just to color but to form, and even to the way the insects hold themselves. The caterpillars that project motionless like dead twigs from the bushes they feed on offer an excellent example of this kind. The cases of imitating objects like bird droppings are rare and exceptional. On this point, Mivart remarks: "As, according to Mr. Darwin's theory, there is a constant tendency to indefinite variation, and as the minute incipient variations will be in all directions, they must tend to neutralize each other, and at first to form such unstable modifications that it is difficult, if not impossible, to see how such indefinite oscillations of infinitesimal beginnings can ever build up a sufficiently appreciable resemblance to a leaf, bamboo, or other object, for natural selection to seize upon and perpetuate."
But in all the cases above, the insects in their original state no doubt already had some rough and accidental resemblance to a common object in the places they frequented. And this isn't at all unlikely, considering the almost infinite number of surrounding objects and the diversity in form and color of the immense number of insects that exist. Since some rough resemblance is necessary for a starting point, we can understand why larger and higher animals don't (with the exception, as far as I know, of one fish) resemble specific objects for protection, but only the surface that commonly surrounds them -- and this mainly in color. Suppose an insect originally happened to somewhat resemble a dead twig or a decayed leaf, and then varied slightly in many ways. All the variations that made the insect look even a little more like that object, and thus helped it escape detection, would be preserved, while other variations would be neglected and eventually lost. Or, if they made the insect look less like the imitated object, they would be eliminated. There would indeed be force in Mivart's objection if we were trying to explain these resemblances independently of natural selection, through mere random variability. But that's not what's being argued, so the objection has no force.
Nor can I see any force in Mivart's difficulty with "the last touches of perfection in the mimicry" -- as in the case described by Wallace of a walking-stick insect (*Ceroxylus laceratus*), which resembles "a stick grown over by a creeping moss or liverwort." The resemblance was so close that a native Dyak insisted the leafy growths were really moss. Insects are preyed on by birds and other enemies whose sight is probably sharper than ours, and every degree of resemblance that helped an insect escape notice or detection would tend to be preserved -- the more perfect the resemblance, the better for the insect. Considering the nature of the differences between species in the group that includes this *Ceroxylus*, there's nothing improbable about this insect having varied in the irregularities on its surface, and these having become more or less green-colored. In every group, the features that differ among the various species are the ones most likely to vary, while the features common to all species in the group (the generic characters) are the most constant.
The Greenland whale is one of the most wonderful animals in the world, and its baleen, or whalebone, one of its most remarkable features. The baleen consists of a row on each side of the upper jaw of about 300 plates or sheets, standing close together at right angles to the long axis of the mouth. Within the main row are some subsidiary rows. The edges and inner margins of all the plates are frayed into stiff bristles, which cover the whole gigantic palate and serve to strain or filter the water, trapping the tiny prey on which these great animals feed. The middle and longest plate in the Greenland whale is ten, twelve, or even fifteen feet long. But in different species of whales, there are gradations in length: the middle plate being four feet in one species, according to Scoresby, three feet in another, eighteen inches in another, and only about nine inches in the rorqual *Balaenoptera rostrata*. The quality of the whalebone also differs among species.
Regarding the baleen, Mivart remarks that if it "had once attained such a size and development as to be at all useful, then its preservation and augmentation within serviceable limits would be promoted by natural selection alone. But how to obtain the beginning of such useful development?" In answer, one might ask: why shouldn't the early ancestors of baleen whales have possessed a mouth constructed something like the ridged beak of a duck? Ducks, like whales, feed by sifting mud and water, and the family has sometimes been called Criblatores, or sifters. I hope I won't be misunderstood as saying that the ancestors of whales actually had mouths ridged like a duck's beak. I only wish to show that this isn't inconceivable, and that the immense plates of baleen in the Greenland whale might have developed from such ridges through finely graduated steps, each useful to its possessor.
The beak of a shoveler duck (*Spatula clypeata*) is a more beautiful and complex structure than the mouth of a whale. The upper mandible is furnished on each side (in the specimen I examined) with a row or comb of 188 thin, elastic plates, beveled at an angle so as to be pointed and placed at right angles to the long axis of the mouth. They arise from the palate and are attached by flexible membrane to the sides of the mandible. Those toward the middle are the longest, about a third of an inch, and they project about fourteen hundredths of an inch below the edge. At their bases is a short subsidiary row of diagonally placed plates. In all these respects, they resemble the plates of baleen in a whale's mouth. But toward the tip of the beak, they differ considerably, projecting inward rather than straight downward. The shoveler's entire head, though incomparably less massive, is about one-eighteenth the length of the head of a moderately large *Balaenoptera rostrata*, in which species the baleen is only nine inches long. So if we were to scale up the shoveler's head to the length of the *Balaenoptera's*, its plates would be six inches long -- two-thirds the length of the baleen in this whale species. The lower mandible of the shoveler duck is furnished with plates of equal length to those above, but finer. In having such plates on the lower jaw, it differs markedly from a whale, which has no baleen on the lower jaw. On the other hand, the tips of these lower plates are frayed into fine bristly points, so they curiously resemble baleen plates. In the genus *Prion*, a member of the distinct petrel family, only the upper mandible is furnished with plates, which are well developed and project below the margin -- so this bird's beak resembles a whale's mouth in this respect.
From the highly developed structure of the shoveler's beak, we can proceed (as I've learned from information and specimens sent to me by Mr. Salvin) without any great gap, as far as sifting ability is concerned, through the beak of *Merganetta armata*, and in some respects through that of *Aix sponsa*, to the beak of the common duck. In the common duck, the plates are much coarser than in the shoveler and are firmly attached to the sides of the mandible. They number only about fifty on each side and don't project below the margin at all. They are flat-topped and edged with translucent, hardened tissue, as if for crushing food. The ridges of the lower mandible are crossed by numerous fine ridges that barely project. Although this beak is far inferior as a sifting device to the shoveler's, everyone knows the common duck constantly uses it for this purpose. There are other species, as I hear from Mr. Salvin, in which the plates are considerably less developed than in the common duck, but I don't know whether they use their beaks for sifting water.
Turning to another group of the same family: in the Egyptian goose (*Chenalopex*), the beak closely resembles that of the common duck, but the plates aren't as numerous, nor as distinct from each other, nor do they project as far inward. Yet this goose, as I'm informed by Mr. E. Bartlett, "uses its bill like a duck by throwing the water out at the corners." Its main food, however, is grass, which it crops like the common goose. In the common goose, the plates of the upper mandible are much coarser than in the duck, nearly fused together, about twenty-seven on each side, and ending upward in tooth-like knobs. The palate is also covered with hard, rounded knobs. The ridges of the lower mandible are serrated with teeth that are much more prominent, coarser, and sharper than in the duck. The common goose doesn't sift water but uses its beak exclusively for tearing or cutting vegetation, for which it's so well fitted that it can crop grass closer than almost any other animal. There are other species of geese, as I hear from Mr. Bartlett, in which the plates are even less developed than in the common goose.
We thus see that a member of the duck family, with a beak built like a common goose's and adapted solely for grazing -- or even one with less well-developed plates -- could be converted by small changes into a species like the Egyptian goose; this into one like the common duck; and finally into one like the shoveler, equipped with a beak almost exclusively adapted for sifting water -- since this bird can hardly use any part of its beak, except the hooked tip, for seizing or tearing solid food. A goose's beak, I might add, could also be converted by small changes into one with prominent, curved teeth, like those of the merganser (a member of the same family), serving the entirely different purpose of catching live fish.
Returning to the whales. *Hyperoodon bidens* has no functional true teeth, but its palate is roughened, according to Lacepede, with small, unequal, hard points of horn. There's therefore nothing improbable in supposing that some early whale-like form had similar horn points on its palate, but more regularly arranged, and which, like the knobs on a goose's beak, helped it seize or tear its food. If so, it can hardly be denied that these points might have been converted through variation and natural selection into plates as well-developed as those of the Egyptian goose, useful both for seizing objects and for sifting water. Then into plates like those of the domestic duck, and onward, until they became as well-built as those of the shoveler, serving exclusively as a sifting apparatus. From this stage -- in which the plates would be two-thirds the length of the baleen in *Balaenoptera rostrata* -- gradations observable in still-living whales lead us onward to the enormous baleen plates of the Greenland whale. There's not the slightest reason to doubt that each step in this progression might have been as useful to certain ancient whales, with the functions of the parts slowly changing during development, as the gradations in the beaks of the different existing members of the duck family are today. We should keep in mind that each species of duck is subjected to a severe struggle for existence, and that every part of its body must be well adapted to its conditions of life.
The Pleuronectidae, or flatfish, are remarkable for their asymmetrical bodies. They rest on one side -- in most species on the left, but in some on the right, and occasionally reversed adult specimens occur. The lower, or resting, surface resembles at first glance the belly of an ordinary fish: it's white, less developed in many ways than the upper side, and the lateral fins are often smaller. But the eyes present the most remarkable feature: they're both placed on the upper side of the head. During early life, however, they sit on opposite sides, and the whole body is symmetrical, with both sides equally colored. Soon the eye that belongs to the lower side begins to glide slowly around the head to the upper side -- but it doesn't pass right through the skull, as was formerly thought. It's obvious that unless the lower eye traveled around this way, it couldn't be used by the fish while lying in its usual position on one side. The lower eye would also have been liable to be scraped against the sandy bottom. That the flatfish are beautifully adapted by their flattened, asymmetrical structure for their way of life is clear from the fact that several species, such as soles and flounders, are extremely common. The main advantages appear to be protection from enemies and ease of feeding on the bottom. The different members of the family present, as Schiodte remarks, "a long series of forms exhibiting a gradual transition from *Hippoglossus pinguis*, which does not considerably alter the shape in which it leaves the egg, to the soles, which are entirely thrown to one side."
Mivart has taken up this case and remarks that a sudden spontaneous shift in the position of the eyes is hardly conceivable -- and I quite agree with him. He then adds: "If the transit was gradual, then how such transit of one eye a minute fraction of the journey towards the other side of the head could benefit the individual is, indeed, far from clear. It seems, even, that such an incipient transformation must rather have been injurious." But he could have found an answer to this objection in the excellent observations published in 1867 by Malm. The flatfish, while very young and still symmetrical, with their eyes on opposite sides of the head, can't stay upright for long because of the excessive depth of their bodies, the small size of their lateral fins, and their lack of a swim bladder. So they soon grow tired and fall to the bottom on one side. While resting this way, they often twist the lower eye upward to see above them, as Malm observed, and they do this so vigorously that the eye is pressed hard against the upper part of the eye socket. The forehead between the eyes consequently becomes, as could clearly be seen, temporarily compressed in width. On one occasion, Malm saw a young fish raise and lower its bottom eye through an angle of about seventy degrees.
We should remember that at this early age, the skull is cartilaginous and flexible, so it readily yields to muscular action. It's also known that in higher animals, even past early youth, the skull will yield and change shape if the skin or muscles are permanently contracted through disease or accident. In lop-eared rabbits, if one ear flops forward and downward, its weight drags forward all the bones of the skull on the same side -- I've published an illustration of this. Malm reports that newly hatched young of perch, salmon, and several other symmetrical fish have the habit of occasionally resting on one side at the bottom. He observed that they often strain their lower eyes so as to look upward, and their skulls are thereby rendered somewhat crooked. These fish, however, are soon able to hold themselves upright, and no permanent effect results. With the flatfish, on the other hand, the older they grow, the more habitually they rest on one side, owing to their increasingly flat bodies, and a permanent effect is thus produced on the shape of the head and the position of the eyes. Judging from analogy, the tendency to this distortion would no doubt be increased through inheritance. Schiodte believes, contrary to some other naturalists, that flatfish are not quite symmetrical even in the embryo. If this is so, we could understand why certain species habitually fall over on the left side while young, and other species on the right side. Malm adds, in support of this view, that the adult *Trachypterus arcticus* -- which is not a flatfish -- rests on its left side at the bottom and swims diagonally through the water. In this fish, the two sides of the head are said to be somewhat different. The great authority on fish, Dr. Gunther, concludes his summary of Malm's paper by noting that "the author gives a very simple explanation of the abnormal condition of the flatfish."
We thus see that the initial stages of the eye's migration from one side of the head to the other -- which Mivart considers would be harmful -- can be attributed to the habit (no doubt beneficial to both the individual and the species) of trying to look upward with both eyes while resting on one side at the bottom. We can also attribute to the inherited effects of use the fact that the mouth in several kinds of flatfish is bent toward the lower surface, with the jaw bones stronger and more effective on this eyeless side of the head than on the other -- for the sake, as Dr. Traquair proposes, of feeding easily on the bottom. Disuse, on the other hand, explains the less developed condition of the whole lower half of the body, including the lateral fins -- though Yarrell thinks the reduced size of these fins is actually advantageous, since "there is so much less room for their action than with the larger fins above." Perhaps the fewer teeth on the upper side of the jaws in the plaice (four to seven, versus twenty-five to thirty on the lower side) can similarly be explained by disuse. From the colorless underside of most fish and many other animals, we may reasonably suppose that the lack of color on the flatfish's bottom side -- whether right or left -- is due to the exclusion of light. But the peculiar speckled appearance of the upper side of the sole, so closely matching the sandy seabed, or the ability of some species (as Pouchet has recently shown) to change color to match the surrounding surface, or the presence of bony bumps on the upper side of the turbot -- these can't be due to light. Here natural selection has probably played a role, as well as in shaping the general body form of these fish and many other features suited to their way of life. We should keep in mind, as I've insisted before, that the inherited effects of increased use -- and perhaps disuse -- will be reinforced by natural selection, since all spontaneous variations in the right direction will be preserved, as will those individuals that inherit to the highest degree the effects of beneficial use. How much to attribute in each case to use, and how much to natural selection, seems impossible to decide.
I can give another example of a structure that apparently owes its origin entirely to use or habit. In some American monkeys, the tip of the tail has been converted into a wonderfully perfect gripping organ, serving as a fifth hand. A reviewer who agrees with Mivart in every detail remarks about this structure: "It is impossible to believe that in any number of ages the first slight incipient tendency to grasp could preserve the lives of the individuals possessing it, or favour their chance of having and of rearing offspring." But there's no need for any such belief. Habit -- and this almost implies that some benefit, great or small, is gained from it -- would in all probability be enough to do the job. Brehm saw the young of an African monkey (*Cercopithecus*) clinging to the underside of their mother by their hands while at the same time hooking their little tails around their mother's tail. Professor Henslow kept some harvest mice (*Mus messorius*) in captivity -- these don't have a structurally prehensile tail -- but he frequently observed them curling their tails around the branches of a bush placed in the cage, thus helping themselves climb. I've received a similar account from Dr. Gunther, who has seen a mouse suspend itself this way. If the harvest mouse had been more strictly tree-dwelling, it might perhaps have developed a structurally prehensile tail, as some other members of the same order have. Why *Cercopithecus*, despite its habits when young, hasn't developed such a tail would be hard to say. It's possible, however, that this monkey's long tail may be more useful as a balancing organ for its prodigious leaps than it would be as a gripping organ.
Mammary glands are shared by the entire class of mammals and are indispensable for their existence. They must therefore have developed at an extremely remote period, and we can know nothing definitive about how they developed. Mivart asks: "Is it conceivable that the young of any animal was ever saved from destruction by accidentally sucking a drop of scarcely nutritious fluid from an accidentally hypertrophied cutaneous gland of its mother? And even if one was so, what chance was there of the perpetuation of such a variation?" But this isn't a fair way to frame the question. Most evolutionists accept that mammals descended from a marsupial form. If so, the mammary glands would have first developed within the marsupial pouch. In the case of the seahorse (*Hippocampus*), the eggs are hatched and the young are reared for a time within a pouch of this kind. The American naturalist Mr. Lockwood believes, from his observations of the young's development, that they are nourished by a secretion from the skin glands of the pouch. Now, with the early ancestors of mammals -- almost before they deserved to be called mammals -- isn't it at least possible that the young were similarly nourished? And if so, the individuals that secreted the most nutritious fluid -- approaching the nature of milk -- would in the long run have reared more well-nourished offspring than those that secreted a poorer fluid. Thus the skin glands, which are the evolutionary equivalents of the mammary glands, would have been improved and made more effective. It's consistent with the widely recognized principle of specialization that the glands over a certain area of the pouch would have become more highly developed than the rest. They would then have formed a breast -- but at first without a nipple, as we see in the platypus (*Ornithorhynchus*), at the base of the mammalian family tree. Through what agency the glands over a certain area became more specialized than the others, I won't pretend to decide -- whether partly through compensatory growth, the effects of use, or natural selection.
The development of mammary glands would have been of no use, and couldn't have been shaped by natural selection, unless the young at the same time were able to take in the secretion. There's no more difficulty in understanding how young mammals instinctively learned to suck than in understanding how unhatched chicks learned to break the eggshell by tapping against it with their specially adapted beaks, or how, just a few hours after hatching, they learn to pick up grains of food. In such cases, the most probable explanation seems to be that the habit was first acquired through practice at a later age and was then passed to offspring at an earlier age. But the young kangaroo is said not to suck -- it only clings to the nipple of its mother, who has the ability to inject milk into the mouth of her helpless, half-formed young. On this point, Mivart remarks: "Did no special provision exist, the young one must infallibly be choked by the intrusion of the milk into the windpipe. But there is a special provision. The larynx is so elongated that it rises up into the posterior end of the nasal passage, and is thus enabled to give free entrance to the air for the lungs, while the milk passes harmlessly on each side of this elongated larynx, and so safely reaches the gullet behind it." Mivart then asks how natural selection could have removed, in the adult kangaroo (and in most other mammals, if we assume they descended from a marsupial form), "this at least perfectly innocent and harmless structure." In answer, it may be suggested that the voice, which is certainly highly important to many animals, could hardly have been used at full strength as long as the larynx entered the nasal passage. And Professor Flower has suggested to me that this structure would have greatly interfered with swallowing solid food.
Now let's turn briefly to the lower divisions of the animal kingdom. The Echinodermata (starfish, sea urchins, and their relatives) are equipped with remarkable organs called pedicellariae, which, when fully developed, consist of a three-pronged forceps -- that is, three serrated arms that fit neatly together, mounted on the tip of a flexible stalk moved by muscles. These forceps can grip any object firmly. Alexander Agassiz has seen a sea urchin rapidly passing particles of waste from forceps to forceps down certain lines of its body, to keep its shell clean. But besides removing debris of all kinds, they clearly serve other functions too, and one of these apparently is defense.
Regarding these organs, Mivart, as on so many previous occasions, asks: "What would be the utility of the first rudimentary beginnings of such structures, and how could such incipient buds have ever preserved the life of a single sea urchin?" He adds: "not even the sudden development of the snapping action would have been beneficial without the freely movable stalk, nor could the latter have been efficient without the snapping jaws, yet no minute, nearly indefinite variations could simultaneously evolve these complex coordinations of structure; to deny this seems to do no less than to affirm a startling paradox." Paradoxical as this may seem to Mivart, three-pronged forceps that are immovably fixed at the base but capable of snapping definitely do exist on some starfish -- and this makes sense if they serve at least partly as a means of defense. Mr. Agassiz, to whose great kindness I'm indebted for much information on this subject, tells me that there are other starfish in which one of the three arms of the forceps is reduced to a support for the other two, and still other genera in which the third arm is completely lost. In *Echinoneus*, the shell is described by M. Perrier as bearing two kinds of pedicellariae -- one resembling those of *Echinus* and the other those of *Spatangus*. Such cases are always interesting because they afford what appear to be sudden transitions, through the loss of one of two states of an organ.
Regarding the steps by which these curious organs evolved, Mr. Agassiz infers from his own research and that of Mr. Muller that pedicellariae in both starfish and sea urchins must undoubtedly be understood as modified spines. This can be inferred from their development in individual organisms, as well as from a long and complete series of gradations across different species and genera -- from simple granules to ordinary spines to perfect three-pronged pedicellariae. The gradation extends even to the way ordinary spines and pedicellariae, with their supporting rods of calcium carbonate, are jointed to the shell. In certain genera of starfish, "the very combinations needed to show that the pedicellariae are only modified branching spines" can be found. We see fixed spines with three equidistant, serrated, movable branches joined near their bases, and higher up on the same spine, three other movable branches. Now, when the latter arise from the summit of a spine, they form, in effect, a crude three-pronged pedicellariae -- and these can be seen on the same spine together with the three lower branches. In this case, the identity between the arms of the pedicellariae and the movable branches of a spine is unmistakable. It's generally accepted that ordinary spines serve as protection. If so, there's no reason to doubt that spines with serrated, movable branches likewise serve for protection -- and they would serve even more effectively as soon as they met together and acted as a gripping or snapping device. Thus every gradation, from an ordinary fixed spine to a fixed pedicellariae, would have been useful.
In certain genera of starfish, these organs, instead of being fixed or mounted on an immovable support, are placed on the tip of a flexible, muscular, though short, stalk. In this case, they probably serve some additional function besides defense. In sea urchins, the steps can be traced by which a fixed spine becomes jointed to the shell and thus made movable. I wish I had space to give a fuller summary of Mr. Agassiz's interesting observations on pedicellariae development. All possible gradations, as he adds, can also be found between the pedicellariae of starfish and the hooks of the Ophiurians (another group of Echinodermata), and again between the pedicellariae of sea urchins and the anchors of the Holothuriae (sea cucumbers), also in the same great class.
Certain compound animals -- or zoophytes, as they've been called -- namely the Polyzoa, are equipped with curious organs called avicularia. These differ greatly in structure across different species. In their most developed form, they curiously resemble a miniature vulture's head and beak, seated on a neck and capable of movement, and the lower jaw is also movable. In one species I observed, all the avicularia on the same branch often moved simultaneously back and forth, with the lower jaw gaping wide open, through an angle of about ninety degrees, in the course of five seconds -- and their movement made the whole colony tremble. When the jaws are touched with a needle, they seize it so firmly that the branch can be shaken by it.
Mivart brings up this case mainly because of the supposed difficulty of organs -- the avicularia of the Polyzoa and the pedicellariae of the Echinodermata -- that he considers "essentially similar," having evolved through natural selection in widely separated divisions of the animal kingdom. But as far as structure goes, I can see no similarity between three-pronged pedicellariae and avicularia. The latter somewhat more closely resemble the claws or pincers of crustaceans, and Mivart might just as reasonably have pointed to this resemblance as a special difficulty -- or even their resemblance to the head and beak of a bird. The avicularia are believed by Mr. Busk, Dr. Smitt, and Dr. Nitsche -- naturalists who have carefully studied this group -- to be homologous with the individual zooids and their cells that make up the colony. The movable lip or lid of the cell corresponds to the lower, movable mandible of the avicularium. Mr. Busk, however, doesn't know of any gradations currently existing between a zooid and an avicularium. It's therefore impossible to guess by what useful gradations one could have been converted into the other. But it by no means follows that such gradations never existed.
Since the pincers of crustaceans somewhat resemble the avicularia of Polyzoa -- both serving as gripping tools -- it may be worth showing that for the former, a long series of useful gradations still exists. In the first and simplest stage, the end segment of a limb closes down either against the flat top of the broad second-to-last segment or against one whole side of it, and can thus grab hold of an object -- but the limb still functions for locomotion. Next, we find one corner of the broad second-to-last segment slightly protruding, sometimes with irregular teeth, against which the end segment closes. As this projection increases in size, with its shape and that of the end segment slightly modified and improved, the pincers become more and more effective, until we end up with an instrument as efficient as a lobster's claw. And all these gradations can actually be traced.
Besides the avicularia, the Polyzoa possess curious organs called vibracula. These generally consist of long bristles, capable of movement and easily stimulated. In one species I examined, the vibracula were slightly curved and serrated along the outer margin, and all of them on the same colony often moved simultaneously -- so that, acting like long oars, they swept a branch rapidly across the glass slide of my microscope. When a branch was placed face-down, the vibracula became entangled and made violent efforts to free themselves. They're thought to serve as a defense, and can be seen, as Mr. Busk notes, "to sweep slowly and carefully over the surface of the colony, removing what might be harmful to the delicate inhabitants of the cells when their tentacles are extended." The avicularia, like the vibracula, probably serve for defense, but they also catch and kill small living animals, which are believed to be swept by the currents within reach of the tentacles of the zooids. Some species have both avicularia and vibracula, some have avicularia alone, and a few have vibracula alone.
It's hard to imagine two structures more different in appearance than a bristle (or vibraculum) and an avicularium that looks like a bird's head. Yet they're almost certainly homologous -- derived from the same source, namely a zooid with its cell. This is why, as Mr. Busk has informed me, these organs grade into each other in some cases. For example, in the avicularia of several species of *Lepralia*, the movable mandible is so elongated and bristle-like that only the presence of the upper or fixed beak identifies it as an avicularium. The vibracula may have developed directly from the cell's lips without passing through an avicularian stage, but it seems more likely that they did pass through that stage, since during the early steps of the transformation, the other parts of the cell, including the zooid inside, could hardly have disappeared all at once. In many cases, the vibracula have a grooved support at their base that seems to represent the fixed beak, though in some species this support is completely absent. This view of how vibracula developed, if correct, is interesting: if all the species with avicularia had gone extinct, no one with the most vivid imagination would ever have guessed that vibracula had originally existed as part of an organ resembling a bird's head, or an irregular box or hood. It's interesting to see two such widely different organs developing from a common origin. And since the movable lip of the cell serves to protect the zooid, there's no difficulty in believing that all the gradations -- from the lip becoming the lower mandible of an avicularium, to its becoming an elongated bristle -- likewise served as protection in different ways and under different circumstances.
In the plant kingdom, Mivart only mentions two cases: the structure of orchid flowers and the movements of climbing plants. Regarding orchids, he says: "The explanation of their origin is deemed thoroughly unsatisfactory -- utterly insufficient to explain the incipient, infinitesimal beginnings of structures which are of utility only when they are considerably developed." Since I've treated this subject fully in another work, I'll give only a few details here on just one of the most striking features of orchid flowers: their pollinia. A pollinium, when fully developed, consists of a mass of pollen grains attached to an elastic foot-stalk or caudicle, which is in turn attached to a small mass of extremely sticky substance. The pollinia are transported by insects from one flower to the stigma of another. In some orchids, there's no caudicle; the pollen grains are merely tied together by fine threads. But since these threads aren't confined to orchids, they needn't be considered here -- though I should mention that at the base of the orchid family tree, in *Cypripedium*, we can see how these threads were probably first developed. In other orchids, the threads join together at one end of the pollen masses, forming the first or beginning trace of a caudicle. That this is the origin of the caudicle, even when it's quite long and fully developed, is supported by the aborted pollen grains that can sometimes be found embedded within the central, solid parts.
Regarding the second key feature -- the small mass of sticky substance attached to the end of the caudicle -- a long series of gradations can be described, each clearly useful to the plant. In most flowers of other groups, the stigma secretes a small amount of sticky substance. In certain orchids, a similar sticky substance is secreted, but in much larger quantities, by one alone of the three stigmas. This stigma, perhaps as a result of the heavy secretion, is rendered sterile. When an insect visits a flower of this kind, it picks up some of the sticky substance and at the same time drags away some pollen grains. From this simple condition -- which differs little from that of countless common flowers -- there are endless gradations to species in which the pollen mass ends in a very short, free caudicle; to others in which the caudicle becomes firmly attached to the sticky substance, with the sterile stigma itself much modified. In this last case, we have a pollinium in its most highly developed and perfect form. Anyone who carefully examines orchid flowers for themselves will not deny the existence of this series of gradations -- from a mass of pollen grains merely tied together by threads, with the stigma barely different from that of ordinary flowers, to a highly complex pollinium beautifully adapted for transport by insects. Nor will they deny that all the gradations in the various species are beautifully adapted, in relation to the general structure of each flower, for fertilization by different insects. In this case, as in almost every other, the inquiry can be pushed further back: how did the stigma of an ordinary flower first become sticky? But since we don't know the full history of any single group of organisms, it's as pointless to ask as it is hopeless to attempt to answer such questions.
Now let's turn to climbing plants. These can be arranged in a long series, from those that simply twine around a support to those I've called leaf-climbers, and to those with tendrils. In these latter two classes, the stems have generally (but not always) lost the power of twining, though they retain the power of revolving, which the tendrils also possess. The gradations from leaf-climbers to tendril-bearers are remarkably close, and certain plants could be placed in either class. But in ascending from simple twiners to leaf-climbers, an important new ability is added: sensitivity to touch. This enables the foot-stalks of the leaves or flowers -- or these modified and converted into tendrils -- to bend around and clasp any object they touch. Anyone who reads my paper on these plants will, I think, agree that all the many gradations in function and structure between simple twiners and tendril-bearers are in each case highly beneficial to the species. For instance, it's clearly a great advantage for a twining plant to become a leaf-climber. And it's likely that every twiner with leaves on long foot-stalks would have developed into a leaf-climber, if those foot-stalks had possessed even the slightest sensitivity to touch.
Since twining is the simplest way of climbing a support and forms the basis of our series, the natural question is: how did plants first acquire this power, which was later improved and enhanced through natural selection? Twining depends, first, on the stems being extremely flexible when young (a feature common to many non-climbing plants), and second, on their continually bending toward all points of the compass in sequence. This movement causes the stems to lean in all directions and to move round and round. As soon as the lower part of a stem strikes an object and is stopped, the upper part keeps bending and revolving, and thus necessarily twines around and up the support. The revolving movement stops after the early growth of each shoot. Since single species and single genera in many widely separated plant families possess the power of revolving and have thus become twiners, they must have acquired this ability independently, rather than inheriting it from a common ancestor. This led me to predict that some slight tendency toward this kind of movement would prove to be quite common among plants that don't climb, and that this would have provided the raw material for natural selection to work on and improve. When I made this prediction, I knew of only one imperfect case: the young flower stalks of a *Maurandia*, which revolved slightly and irregularly, like the stems of twining plants, but without making any use of this habit. Soon afterward, Fritz Muller discovered that the young stems of an *Alisma* and a *Linum* -- plants that don't climb and are widely separated in the natural system -- revolved plainly, though irregularly. He says he has reason to suspect this occurs in some other plants too. These slight movements appear to be of no use to these plants; at any rate, they're of no use at all for climbing, which is our concern. Nevertheless, we can see that if the stems of these plants had been flexible, and if under their conditions it had been advantageous to climb to a height, then the habit of slight, irregular revolving might have been increased and put to use through natural selection, until they were converted into fully developed twiners.
Regarding the sensitivity to touch in the foot-stalks of leaves and flowers, and in tendrils, nearly the same points apply as for the revolving movements of twiners. Since a vast number of species across widely different groups have this kind of sensitivity, it ought to be found in a beginning state in many plants that haven't become climbers. This is indeed the case. I observed that the young flower stalks of the *Maurandia* mentioned above curved slightly toward the side that was touched. Morren found that in several species of *Oxalis*, the leaves and their stalks moved, especially after exposure to hot sun, when they were gently and repeatedly touched or when the plant was shaken. I repeated these observations on some other *Oxalis* species with the same result. In some, the movement was distinct but best seen in young leaves; in others, it was extremely slight. It's a more important fact that, according to the eminent authority Hofmeister, the young shoots and leaves of all plants move after being shaken. And in climbing plants, as we know, only the young, growing foot-stalks and tendrils are sensitive.
It's scarcely possible that these slight movements caused by a touch or shake in the young, growing organs of plants are of any functional importance to them. But plants do possess, in response to various stimuli, powers of movement that are clearly important to them -- for instance, movement toward light (and more rarely away from it), or against the pull of gravity (and more rarely with it). When the nerves and muscles of an animal are stimulated by an electric shock or by the absorption of strychnine, the resulting movements can be called an incidental result, since the nerves and muscles weren't made specially sensitive to these stimuli. Similarly with plants: because they have the power to move in response to certain stimuli, they are incidentally stimulated by a touch or shake. So there's no great difficulty in accepting that in leaf-climbers and tendril-bearers, this tendency has been seized upon and enhanced by natural selection. It's likely, however, for reasons I've discussed in my paper, that this would have occurred only in plants that had already acquired the power of revolving and had thus become twiners.
I've already tried to explain how plants became twiners -- by the enhancement of a tendency to slight, irregular revolving movements that were initially of no use. This movement, like the movement caused by touch, is the incidental result of the power of movement gained for other, beneficial purposes. Whether natural selection has been aided by the inherited effects of use in the gradual development of climbing plants, I won't pretend to decide. But we know that certain periodic movements, like the so-called sleep of plants, are governed by habit.
I've now considered enough -- perhaps more than enough -- of the cases carefully selected by a skilled naturalist to prove that natural selection can't account for the beginning stages of useful structures. And I've shown, I hope, that there's no great difficulty on this point. A good opportunity has been provided to expand somewhat on gradations of structure, often associated with unexpected changes in function -- an important subject that wasn't treated at sufficient length in earlier editions of this work. I'll now briefly summarize the foregoing cases.
With the giraffe, the continued preservation of the tallest individuals of some extinct high-browsing ruminant -- those with the longest necks, legs, and so on, who could feed a little above the average height -- and the continued death of those that couldn't reach as high, would have been sufficient to produce this remarkable animal. But the prolonged use of all the parts, combined with inheritance, would have contributed significantly to their coordination. With the many insects that mimic various objects, there's nothing improbable in the belief that an accidental resemblance to some common object was in each case the foundation for natural selection's work, which was then perfected through the occasional preservation of slight variations that improved the resemblance. This would have continued as long as the insect kept varying and as long as increasingly perfect resemblance helped it escape sharp-eyed enemies. In certain whale species, there's a tendency to form irregular little horn points on the palate. It seems well within the scope of natural selection to preserve all favorable variations until the points were converted first into ridged knobs or teeth (like those on a goose's beak), then into short plates (like those of the domestic duck), then into plates as perfect as the shoveler's -- and finally into the gigantic baleen plates of the Greenland whale. In the duck family, the plates are first used as teeth, then partly as teeth and partly as a sifting apparatus, and finally almost exclusively for sifting.
With structures like these horn plates or whalebone, habit or use can have done little or nothing toward their development, as far as we can judge. The movement of the flatfish's lower eye to the upper side of the head, on the other hand, and the formation of a prehensile tail, can be attributed almost entirely to continued use combined with inheritance. Regarding the mammary glands of higher animals, the most probable theory is that originally the skin glands over the whole surface of a marsupial pouch secreted a nutritious fluid, and that these glands were improved in function through natural selection and concentrated into a limited area, forming a breast. There's no more difficulty in understanding how the branching spines of some ancient echinoderm, which served as protection, were developed through natural selection into three-pronged pedicellariae, than in understanding the development of crustacean pincers through slight, useful modifications of the last and next-to-last segments of a limb that was originally used only for walking. In the avicularia and vibracula of the Polyzoa, we have widely different-looking organs developed from the same source. And with the vibracula, we can understand how the successive gradations might have been useful. With the pollinia of orchids, the threads that originally served to tie together the pollen grains can be traced joining into caudicles, and the steps can likewise be followed by which sticky substance -- such as that secreted by the stigmas of ordinary flowers, and still serving nearly the same purpose -- became attached to the free ends of the caudicles. All these gradations are clearly beneficial to the plants in question. Regarding climbing plants, I needn't repeat what has just been said.
It has often been asked: if natural selection is so powerful, why hasn't this or that structure been gained by certain species, when it would apparently be advantageous? But it's unreasonable to expect a precise answer to such questions, given our ignorance of the past history of each species and of the conditions that currently determine its numbers and range. In most cases, only general reasons can be given, though in a few, specific ones can be offered. Adapting a species to new habits of life requires many coordinated modifications, and it may often have happened that the necessary parts didn't vary in the right way or to the right degree. Many species must have been prevented from increasing in numbers by destructive forces unrelated to the structures that we imagine would be beneficial, making it seem as though natural selection failed to produce them. In such cases, since the struggle for life didn't depend on those structures, they couldn't have been acquired through natural selection. In many cases, complex and long-lasting conditions of a special nature are needed for a structure to develop, and the right conditions may rarely have come together. The belief that any structure we think (often mistakenly) would be beneficial to a species must necessarily be acquired under all circumstances through natural selection contradicts what we understand about how natural selection works. Mivart doesn't deny that natural selection has accomplished something, but he considers it "demonstrably insufficient" to account for the phenomena I explain by its agency. His main arguments have now been considered, and the rest will be addressed later. They seem to me to fall far short of demonstrations, and to carry little weight compared to the evidence favoring the power of natural selection, aided by the other mechanisms I've often specified. I should add that some of the facts and arguments I've used here have been put forward for the same purpose in an able article recently published in the *Medico-Chirurgical Review*.
Today almost all naturalists accept evolution in some form. Mivart believes that species change through "an internal force or tendency," about which, he admits, nothing is known. That species have a capacity for change will be granted by all evolutionists. But there's no need, it seems to me, to invoke any internal force beyond the tendency to ordinary variability, which through human selection has produced many well-adapted domestic breeds, and which, through natural selection, would equally well produce natural varieties or species by graduated steps. The final result will generally have been -- as I've already explained -- an advance in organization, though in some cases a decline.
Mivart is also inclined to believe, and some naturalists agree with him, that new species appear "with suddenness and by modifications appearing at once." For instance, he supposes that the differences between the extinct three-toed *Hipparion* and the horse arose suddenly. He thinks it difficult to believe that a bird's wing "was developed in any other way than by a comparatively sudden modification of a marked and important kind" -- and he apparently extends this view to the wings of bats and pterodactyls. This conclusion, which implies major breaks or discontinuity in the series, seems to me improbable in the highest degree.
Everyone who believes in slow and gradual evolution will of course admit that the changes between species may have been as abrupt and as large as any single variation we encounter in nature or even under domestication. But since species are more variable when domesticated or cultivated than in their natural state, it's unlikely that such great, abrupt variations have often occurred in nature as are occasionally known to arise under domestication. Several of these latter variations can be attributed to reversion, and the characters that reappear were probably, in many cases, originally gained gradually. A still greater number must be called monstrosities -- such as six-fingered humans, porcupine-skinned humans, Ancon sheep, Niata cattle, and so on. Since these differ so widely from natural species, they shed very little light on our subject. Setting aside such cases of abrupt variation, the few that remain would at best constitute, if found in nature, doubtful species closely related to their parent types.
My reasons for doubting whether natural species have changed as abruptly as domestic breeds sometimes have, and for entirely disbelieving that they've changed in the dramatic way Mivart suggests, are as follows. Based on experience, abrupt, strongly marked variations occur in our domestic animals singly and at rather long intervals. If such variations occurred in nature, they would be liable -- as I've previously explained -- to be lost by accidental death and by subsequent crossbreeding. And this is known to happen under domestication, unless such variations are specially preserved and separated by human care. Therefore, for a new species to suddenly appear as Mivart envisions, it's almost necessary to believe -- against all analogy -- that several wonderfully changed individuals appeared simultaneously in the same area. This difficulty, as with unconscious selection by humans, is avoided by the theory of gradual evolution, through the preservation of a large number of individuals that varied more or less in any favorable direction, and the death of a large number that varied in the opposite direction.
That many species have evolved in an extremely gradual manner can hardly be doubted. The species and even the genera of many large natural families are so closely related that it's difficult to tell many of them apart. On every continent, traveling from north to south, from lowland to upland, and so on, we encounter hosts of closely related or representative species, as we do on certain distinct continents that we have reason to believe were formerly connected. (In making these and the following remarks, I'm compelled to touch on subjects I'll discuss later.) Look at the many outlying islands around a continent, and see how many of their inhabitants can only be ranked as doubtful species. The same is true if we look to past times and compare species that have just disappeared with those still living in the same areas, or if we compare fossil species embedded in successive layers of the same geological formation. It's clear that multitudes of species are related in the closest way to other species that still exist or have recently existed, and it's hard to maintain that such species developed abruptly or suddenly. And we shouldn't forget that when we look at specific features of related species, rather than at distinct species, numerous wonderfully fine gradations can be traced connecting very different structures.
Many large groups of facts are intelligible only if species have evolved by very small steps. For instance, the fact that species in larger genera are more closely related to each other and show a greater number of varieties than species in smaller genera. The former are also grouped in small clusters, like varieties around species, and they show other parallels with varieties, as was demonstrated in our second chapter. On this same principle, we can understand why the features that define species are more variable than those that define genera, and why parts developed to an extraordinary degree are more variable than other parts of the same species. Many analogous facts, all pointing in the same direction, could be added.
Although many species have almost certainly been produced by steps no greater than those separating fine varieties, it might be argued that some developed in a different, abrupt manner. Such a claim, however, shouldn't be made without strong evidence. The vague and in some respects false analogies -- as Mr. Chauncey Wright has shown them to be -- that have been put forward in support of this view, such as the sudden crystallization of inorganic substances, or a faceted sphere tumbling from one facet to another, hardly deserve consideration. One class of evidence, however -- the sudden appearance of new and distinct life forms in our geological record -- does at first glance support the idea of abrupt development. But the value of this evidence depends entirely on the completeness of the geological record for remote periods of the world's history. If the record is as fragmentary as many geologists strongly assert, there's nothing surprising about new forms appearing as if they developed suddenly.
Unless we accept transformations as extraordinary as those advocated by Mivart -- such as the sudden development of bird or bat wings, or the sudden conversion of a *Hipparion* into a horse -- the belief in abrupt modification sheds hardly any light on the gaps in our geological record. But against the belief in such abrupt changes, embryology raises a powerful objection. It's well known that the wings of birds and bats, and the legs of horses and other mammals, are indistinguishable at an early embryonic stage and become differentiated only by imperceptibly fine steps. Embryological resemblances of all kinds can be explained -- as we'll see later -- by the idea that the ancestors of existing species varied after their early youth and passed their newly acquired features to their offspring at a corresponding age. The embryo is thus left nearly unchanged and serves as a record of the species' past condition. This is why existing species during the early stages of their development so often resemble ancient, extinct forms belonging to the same class. On this view of what embryological resemblances mean -- and indeed on any view -- it's incredible that an animal could have undergone such dramatic and abrupt transformations as those suggested, yet bear not even a trace in its embryonic development of any sudden change, with every detail of its structure developing instead by imperceptibly fine steps.
Anyone who believes that some ancient form was transformed suddenly through an internal force or tendency into, say, a creature with wings, will be almost forced to assume -- against all analogy -- that many individuals varied at the same time. It can't be denied that such abrupt, dramatic changes of structure are entirely different from what most species appear to have undergone. Such a person will further be forced to believe that many structures beautifully adapted to all the other parts of the same creature and to the surrounding conditions were suddenly produced. And for such complex, wonderful mutual adaptations, they won't be able to offer even a shadow of an explanation. They will be forced to admit that these great, sudden transformations left no trace in the embryo. To accept all this is, it seems to me, to enter the realm of miracles and to leave the realm of science.
Many instincts are so remarkable that their development will probably seem to the reader like a difficulty big enough to overthrow my entire theory. Let me say upfront that I'm not concerned here with the origin of mental abilities, any more than I am with the origin of life itself. We're dealing only with the differences in instinct and other mental abilities among animals of the same class.
I won't try to define instinct. It would be easy to show that several distinct mental actions are commonly lumped together under this term, but everyone understands what we mean when we say that instinct drives the cuckoo to migrate and to lay her eggs in other birds' nests. An action that we ourselves need experience to perform, but which an animal -- especially a very young one -- performs without experience, and which many individuals perform in the same way without knowing why they do it, is usually called instinctive. But I could show that none of these characteristics are universal. A small dose of judgment or reason, as the naturalist Pierre Huber put it, often comes into play even in animals low on the scale of nature.
Frederick Cuvier and several of the older philosophers compared instinct with habit. This comparison gives, I think, an accurate picture of the mental state during an instinctive action, but not necessarily of how it originated. Think about how unconsciously we perform many habitual actions -- sometimes even against our conscious will! Yet habits can be modified by will or reason. Habits easily become linked to other habits, to certain times of day, and to states of the body. Once acquired, they often remain constant throughout life. Several other similarities between instincts and habits could be pointed out. Just as when repeating a well-known song, so with instincts: one action follows another in a kind of rhythm. If a person is interrupted while singing a song or reciting something from memory, they're generally forced to go back to pick up the habitual train of thought. Pierre Huber found the same was true with a caterpillar that makes a very complicated cocoon. If he took a caterpillar that had completed its cocoon up to, say, the sixth stage of construction and placed it into a cocoon completed only up to the third stage, the caterpillar simply re-performed the fourth, fifth, and sixth stages. If, however, a caterpillar was taken out of a cocoon made up to the third stage and placed into one finished up to the sixth stage -- so that much of its work was already done for it -- far from benefiting, it was deeply confused. In order to complete its cocoon, it seemed forced to start from the third stage where it had left off, and so it tried to redo the already finished work.
If we suppose any habitual action to become inherited -- and it can be shown that this does sometimes happen -- then the resemblance between what was originally a habit and an instinct becomes so close that you can't tell them apart. If Mozart, instead of playing the piano at three years old with remarkably little practice, had played a tune with no practice at all, he could truly be said to have done so instinctively. But it would be a serious mistake to suppose that most instincts have been acquired by habit in a single generation and then passed on to the next. It can be clearly shown that the most remarkable instincts we know of -- namely, those of the honeybee and of many ants -- could not possibly have been acquired by habit.
Everyone will agree that instincts are as important as bodily structures for the survival of each species under its current conditions. Under changed conditions, it's at least possible that slight modifications of instinct might be beneficial to a species. And if instincts do vary even a little, then I see no difficulty in natural selection preserving and steadily accumulating variations of instinct to any extent that proves beneficial. This, I believe, is how all the most complex and remarkable instincts have originated. Just as modifications of bodily structure arise from and are increased by use or habit, and are reduced or lost by disuse, I have no doubt the same has happened with instincts. But I believe that the effects of habit are in many cases less important than the effects of natural selection acting on what we might call spontaneous variations of instinct -- that is, variations produced by the same unknown causes that produce slight differences in bodily structure.
No complex instinct can possibly arise through natural selection except by the slow and gradual accumulation of numerous small but beneficial variations. So, as with bodily structures, we should expect to find in nature not the actual transitional steps by which each complex instinct was acquired -- those could only be found in the direct ancestors of each species -- but we should find among related species some evidence of such gradations. Or at least we should be able to show that gradations of some kind are possible. And this we certainly can do. I've been surprised to find -- making allowance for the fact that animal instincts have been little observed except in Europe and North America, and that no instincts are known among extinct species -- how commonly gradations leading to the most complex instincts can be discovered. Changes of instinct may sometimes be helped along when a species has different instincts at different periods of life, at different seasons, or under different circumstances. In such cases, either one instinct or the other might be preserved by natural selection. And such cases of diverse instincts within the same species can be shown to occur in nature.
Again, as with bodily structure and in keeping with my theory, the instinct of each species serves its own good, but has never -- as far as we can tell -- been produced for the exclusive benefit of others. One of the strongest cases of an animal apparently acting solely for another's benefit is that of aphids voluntarily yielding their sweet secretion to ants, as Huber first observed. That they do so voluntarily, the following facts show. I removed all the ants from a group of about a dozen aphids on a dock plant and kept the ants away for several hours. After this, I felt sure the aphids would need to excrete. I watched them for some time through a magnifying glass, but not one excreted. I then tickled and stroked them with a hair, imitating as best I could the way ants use their antennae -- but still not one excreted. Then I allowed an ant to visit them, and it immediately seemed, by the eager way it ran about, to realize what a rich flock it had discovered. It began to stroke the abdomen of first one aphid and then another with its antennae, and each, as soon as it felt the touch, immediately lifted its abdomen and released a clear drop of sweet juice, which the ant eagerly devoured. Even the very young aphids behaved this way, showing the action was instinctive, not learned from experience. It's clear from Huber's observations that the aphids show no dislike of the ants: if the ants aren't present, the aphids are eventually forced to eject their secretion on their own. But since the secretion is extremely sticky, it's no doubt convenient for the aphids to have it removed. So they probably don't excrete solely for the ants' benefit. Although there's no evidence that any animal acts exclusively for the good of another species, each species tries to take advantage of the instincts of others, just as each takes advantage of the weaker bodily structure of other species. Certain instincts also cannot be considered absolutely perfect, but since the details of this and similar points aren't essential, I'll pass over them here.
Since some degree of variation in instincts under natural conditions, and the inheritance of those variations, are essential for natural selection to work, I should give as many examples as possible. But lack of space prevents me. I can only state that instincts certainly do vary -- for instance, the migratory instinct varies in both extent and direction, and is sometimes lost entirely. The same is true of birds' nests, which vary partly based on the locations chosen and the climate of the country, but often for reasons entirely unknown to us. The ornithologist Audubon gave several remarkable examples of differences in the nests of the same species in the northern and southern United States. It has been asked: if instinct is variable, why hasn't it given the bee "the ability to use some other material when wax runs short?" But what other natural material could bees use? They will work, as I have seen, with wax hardened with vermilion or softened with lard. The horticulturist Andrew Knight observed that his bees, instead of laboriously collecting propolis, used a cement of wax and turpentine with which he had covered stripped trees. It has also been shown that bees, instead of searching for pollen, will gladly use a very different substance -- namely, oatmeal. Fear of any particular predator is certainly an instinctive quality, as can be seen in nestling birds, though it's strengthened by experience and by seeing fear of the same predator in other animals. The fear of humans is slowly acquired, as I've shown elsewhere, by various animals on remote islands. We see an example of this even in England, in the greater wariness of all our large birds compared to our small ones, because the large birds have been most persecuted by people. We can safely attribute this greater wariness to that cause, for on uninhabited islands large birds are no more fearful than small ones. And the magpie, so wary in England, is tame in Norway, just as the hooded crow is tame in Egypt.
That the mental qualities of animals of the same kind, born in the wild, vary a great deal could be shown by many facts. Several cases could also be given of occasional and unusual habits in wild animals, which, if advantageous to the species, might have given rise through natural selection to new instincts. But I'm well aware that these general statements, without detailed evidence, can make only a weak impression on the reader's mind. I can only repeat my assurance that I don't speak without good evidence.
The possibility -- or even probability -- of inherited variations of instinct in nature will be strengthened by briefly considering a few cases under domestication. This will let us see the roles that habit and the selection of so-called spontaneous variations have played in modifying the mental qualities of our domestic animals. It's well known how much domestic animals vary in their mental qualities. With cats, for instance, one naturally takes to catching rats and another mice, and these tendencies are known to be inherited. One cat, according to Mr. St. John, always brought home game birds; another brought hares or rabbits; and another hunted on marshy ground and almost nightly caught woodcocks or snipes. Many curious and authentic examples could be given of various shades of temperament and preference, and likewise of the oddest tricks, linked to certain states of mind or times of day. But let's look at the familiar case of dog breeds. There's no doubt that young pointers -- I've seen striking examples myself -- will sometimes point and even back other dogs the very first time they're taken out. Retrieving is certainly inherited to some degree by retrievers. And shepherd dogs have an inherited tendency to run around a flock of sheep rather than straight at them. I can't see how these actions -- performed without experience by the young, performed in nearly the same way by each individual, performed with eager delight by each breed, and performed without understanding the purpose (for the young pointer can no more know that he points to help his master than the white butterfly knows why she lays her eggs on a cabbage leaf) -- I can't see how these actions differ in any essential way from true instincts. If we were to see one kind of wolf, when young and without any training, stand motionless like a statue as soon as it scented its prey and then slowly creep forward with a distinctive gait, and another kind of wolf rushing around a herd of deer rather than straight at them, driving them to a distant point, we would certainly call these actions instinctive. Domestic instincts, as we might call them, are certainly far less fixed than natural instincts. But they've been shaped by far less rigorous selection and have been passed down for an incomparably shorter period, under less stable conditions of life.
How strongly these domestic instincts, habits, and dispositions are inherited -- and how curiously they become blended -- is well shown when different breeds of dogs are crossed. A cross with a bulldog, for example, has been known to affect the courage and stubbornness of greyhounds for many generations. And a cross with a greyhound has given a whole family of shepherd dogs a tendency to chase hares. These domestic instincts, when tested by crossing, resemble natural instincts, which similarly become curiously blended and for a long time show traces of both parents' instincts. For example, Le Roy described a dog whose great-grandfather was a wolf, and this dog showed a trace of its wild ancestry in only one way: by not coming in a straight line to his master when called.
Domestic instincts are sometimes described as actions that became inherited solely from long-continued and forced habit, but this isn't true. No one would ever have thought of teaching -- and probably couldn't have taught -- the tumbler pigeon to tumble, an action which, as I've witnessed, is performed by young birds that have never seen a pigeon tumble. We can believe that some individual pigeon showed a slight tendency to this strange habit, and that the long-continued selection of the best individuals in successive generations made tumblers what they now are. Near Glasgow there are house-tumblers, as I hear from Mr. Brent, that can't fly eighteen inches high without going head over heels. It's doubtful whether anyone would have thought of training a dog to point if some dog hadn't naturally shown a tendency in that direction -- and this is known to happen occasionally. I once saw it in a pure terrier. The act of pointing is probably, as many have thought, just the exaggerated pause of an animal about to spring on its prey. Once the first tendency to point appeared, methodical selection and the inherited effects of forced training in each successive generation would soon complete the work. And unconscious selection is still going on, as each person tries to get dogs that stand and hunt best, without intending to improve the breed. On the other hand, habit alone has sometimes been enough. Hardly any animal is more difficult to tame than a young wild rabbit; hardly any is tamer than a young domestic rabbit. But I can hardly suppose that domestic rabbits have often been selected for tameness alone. So we must attribute at least the greater part of the inherited change -- from extreme wildness to extreme tameness -- to habit and long-continued close confinement.
Natural instincts are lost under domestication. A remarkable example is seen in those breeds of hens that very rarely or never become "broody" -- that is, they never want to sit on their eggs. Familiarity alone keeps us from seeing how greatly and permanently the minds of our domestic animals have been modified. It's hard to doubt that the love of humans has become instinctive in the dog. All wolves, foxes, jackals, and cat species, when kept tame, are eager to attack poultry, sheep, and pigs. This tendency has been found incurable in dogs brought home as puppies from countries like Tierra del Fuego and Australia, where the indigenous people don't keep these domestic animals. How rarely, on the other hand, do our domesticated dogs, even when quite young, need to be taught not to attack poultry, sheep, and pigs! They do occasionally make an attack, of course, and are then punished -- and if not cured, they're put down. So habit and some degree of selection have probably worked together to civilize our dogs by inheritance. On the other hand, young chickens have completely lost, through habit, the fear of dogs and cats that was no doubt originally instinctive. Captain Hutton tells me that the young chickens of the original wild stock, Gallus bankiva, when reared in India under a domestic hen, are at first extremely wild. The same is true of young pheasants reared in England under a hen. It's not that chickens have lost all fear -- only their fear of dogs and cats. If the hen gives her danger call, they will run (especially young turkeys) from under her and hide in the surrounding grass or thickets. This is clearly done for the instinctive purpose of allowing their mother to fly away, as we see in wild ground birds. But this instinct retained by our chickens has become useless under domestication, because the mother hen has almost lost the power of flight through disuse.
So we can conclude that under domestication, instincts have been acquired and natural instincts have been lost -- partly through habit and partly through humans selecting and accumulating, over successive generations, particular mental habits and actions that first appeared from what we must in our ignorance call accident. In some cases, forced habit alone has been enough to produce inherited mental changes. In other cases, forced habit has done nothing and everything has been the result of selection, pursued both deliberately and unconsciously. But in most cases, habit and selection have probably worked together.
We'll probably best understand how instincts in nature have been modified by selection by looking at a few specific cases. I'll choose just three: the instinct that drives the cuckoo to lay her eggs in other birds' nests; the slave-making instinct of certain ants; and the cell-building ability of the honeybee. These last two have been generally and rightly ranked by naturalists as the most remarkable of all known instincts.
Some naturalists believe that the immediate cause of the cuckoo's egg-laying instinct is that she lays her eggs not daily, but at intervals of two or three days. If she were to make her own nest and sit on her own eggs, the first ones laid would have to go unincubated for a while, or there would be eggs and young birds of different ages in the same nest. If so, the process of laying and hatching might be inconveniently long, especially since she migrates very early. And the first-hatched young would probably have to be fed by the male alone. But the American cuckoo is in exactly this situation, for she makes her own nest and has eggs and young of different ages all at the same time. It has been both claimed and denied that the American cuckoo occasionally lays her eggs in other birds' nests, but I've recently heard from Dr. Merrill, of Iowa, that he once found in Illinois a young cuckoo together with a young jay in the nest of a blue jay (Garrulus cristatus). Since both were nearly fully feathered, there could be no mistake in identifying them. I could also give several examples of various birds known to occasionally lay their eggs in other birds' nests.
Now let's suppose that the ancient ancestor of our European cuckoo had the habits of the American cuckoo and occasionally laid an egg in another bird's nest. If the mother bird benefited from this occasional habit -- by being able to migrate earlier, or for any other reason -- or if the young were made more vigorous by exploiting the misguided instinct of another species rather than being raised by their own mother (who could hardly avoid being burdened by having eggs and young of different ages at the same time), then either the old birds or the foster-raised young would gain an advantage. And we'd expect that the young raised this way would be likely to follow, by inheritance, their mother's occasional and unusual habit, and would in turn be inclined to lay their eggs in other birds' nests, and thus be more successful in raising their young. Through a continued process of this kind, I believe the strange instinct of our cuckoo was generated. It has also recently been established on good evidence, by Adolf Muller, that the cuckoo occasionally lays her eggs on bare ground, sits on them, and feeds her young. This rare event is probably a case of reversion to the long-lost original instinct of nest-building.
It has been objected that I haven't discussed other related instincts and structural adaptations in the cuckoo that are said to be necessarily coordinated. But speculation about an instinct known to us in only a single species is pointless, because until recently we had no facts to guide us. Until recently, only the instincts of the European and non-parasitic American cuckoo were known. Now, thanks to Mr. Ramsay's observations, we've learned something about three Australian species that lay their eggs in other birds' nests. The main points to note are three. First, the common cuckoo, with rare exceptions, lays only one egg per nest, so the large and voracious young bird gets plenty of food. Second, the eggs are remarkably small -- no bigger than a skylark's, even though a skylark is about one-quarter the size of a cuckoo. That this small egg size is a real adaptation we can infer from the fact that the non-parasitic American cuckoo lays full-sized eggs. Third, the young cuckoo, soon after birth, has the instinct, the strength, and a properly shaped back for ejecting its foster-siblings, who then die of cold and hunger. This has been boldly called a "beneficent arrangement" so that the young cuckoo may get enough food, and its foster-siblings may die before they've developed much feeling!
Turning now to the Australian species: although these birds generally lay only one egg per nest, it's not rare to find two or even three eggs in the same nest. In the bronze cuckoo, the eggs vary greatly in size, from eight to ten lines in length. Now, if it had been advantageous to this species to lay eggs even smaller than those it currently lays -- either to better deceive certain foster-parents, or more likely to hatch in a shorter period (since there seems to be a relationship between egg size and incubation time) -- then there's no difficulty in believing that a variety or species might have evolved that laid smaller and smaller eggs, since these would have been more safely hatched and reared. Mr. Ramsay notes that two of the Australian cuckoos, when they lay their eggs in an open nest, show a clear preference for nests containing eggs similar in color to their own. The European species apparently shows some tendency toward a similar instinct, but doesn't always follow it, as shown by her laying dull, pale-colored eggs in the nest of the hedge warbler, which has bright greenish-blue eggs. Had our cuckoo always displayed this instinct, it would certainly have been added to the list of instincts supposedly acquired all at once. The eggs of the Australian bronze cuckoo vary, according to Mr. Ramsay, to an extraordinary degree in color. So in this respect, as well as in size, natural selection might have secured and locked in any advantageous variation.
In the case of the European cuckoo, the foster-parents' own offspring are typically ejected from the nest within three days after the cuckoo hatches. Since the cuckoo at this age is in an utterly helpless state, the ornithologist Mr. Gould was initially inclined to think that the ejection was done by the foster-parents themselves. But he has now received a trustworthy account of a young cuckoo that was actually seen -- while still blind and unable even to hold up its own head -- in the act of ejecting its foster-siblings. One of them was placed back in the nest by the observer, and was thrown out again. As for how this strange and cruel instinct was acquired: if it was very important for the young cuckoo to receive as much food as possible soon after birth (as is probably the case), I see no special difficulty in its having gradually acquired, over successive generations, the blind urge, the strength, and the bodily structure needed for the work of ejection. Those cuckoos with the best-developed habits and structure for this would be the most securely raised. The first step toward acquiring this instinct might have been mere restlessness on the part of the young bird when somewhat older and stronger. The habit would then have been improved and passed on to an earlier age. I see no more difficulty in this than in the unhatched young of other birds acquiring the instinct to break through their own shells, or in young snakes having, as the anatomist Owen noted, a temporary sharp tooth in their upper jaws for cutting through tough eggshells. For if each body part is subject to individual variation at all ages, and these variations tend to be inherited at a corresponding or earlier age -- propositions that can't be disputed -- then the instincts and structure of the young could be slowly modified just as surely as those of the adult. Both cases must stand or fall together with the whole theory of natural selection.
Some species of Molothrus, a completely different genus of American birds related to our starlings, have parasitic habits like those of the cuckoo. The species present an interesting gradation in the perfection of their instincts. The sexes of Molothrus badius, according to the excellent observer Mr. Hudson, sometimes live promiscuously together in flocks and sometimes pair off. They either build their own nest or seize one belonging to another bird, occasionally throwing out the original nestlings. They either lay their eggs in the captured nest, or -- oddly enough -- build one of their own on top of it. They usually sit on their own eggs and raise their own young, but Mr. Hudson says they're probably occasionally parasitic, since he has seen the young of this species following adults of a different kind and begging to be fed.
The parasitic habits of another species, Molothrus bonariensis, are much more highly developed but still far from perfect. This bird, as far as we know, invariably lays its eggs in the nests of other species. But remarkably, several sometimes start to build an irregular, untidy nest of their own, placed in absurdly unsuitable spots, like on the leaves of a large thistle. They never, as far as Mr. Hudson has determined, actually complete a nest for themselves. They often lay so many eggs -- from fifteen to twenty -- in the same foster nest that few or none can be hatched. They also have the extraordinary habit of pecking holes in the eggs, whether their own species' or their foster-parents', that they find in the captured nests. They also drop many eggs on bare ground, where they're wasted.
A third species, Molothrus pecoris of North America, has acquired instincts as perfect as those of the cuckoo, for it never lays more than one egg in a foster nest, so the young bird is securely raised. Mr. Hudson is a strong disbeliever in evolution, but he appears to have been so struck by the imperfect instincts of Molothrus bonariensis that he quotes my words and asks: "Must we consider these habits, not as specially endowed or created instincts, but as small consequences of one general law, namely, transition?"
Various birds, as I've already mentioned, occasionally lay their eggs in the nests of other birds. This habit is not uncommon in the Gallinaceae (the chicken-like birds) and sheds some light on the singular instinct of the ostrich. In this family, several hens join together and lay a few eggs first in one nest, then in another, and these are hatched by the males. This instinct can probably be explained by the fact that the hens lay a large number of eggs, but, as with the cuckoo, at intervals of two or three days. The instinct of the American ostrich, however, like that of Molothrus bonariensis, hasn't yet been perfected. A surprising number of eggs lie scattered over the plains, so that in one day's hunting I picked up no fewer than twenty lost and wasted eggs.
Many bees are parasitic and regularly lay their eggs in the nests of other kinds of bees. This case is even more remarkable than the cuckoo's, because these bees have had not only their instincts but their body structure modified in line with their parasitic habits: they don't possess the pollen-collecting apparatus that would be essential if they stored up food for their own young. Some species of Sphegidae (wasp-like insects) are likewise parasitic. The entomologist M. Fabre has recently shown good reason to believe that although Tachytes nigra generally makes its own burrow and stocks it with paralyzed prey for its larvae, when this insect finds a burrow already made and stocked by another wasp, it takes advantage of the prize and becomes a parasite for the occasion. In this case, as with Molothrus or the cuckoo, I see no difficulty in natural selection making an occasional habit permanent, if it benefits the species, and if the insect whose nest and stored food are stolen isn't driven to extinction as a result.
This remarkable instinct was first discovered in Formica (Polyerges) rufescens by the naturalist Pierre Huber, an even better observer than his celebrated father. This ant is absolutely dependent on its slaves. Without their help, the species would certainly go extinct within a single year. The males and fertile females do no work of any kind, and the workers or sterile females, though extremely energetic and courageous in capturing slaves, do no other work. They're incapable of building their own nests or feeding their own larvae. When the old nest becomes inconvenient and they need to migrate, it's the slaves that decide on the migration and actually carry their masters in their jaws. The masters are so utterly helpless that when Huber shut up thirty of them without a slave, but with plenty of their favorite food and with their larvae and pupae to stimulate them to work, they did nothing. They couldn't even feed themselves, and many starved to death. Huber then introduced a single slave (F. fusca), and she instantly went to work, fed and saved the survivors, made some cells, tended the larvae, and put everything in order. What could be more extraordinary than these well-documented facts? If we knew of no other slave-making ant, it would be hopeless to speculate how such a remarkable instinct could have evolved.
Another species, Formica sanguinea, was also first identified by Pierre Huber as a slave-making ant. This species is found in southern England, and its habits have been studied by Mr. F. Smith of the British Museum, to whom I owe a great deal for information on this and other subjects. Although I fully trusted Huber's and Smith's accounts, I tried to approach the subject with a skeptical mindset, since anyone might be excused for doubting the existence of so extraordinary an instinct as that of making slaves. So I'll describe my own observations in some detail.
I opened fourteen nests of F. sanguinea and found a few slaves in all of them. Males and fertile females of the slave species (F. fusca) are found only in their own independent colonies and have never been observed in the nests of F. sanguinea. The slaves are black and no more than half the size of their red masters, so the contrast in appearance is striking. When the nest is slightly disturbed, the slaves occasionally come out and, like their masters, are clearly agitated and defend the nest. When the nest is heavily disturbed and the larvae and pupae are exposed, the slaves work energetically alongside their masters to carry them to safety. Clearly, the slaves feel quite at home.
During June and July, over three successive years, I spent many hours watching several nests in Surrey and Sussex and never saw a slave either leave or enter a nest. Since slaves are very few in number during these months, I thought they might behave differently when more numerous. But Mr. Smith tells me that he has watched the nests at various hours during May, June, and August, both in Surrey and Hampshire, and has never seen the slaves -- though present in large numbers in August -- either leave or enter the nest. He considers them strictly household slaves. The masters, on the other hand, can constantly be seen bringing in building materials and food of all kinds.
During 1860, however, in July, I came across a colony with an unusually large stock of slaves, and I observed a few slaves mingled with their masters leaving the nest and marching along the same path to a tall Scotch fir tree twenty-five yards away, which they climbed together, probably in search of aphids or scale insects. According to Huber, who had ample opportunities for observation, the slaves in Switzerland habitually work alongside their masters in building the nest, and they alone open and close the doors morning and evening. As Huber specifically states, their main task is to search for aphids. This difference in the usual habits of masters and slaves in the two countries probably depends simply on the slaves being captured in greater numbers in Switzerland than in England.
One day I was lucky enough to witness a migration of F. sanguinea from one nest to another, and it was a fascinating spectacle to see the masters carefully carrying their slaves in their jaws -- instead of being carried by them, as happens with F. rufescens. Another day my attention was caught by about twenty of the slave-makers hovering around the same spot, clearly not searching for food. They approached and were vigorously fought off by an independent colony of the slave species (F. fusca). Sometimes as many as three of these ants were clinging to the legs of a single slave-making F. sanguinea. The slave-makers ruthlessly killed their small opponents and carried their dead bodies as food back to their nest, twenty-nine yards away. But they were prevented from capturing any pupae to raise as slaves. I then dug up a small batch of F. fusca pupae from another nest and placed them on bare ground near the battle site. They were eagerly seized and carried off by the raiders, who perhaps fancied that they had been victorious in their recent fight after all.
At the same time, I placed nearby a small batch of pupae of another species, F. flava, with a few of these little yellow ants still clinging to the fragments of their nest. This species is sometimes, though rarely, enslaved, as Mr. Smith has described. Although very small, it's extremely courageous, and I've seen it ferociously attack other ants. In one case, I was surprised to find an independent colony of F. flava under a stone beneath a nest of the slave-making F. sanguinea. When I accidentally disturbed both nests, the little ants attacked their big neighbors with astonishing courage.
Now I was curious to find out whether F. sanguinea could distinguish the pupae of F. fusca, which they regularly enslave, from those of the little and fierce F. flava, which they rarely capture. It was clear that they could tell the difference immediately. As we've seen, they eagerly and instantly seized the pupae of F. fusca. But they were clearly terrified when they came across the pupae -- or even the soil from the nest -- of F. flava, and quickly ran away. After about a quarter of an hour, however, shortly after all the little yellow ants had crawled away, they took heart and carried off the pupae.
One evening I visited another colony of F. sanguinea and found a number of these ants returning home and entering their nest carrying the dead bodies of F. fusca (showing this was not a migration) and numerous pupae. I traced a long file of ants burdened with their plunder for about forty yards back to a very thick clump of heath, from which I saw the last individual of F. sanguinea emerge carrying a pupa. But I wasn't able to find the raided nest in the thick undergrowth. The nest must have been close by, however, because two or three F. fusca individuals were rushing about in extreme agitation, and one was perched motionless with its own pupa in its mouth on top of a sprig of heath -- a picture of despair over its devastated home.
Such are the facts -- though they didn't need confirmation by me -- regarding the remarkable instinct of making slaves. Notice what a contrast the instinctive habits of F. sanguinea present compared to those of the continental F. rufescens. The latter doesn't build its own nest, doesn't determine its own migrations, doesn't collect food for itself or its young, and can't even feed itself: it's absolutely dependent on its numerous slaves. Formica sanguinea, on the other hand, has far fewer slaves, and in early summer extremely few. The masters decide when and where to form a new nest, and when they migrate, the masters carry the slaves. Both in Switzerland and England, the slaves seem to have sole charge of the larvae, and the masters alone go on slave-making raids. In Switzerland, the slaves and masters work together building and bringing materials for the nest. Both -- but mainly the slaves -- tend and "milk," as we might say, their aphids, and so both collect food for the community. In England, the masters alone usually leave the nest to gather building materials and food for themselves, their slaves, and larvae. So the masters in England receive much less service from their slaves than they do in Switzerland.
How did the instinct of F. sanguinea originate? I won't pretend to guess. But since ants that are not slave-makers will, as I've seen, carry off pupae of other species when they're scattered near their nests, it's possible that pupae originally stored as food might sometimes survive and develop. The foreign ants thus unintentionally raised would then follow their own instincts and do whatever work they could. If their presence proved useful to the species that had seized them -- if it was more advantageous to capture workers than to produce them -- the habit of collecting pupae, originally for food, might through natural selection be strengthened and made permanent for the very different purpose of raising slaves. Once acquired, even if carried out to a much lesser extent than in our British F. sanguinea (which, as we've seen, gets less help from its slaves than the same species in Switzerland), natural selection might increase and modify the instinct -- always assuming each modification to be useful to the species -- until an ant was formed as completely dependent on its slaves as Formica rufescens.
I won't go into minute details on this subject but will simply outline the conclusions I've reached. You'd have to be a dull person to examine the exquisite structure of a honeycomb, so beautifully adapted to its purpose, without enthusiastic admiration. Mathematicians tell us that bees have essentially solved a difficult geometric problem, making their cells the exact shape to hold the greatest possible amount of honey while using the least possible amount of precious wax in construction. It's been noted that a skilled craftsman with the right tools and measurements would find it very difficult to make cells of wax with the correct shape, even though this is accomplished by a crowd of bees working in a dark hive. No matter how much instinct you grant them, it seems at first completely inconceivable how they can make all the necessary angles and surfaces, or even tell when they've got them right. But the difficulty is not nearly as great as it first appears. All this beautiful work can be shown, I think, to follow from a few simple instincts.
I was led to investigate this subject by Mr. Waterhouse, who showed that the shape of each cell is closely related to the presence of neighboring cells. The following view may perhaps be considered only a modification of his theory. Let's look at the great principle of gradation and see whether nature reveals her method of work.
At one end of a short series, we have bumblebees, which use their old cocoons to hold honey, sometimes adding short tubes of wax, and also making separate, very irregular rounded cells of wax. At the other end, we have the cells of the honeybee, arranged in a double layer. Each cell, as is well known, is a hexagonal prism with the basal edges of its six sides beveled to form an inverted pyramid of three rhombuses. These rhombuses have specific angles, and the three that form the pyramidal base of a single cell on one side of the comb fit into the bases of three adjacent cells on the opposite side.
Between these two extremes -- the perfection of the honeybee's cells and the simplicity of the bumblebee's -- we have the cells of the Mexican Melipona domestica, carefully described and illustrated by Pierre Huber. The Melipona itself is intermediate in structure between the honeybee and bumblebee, but more closely related to the latter. It forms a nearly regular waxen comb of cylindrical cells in which the young are hatched, plus some large cells of wax for storing honey. These honey cells are nearly spherical and nearly equal in size, and they're clustered into an irregular mass. But the key point is this: these cells are always built close enough together that they would intersect or break into each other if the spheres were completed. But this never happens, because the bees build perfectly flat walls of wax between the spheres at the points where they would overlap. So each cell consists of an outer spherical portion plus two, three, or more flat surfaces, depending on how many other cells it touches. When one cell rests on three others -- which, because the spheres are nearly the same size, is very often the case -- the three flat surfaces form a pyramid. This pyramid, as Huber noted, is clearly a rough version of the three-sided pyramidal base of a honeybee's cell. Just as in the honeybee's cells, the three flat surfaces of any one cell also form part of the construction of three neighboring cells. The Melipona obviously saves wax and, more importantly, labor by building this way, because the flat walls between adjacent cells aren't doubled -- they're the same thickness as the outer spherical portions, yet each flat wall forms part of two cells.
Thinking about this case, it occurred to me that if the Melipona made its spheres at a set distance from each other, made them equal in size, and arranged them symmetrically in a double layer, the resulting structure would be as perfect as the comb of the honeybee. I wrote to Professor Miller of Cambridge, and this mathematician kindly reviewed the following statement, drawn up from his information, and confirmed that it is strictly correct:
If a number of equal spheres are placed with their centers in two parallel layers, with the center of each sphere at a distance of radius times the square root of 2 (or radius times 1.41421), or at some lesser distance, from the centers of the six surrounding spheres in the same layer, and at the same distance from the centers of the adjacent spheres in the other parallel layer -- then, if planes of intersection between the spheres in both layers are drawn, the result will be a double layer of hexagonal prisms joined together by pyramidal bases made of three rhombuses. And the rhombuses and the sides of the hexagonal prisms will have every angle exactly the same as the best measurements that have been made of honeybee cells.
But I'm told by Professor Wyman, who has made many careful measurements, that the accuracy of the bee's workmanship has been greatly exaggerated -- so much so that whatever the ideal form of the cell may be, it's rarely if ever actually achieved.
So we can safely conclude that if we could slightly modify the instincts the Melipona already possesses -- instincts that are not particularly remarkable in themselves -- this bee would build a structure as wonderfully perfect as the honeybee's. We'd need to suppose the Melipona could form truly spherical cells of equal sizes -- and this wouldn't be very surprising, since she already does so approximately, and since many insects can bore perfectly cylindrical tunnels in wood, apparently by turning around a fixed point. We'd need to suppose she arranges her cells in flat layers, as she already does with her cylindrical cells. And we'd need to suppose -- and this is the greatest difficulty -- that she can somehow judge accurately how far to stand from her fellow workers when several are making their spheres. But she already has some ability to judge distance, since she always makes her spheres close enough that they partially intersect, and then she joins the intersection points with perfectly flat surfaces. Through such modifications of instincts that aren't very remarkable in themselves -- hardly more remarkable than those that guide a bird to build its nest -- I believe the honeybee has acquired, through natural selection, her matchless architectural powers.
But this theory can be tested by experiment. Following the example of Mr. Tegetmeier, I separated two combs and placed between them a long, thick, rectangular strip of wax. The bees immediately began excavating tiny circular pits in it. As they deepened these little pits, they widened them until they became shallow basins, appearing to the eye as perfectly true parts of a sphere, about the diameter of a cell. It was fascinating to observe that wherever several bees had begun excavating these basins close together, they had started their work at just the right distance from each other so that by the time the basins reached the width of an ordinary cell, and were about one-sixth the depth of the sphere they formed part of, their rims intersected. As soon as this happened, the bees stopped excavating and began building flat walls of wax along the lines of intersection. Each hexagonal prism was thus built on the scalloped edge of a smooth basin, rather than on the straight edges of a three-sided pyramid as in ordinary cells.
I then placed in the hive, instead of a thick rectangular piece of wax, a thin, narrow, knife-edged ridge colored with vermilion. The bees immediately began excavating little basins on both sides, close together, just as before. But the ridge of wax was so thin that the bottoms of the basins, if excavated to the same depth as in the first experiment, would have broken through from opposite sides. The bees didn't let this happen: they stopped excavating in time. So the basins, once they had been slightly deepened, came to have flat bottoms. These flat bottoms, formed by thin little plates of ungnawed vermilion wax, were positioned -- as far as the eye could judge -- exactly along the planes of imaginary intersection between the basins on the opposite side of the ridge. In some parts, only small portions of a rhombic plate were left between opposing basins; in other parts, large portions. The work wasn't neatly done, given the unnatural conditions. The bees must have worked at very nearly the same rate on both sides of the vermilion wax ridge in order to have succeeded in leaving flat plates between the basins by stopping exactly at the planes of intersection.
Given how flexible thin wax is, I don't see any difficulty in the bees, while working on the two sides of a strip of wax, being able to tell when they've gnawed the wax to the right thinness, and then stopping. In ordinary combs, it seemed to me that the bees don't always succeed in working at exactly the same rate from opposite sides. I've noticed half-completed rhombuses at the base of a just-started cell that were slightly concave on one side (where I suppose the bees had excavated too fast) and convex on the opposite side (where they'd worked more slowly). In one clear case, I put the comb back into the hive, let the bees work for a short time, and examined the cell again. I found that the rhombic plate had been completed and was now perfectly flat. It was absolutely impossible, given the extreme thinness of the plate, that they could have achieved this by gnawing away the convex side. I suspect that in such cases, bees standing in the opposing cells push and bend the soft, warm wax (which, as I've tried, is easily done) into the correct intermediate plane, and so flatten it.
From the experiment with the vermilion wax ridge, we can see that if bees were to build a thin wall of wax for themselves, they could make their cells the proper shape by standing at the right distance from each other, excavating at the same rate, and trying to make equal spherical hollows while never letting the spheres break through. Now bees -- as you can clearly see by examining the edge of a growing comb -- do make a rough surrounding wall or rim all around the comb. They gnaw this away from opposite sides, always working in circles as they deepen each cell. They don't build the whole three-sided pyramidal base of any one cell at the same time, but only the one rhombic plate on the extreme growing edge, or two plates, as the case may be. And they never complete the upper edges of the rhombic plates until the hexagonal walls have been started. Some of these statements differ from those of the justly celebrated elder Huber, but I'm convinced of their accuracy. If I had space, I could show they're consistent with my theory.
Huber stated that the very first cell is excavated from a little parallel-sided wall of wax. This isn't, as far as I've seen, strictly correct: the first beginning is always a little hood of wax. But I won't go into those details here. We can see how important a role excavation plays in cell construction. But it would be a serious mistake to suppose that bees can't also build up a rough wall of wax in the right place -- that is, along the plane of intersection between two adjacent spheres. I have several specimens clearly showing they can do this. Even in the rough surrounding rim of a growing comb, curves can sometimes be seen that correspond in position to the planes of the rhombic base plates of future cells. But the rough wall of wax always has to be finished off by extensive gnawing on both sides.
The way bees build is curious: they always make the first rough wall ten to twenty times thicker than the incredibly thin finished wall of the cell that will ultimately remain. We can understand how they work by imagining masons first piling up a broad ridge of cement, then cutting it away equally on both sides near the ground until a smooth, very thin wall is left in the middle -- with the masons always piling the cut-away cement, plus fresh cement, on top of the ridge. The result would be a thin wall steadily growing upward but always topped by a massive cap. Because all cells -- both newly started and completed ones -- are crowned by a strong capping of wax, the bees can cluster and crawl over the comb without damaging the delicate hexagonal walls. These walls, as Professor Miller has kindly measured for me, vary considerably in thickness, averaging (from twelve measurements near the border of the comb) 1/352 of an inch. The basal rhombic plates are thicker, in roughly a three-to-two ratio, averaging (from twenty-one measurements) 1/229 of an inch. Through this remarkable building method, strength is continually given to the comb with the greatest ultimate economy of wax.
It might seem to add to the difficulty that many bees all work together -- one bee working briefly at one cell then moving to another, so that, as Huber noted, as many as twenty individuals work even on the very first cell. I was able to demonstrate this by coating the edges of the hexagonal walls of a single cell, or the outer rim of a growing comb, with an extremely thin layer of melted vermilion wax. I invariably found that the color was spread with the greatest delicacy by the bees -- as delicately as a painter could have done with a brush -- with tiny bits of the colored wax taken from the spot where it had been placed and worked into the growing edges of cells all around. The construction process seems to be a kind of balance struck among many bees, all instinctively standing at the same relative distance from each other, all trying to scoop out equal spheres, and then building up -- or leaving ungnawed -- the planes of intersection between these spheres. It was really fascinating to see, in difficult cases like where two pieces of comb met at an angle, how often the bees would tear down and rebuild the same cell in different ways, sometimes reverting to a shape they had initially rejected.
When bees have a surface to stand on in their proper working positions -- for instance, a slip of wood placed directly under the middle of a comb growing downward, so the comb has to be built over one face of the slip -- in this case, the bees can lay the foundation of one wall of a new hexagon in exactly the right place, projecting beyond the other completed cells. It's enough for the bees to stand at their proper relative distances from each other and from the walls of the last completed cells. Then, by tracing imaginary spheres, they can build a wall in between two adjacent spheres. But as far as I've seen, they never gnaw away and finish off the angles of a cell until a large portion of both that cell and the neighboring cells has been built. This ability of bees to lay down a rough wall in the correct position between two just-started cells is important, because it relates to a fact that at first seems to undermine the theory I've described: namely, that the cells on the extreme edge of wasp combs are sometimes perfectly hexagonal. But I don't have space to go into this subject. Nor does there seem to me any great difficulty in a single insect (like a queen wasp) making hexagonal cells, if she were to work alternately on the inside and outside of two or three cells started at the same time, always standing at the right relative distance from the parts of the cells just begun, tracing spheres or cylinders, and building up the intermediate surfaces.
Since natural selection works only by accumulating slight modifications of structure or instinct, each beneficial to the individual under its conditions of life, it's reasonable to ask: how could a long and gradual succession of modified building instincts, all tending toward the present perfect plan of construction, have benefited the ancestors of the honeybee? I think the answer isn't hard. Cells built like those of the bee or the wasp gain in strength and save greatly on labor, space, and building materials. As for wax production, it's known that bees are often hard pressed to gather enough nectar. Mr. Tegetmeier tells me that experiments have shown that a hive of bees consumes twelve to fifteen pounds of dry sugar to produce a single pound of wax. So a tremendous quantity of nectar must be collected and consumed for the wax needed to build their combs. Moreover, many bees have to sit idle for days during the secretion process. A large store of honey is essential to support a large colony of bees through the winter, and the security of the hive depends mainly on having a large number of bees. So saving wax (which means saving honey, which means saving the time spent collecting it) must be a key factor in any bee family's success.
Of course, a species' success may depend on the number of its predators or parasites, or on entirely different factors, and may have nothing to do with how much honey the bees can collect. But let's suppose that the amount of honey often has determined -- as it probably has -- whether a bee related to our bumblebee could thrive in large numbers in a given area. And let's further suppose that the colony lives through the winter and therefore needs a honey store. In that case, there's no doubt that it would be an advantage to our imaginary bumblebee if a slight modification of her instincts led her to build her wax cells closer together, so they overlapped a bit -- because even a single shared wall between two adjacent cells would save some labor and wax. So it would be progressively more and more advantageous for our bumblebees to make their cells more regular, nearer together, and clustered into a mass, like the cells of the Melipona. In that case, much of the outer surface of each cell would serve as a boundary for adjacent cells, and much labor and wax would be saved. Similarly, it would benefit the Melipona to make her cells closer together and more regular in every way, because then, as we've seen, the spherical surfaces would disappear entirely, replaced by flat ones. The Melipona would make a comb as perfect as the honeybee's. Beyond this stage of architectural perfection, natural selection could not lead, for the honeybee's comb, as far as we can tell, is absolutely perfect in its economy of labor and wax.
So I believe the most remarkable of all known instincts -- that of the honeybee -- can be explained by natural selection taking advantage of numerous, successive, slight modifications of simpler instincts. Natural selection has, by slow degrees, more and more perfectly led the bees to scoop out equal spheres at a given distance from each other in a double layer, and to build up and excavate the wax along the planes of intersection. The bees, of course, no more know that they're making spheres at one particular distance from each other than they know the precise angles of the hexagonal prisms and the basal rhombic plates. The driving force behind the process of natural selection has been the construction of cells of the right strength, size, and shape for the larvae -- achieved with the greatest possible economy of labor and wax. The individual swarm that built the best cells with the least labor and the least waste of honey in producing wax succeeded best, passed on their newly acquired economical instincts to new swarms, and those swarms in turn had the best chance of succeeding in the struggle for existence.
It has been objected to the above view of the origin of instincts that "variations of structure and of instinct must have been simultaneous and perfectly adjusted to each other, since a modification in one without an immediate corresponding change in the other would have been fatal." The force of this objection rests entirely on the assumption that the changes in instinct and structure are sudden. To illustrate: consider the great titmouse (Parus major), mentioned in a previous chapter, which often holds yew seeds between its feet on a branch and hammers with its beak to get at the kernel. What special difficulty would there be in natural selection preserving all the slight individual variations in beak shape that were better and better adapted to cracking open seeds, until a beak was formed as well-suited for this purpose as the nuthatch's -- at the same time that habit, or necessity, or spontaneous changes in taste led the bird to become more and more of a seed-eater? In this case, the beak is supposed to be slowly modified by natural selection in response to slowly changing habits or taste. But suppose the titmouse's feet also grew larger, from correlated development with the beak, or from some other unknown cause. It's not unlikely that such larger feet would lead the bird to climb more and more until it acquired the remarkable climbing instinct and ability of the nuthatch. In this case, a gradual change of structure leads to changed instinctive habits.
To take another case: few instincts are more remarkable than the one that leads the swift of the Eastern Islands to make its nest entirely of thickened saliva. Some birds build their nests of mud, thought to be moistened with saliva. One of the swifts of North America makes its nest, as I've seen, of sticks glued together with saliva, and even with flakes of this substance. Is it really so unlikely that the natural selection of individual swifts that produced more and more saliva should eventually produce a species with instincts leading it to abandon other materials and make its nest exclusively of thickened saliva? And so with other cases. It must be admitted, however, that in many instances we can't tell whether instinct or structure changed first.
No doubt many instincts that are very hard to explain could be brought up against the theory of natural selection -- cases where we can't see how an instinct could have originated; cases where no intermediate steps are known to exist; cases of instincts so trivial they could hardly have been shaped by natural selection; cases of instincts almost identical in animals so far apart on the tree of life that we can't explain the similarity by inheritance from a common ancestor and must believe they were independently acquired through natural selection. I won't go into all these cases here but will focus on one special difficulty that at first seemed to me insurmountable and actually fatal to the whole theory. I mean the sterile females in insect colonies: these sterile workers often differ widely in instinct and in structure from both the males and fertile females, and yet, being sterile, they can't pass on their traits to offspring.
This subject deserves to be discussed at great length, but I'll take only a single case: that of worker or sterile ants. How the workers became sterile is a difficulty, but not a much greater one than any other striking structural modification. It can be shown that some insects and other jointed animals in nature occasionally become sterile. If such insects were social, and it was beneficial to the colony for a number to be born each year that could work but couldn't reproduce, I see no particular difficulty in this having been brought about through natural selection. But I must pass over this preliminary difficulty.
The great difficulty lies in the fact that worker ants differ widely from both males and fertile females in structure -- in the shape of the thorax, in lacking wings, and sometimes in lacking eyes -- and in instinct. As far as instinct alone goes, the extraordinary difference between workers and fertile females would have been even better illustrated by the honeybee. If a worker ant or other sterile insect had been an ordinary animal, I would have unhesitatingly assumed that all its characteristics had been slowly acquired through natural selection -- by individuals born with slight beneficial modifications that were inherited by their offspring, who then varied further and were selected again, and so on. But with the worker ant, we have an insect differing greatly from its parents yet absolutely sterile, so it could never have passed on successively acquired modifications of structure or instinct to offspring. You might well ask: how is it possible to reconcile this with the theory of natural selection?
First, remember that we have countless examples, both in domestic animals and in nature, of all sorts of inherited structural differences linked to certain ages and to either sex. We have differences linked not just to one sex but to the short period when the reproductive system is active, as in the mating plumage of many birds and the hooked jaws of the male salmon. We even have slight differences in the horns of different cattle breeds related to an artificially altered state of the male: oxen of certain breeds have longer horns than oxen of other breeds, relative to the horn length of both bulls and cows of those same breeds. So I see no great difficulty in any trait becoming linked to the sterile condition of certain members of insect colonies. The difficulty lies in understanding how such linked modifications of structure could have been slowly accumulated by natural selection.
This difficulty, though seemingly insurmountable, is lessened -- or as I believe, disappears -- when you remember that selection can be applied to the family as well as to the individual, and can thus achieve the desired result. Cattle breeders want the flesh and fat to be well-marbled together. An animal with this quality has been slaughtered, but the breeder has gone with confidence to the same stock and succeeded. Such is the power of selection that a breed of cattle always producing oxen with extraordinarily long horns could probably be formed by carefully watching which individual bulls and cows, when mated, produced oxen with the longest horns -- even though no ox would ever have reproduced.
Here's a better and real example. According to M. Verlot, some varieties of the double annual stock plant, from having been long and carefully selected to the right degree, always produce a large proportion of seedlings bearing double and completely sterile flowers, but they also yield some single and fertile plants. These fertile plants, by which alone the variety can be propagated, can be compared to the fertile male and female ants, and the double sterile plants to the sterile workers of the same colony. As with the stock varieties, so with social insects: selection has been applied to the family, not the individual, for the sake of a useful result.
So we can conclude that slight modifications of structure or instinct, linked to the sterile condition of certain community members, have proved advantageous. Consequently, the fertile males and females have thrived and passed on to their fertile offspring a tendency to produce sterile members with the same modifications. This process must have been repeated many times until the enormous difference between fertile and sterile females of the same species -- which we see in many social insects -- was produced.
But we haven't yet touched on the peak of the difficulty: the fact that the sterile workers of several ant species differ not only from the fertile females and males but from each other, sometimes to an almost incredible degree, and are thus divided into two or even three castes. These castes generally don't grade into each other but are perfectly distinct -- as different from each other as any two species of the same genus, or even any two genera of the same family. In Eciton, for example, there are worker and soldier castes with extraordinarily different jaws and instincts. In Cryptocerus, the workers of one caste alone carry a remarkable sort of shield on their heads, whose function is completely unknown. In the Mexican Myrmecocystus, the workers of one caste never leave the nest. They're fed by workers of another caste and have enormously swollen abdomens that secrete a kind of honey, taking the place of the honeydew produced by the aphids -- or "domestic cattle," as we might call them -- that our European ants guard and keep captive.
It will certainly seem that I have an overweening confidence in natural selection when I don't admit that such remarkable and well-established facts destroy the theory outright. In the simpler case of sterile insects all of one caste, which I believe have been made different from the fertile males and females through natural selection, we can conclude by analogy with ordinary variation that the successive, slight, beneficial modifications didn't first arise in all the sterile individuals in the same nest but only in some few. Through the survival of colonies whose females produced the most sterile individuals with the advantageous modification, all the sterile workers eventually came to have these traits.
According to this view, we should occasionally find, in the same nest, sterile insects showing gradations of structure. And we do find this, even fairly often, considering how few sterile insects outside Europe have been carefully examined. Mr. F. Smith has shown that the sterile workers of several British ants differ surprisingly from each other in size and sometimes in color, and that the extreme forms can be linked by intermediates taken from the same nest. I've compared such perfect gradations myself. Sometimes the larger or the smaller workers are the most numerous, or both large and small are numerous while those of intermediate size are scarce. Formica flava has larger and smaller workers with a few of intermediate size. In this species, as Mr. Smith has observed, the larger workers have simple eyes (ocelli), which, though small, are clearly visible, while the smaller workers have their ocelli in a rudimentary state. Having carefully dissected several specimens of these workers, I can confirm that the eyes are far more rudimentary in the smaller workers than can be explained merely by their proportionally smaller size. And I fully believe -- though I dare not state it with complete certainty -- that the workers of intermediate size have their ocelli in an exactly intermediate condition.
So here we have two groups of sterile workers in the same nest, differing not only in size but in their organs of vision, yet connected by a few members in an intermediate state. Let me add a thought experiment: if the smaller workers had been the most useful to the colony, and those males and females had been continually selected that produced more and more of the smaller workers until all the workers were of this type, we would then have had an ant species with sterile workers in nearly the same condition as those of Myrmica. For the workers of Myrmica don't even have rudiments of ocelli, though the male and female ants of this genus have well-developed ones.
I can give one more case. So confident was I of occasionally finding gradations of important structures between different castes of sterile workers in the same species that I gladly accepted Mr. F. Smith's offer of numerous specimens from the same nest of the driver ant (Anomma) of West Africa. The reader will perhaps best appreciate the amount of difference in these workers through this strictly accurate comparison: the difference was as if we were to see a set of workers building a house, of whom many were five feet four inches tall and many were sixteen feet tall -- but we must also suppose that the larger workers had heads four times as big (instead of three times) as those of the smaller ones, and jaws nearly five times as big. The jaws of the various-sized worker ants also differed wonderfully in shape and in the form and number of teeth. But the important fact for us is that although the workers can be grouped into distinct size classes, they grade imperceptibly into each other, as do their widely different jaw structures. I speak confidently on this last point, as Sir J. Lubbock made drawings for me, using a camera lucida, of the jaws I dissected from workers of the different sizes. The naturalist Mr. Bates, in his fascinating book The Naturalist on the Amazons, has described similar cases.
With these facts before me, I believe that natural selection, by acting on the fertile parents, could produce a species that regularly generates sterile workers all of large size with one form of jaw, or all of small size with very different jaws, or -- and this is the greatest difficulty -- one set of workers of one size and structure alongside another set of workers of a different size and structure. A graded series would first have been formed, as in the case of the driver ant, and then the extreme forms would have been produced in greater and greater numbers through the survival of the parents that generated them, until no intermediates were produced.
An analogous explanation has been given by Wallace for the equally complex case of certain Malayan butterflies that regularly appear in two or even three distinct female forms, and by Fritz Muller for certain Brazilian crustaceans that similarly appear in two widely distinct male forms. But this subject needn't be discussed further here.
I've now explained how, I believe, the remarkable fact of two distinctly defined castes of sterile workers existing in the same nest -- both widely different from each other and from their parents -- has originated. We can see how their production may have been useful to a social community of ants, on the same principle that the division of labor is useful to human civilization. Ants, however, work by inherited instincts and inherited bodily tools, while humans work by acquired knowledge and manufactured instruments. But I must confess that, with all my faith in natural selection, I would never have expected this principle to be so powerfully effective had the case of these sterile insects not led me to this conclusion. I've therefore discussed this case at some length -- though still not enough -- to show the power of natural selection, and also because this is by far the most serious specific difficulty my theory has encountered.
The case is also very interesting because it proves that in animals, as in plants, any amount of modification can be achieved by the accumulation of numerous small spontaneous variations that are in any way beneficial, without exercise or habit playing any role. For peculiar habits confined to sterile female workers, no matter how long they might be followed, could not possibly affect the males and fertile females, which alone leave descendants. I'm surprised that no one has used this powerful case of sterile insects as an argument against the well-known doctrine of inherited habit, as proposed by Lamarck.
In this chapter I've tried briefly to show that the mental qualities of our domestic animals vary, and that the variations are inherited. Even more briefly, I've tried to show that instincts vary slightly in nature. No one will dispute that instincts are of the highest importance to each animal. Therefore, there's no real difficulty in natural selection accumulating, under changing conditions of life, slight modifications of instinct that are in any way useful. In many cases, habit or use and disuse have probably played a role as well. I don't claim that the facts given in this chapter greatly strengthen my theory. But none of the difficult cases, to the best of my judgment, destroy it.
On the other hand, the fact that instincts are not always absolutely perfect and are prone to mistakes; that no instinct can be shown to have been produced for the good of other animals, even though animals do exploit each other's instincts; that the principle in natural history of "Nature does not make leaps" applies to instincts as well as to bodily structure, and is plainly explained by the views I've described but is otherwise inexplicable -- all of this supports the theory of natural selection.
This theory is also strengthened by some other facts about instincts -- for example, the common case of closely related but distinct species living in distant parts of the world under very different conditions, yet often retaining nearly the same instincts. We can understand, through the principle of inheritance, how the thrush of tropical South America lines its nest with mud in the same distinctive way as our British thrush. We can understand how the hornbills of Africa and India have the same extraordinary instinct of plastering up and imprisoning the female in a hole in a tree, with only a small opening through which the male feeds her and their young after they hatch. We can understand how the male wrens (Troglodytes) of North America build "decoy nests" to roost in, just like the males of our own wrens -- a habit completely unlike that of any other known bird.
Finally, it may not be a logical deduction, but to my mind it is far more satisfying to view instincts like the young cuckoo ejecting its foster-siblings, ants making slaves, and the larvae of parasitic wasps feeding within the living bodies of caterpillars -- not as specially endowed or divinely created instincts, but as small consequences of one general law leading to the advancement of all living things: namely, multiply, vary, let the strongest live and the weakest die.
The view most naturalists hold is that species have been specially designed to be sterile when crossed, in order to prevent them from blending together. This certainly seems plausible at first -- after all, species living side by side could hardly stay distinct if they could freely interbreed. This subject matters a great deal for our discussion, especially because the sterility of species when first crossed, and the sterility of the hybrid offspring they produce, cannot have been acquired through the preservation of successively greater degrees of sterility, as I'll show. Instead, it's an incidental byproduct of differences in the reproductive systems of the parent species.
When dealing with this subject, two fundamentally different classes of facts have generally been lumped together: the sterility of species when they're first crossed, and the sterility of the hybrids produced from those crosses.
Pure species obviously have their reproductive organs in perfect working order, yet when crossed they produce few or no offspring. Hybrids, on the other hand, have functionally impotent reproductive organs, as you can clearly see from the state of the male element in both plants and animals -- even though the reproductive organs themselves are structurally perfect, as far as the microscope can reveal. In the first case, the two sexual elements that combine to form the embryo are both perfect. In the second case, they're either undeveloped or imperfectly developed. This distinction matters when we consider the cause of the sterility that's common to both cases. The distinction has probably been overlooked because sterility in both cases was treated as a special endowment, beyond the reach of our reasoning powers.
The fertility of varieties -- that is, forms known or believed to have descended from common parents -- when crossed, and the fertility of their hybrid offspring, is equally important for my theory as the sterility of species. For it seems to draw a broad and clear line between varieties and species.
First, let's look at the sterility of species when crossed and of their hybrid offspring. It's impossible to study the papers and books of those two meticulous and admirable observers, the botanists Kolreuter and Gartner, who practically devoted their lives to this subject, without being deeply impressed by how general some degree of sterility is. Kolreuter treats the rule as universal -- but he cuts the knot by simply ranking as varieties any two forms that proved fully fertile together, which happened in ten cases, even though most authors considered those forms distinct species. Gartner likewise treats the rule as universal, and he disputes the complete fertility of Kolreuter's ten cases. But in these and many other instances, Gartner has to carefully count seeds to show that there's any degree of sterility at all. He always compares the maximum number of seeds produced by two species when first crossed, and the maximum produced by their hybrid offspring, against the average number produced by both pure parent species under natural conditions. But serious sources of error come into play here: a plant being hybridized has to be castrated, and -- what's often more important -- it has to be isolated to prevent insects from bringing pollen from other plants. Nearly all the plants Gartner experimented on were potted and kept in a room in his house. There's no doubt that these procedures are often harmful to a plant's fertility, because Gartner's own tables show about twenty cases where he castrated plants and artificially fertilized them with their own pollen, and -- excluding cases like the Leguminosae, where manipulation is known to be tricky -- half of these twenty plants had their fertility reduced to some degree. Moreover, since Gartner repeatedly crossed some forms, like the common red and blue pimpernels (Anagallis arvensis and coerulea), which the best botanists rank as mere varieties, and found them absolutely sterile, we may well doubt whether many species are really as sterile when crossed as he believed.
It's certain, on one hand, that the sterility of various species when crossed differs enormously in degree and grades away so imperceptibly, and on the other hand, that the fertility of pure species is so easily affected by various circumstances, that for all practical purposes it's extremely difficult to say where perfect fertility ends and sterility begins. I think no better evidence of this could be asked for than the fact that the two most experienced observers who ever lived -- Kolreuter and Gartner -- arrived at completely opposite conclusions about some of the very same forms. It's also highly instructive to compare -- though I don't have space to go into the details here -- the evidence our best botanists put forward about whether certain doubtful forms should be ranked as species or varieties, with the evidence from fertility experiments by different hybridizers, or by the same observer working in different years. This comparison shows that neither sterility nor fertility provides any certain distinction between species and varieties. The evidence from this source grades away and is just as doubtful as the evidence from other structural and constitutional differences.
Regarding the sterility of hybrids in successive generations: although Gartner was able to rear some hybrids for six or seven generations (and in one case for ten), carefully preventing them from crossing with either pure parent, he states positively that their fertility never increases but generally decreases, often greatly and suddenly. Regarding this decrease, we should first note that when any structural or constitutional trait is shared by both parents, it's often passed on in an amplified form to the offspring -- and in hybrid plants, both sexual elements are already somewhat affected. But I believe their fertility has been reduced in nearly all these cases by an additional cause: too much inbreeding. I've conducted so many experiments and gathered so many facts showing that an occasional cross with a different individual or variety boosts the vigor and fertility of offspring, while very close inbreeding reduces them, that I can't doubt this conclusion. Experimenters rarely raise hybrids in large numbers, and since the parent species or other related hybrids are usually growing in the same garden, insect visits have to be carefully prevented during the flowering season. So hybrids left to themselves will generally be fertilized each generation by pollen from the same flower -- and this would probably harm their fertility, which is already reduced by their hybrid origin. I'm strengthened in this view by a remarkable observation that Gartner made repeatedly: if even the less fertile hybrids are artificially fertilized with hybrid pollen of the same kind, their fertility -- despite the frequent harmful effects of manipulation -- sometimes clearly increases, and goes on increasing. Now, during artificial fertilization, pollen is just as often taken by chance (as I know from my own experience) from the anthers of a different flower as from the flower being fertilized. So a cross between two flowers, though probably often on the same plant, would happen this way. Moreover, whenever complicated experiments are underway, a careful observer like Gartner would have castrated his hybrids, ensuring a cross with pollen from a distinct flower each generation -- either from the same plant or from another plant of the same hybrid type. And so the strange fact that fertility increases in successive generations of artificially fertilized hybrids, compared to those that self-fertilize spontaneously, can, I believe, be explained by the avoidance of too-close inbreeding.
Now let's turn to the results of a third highly experienced hybridizer: the Reverend W. Herbert. He is just as emphatic in his conclusion that some hybrids are perfectly fertile -- as fertile as the pure parent species -- as Kolreuter and Gartner are that some degree of sterility between distinct species is a universal law of nature. He experimented on some of the very same species as Gartner. The difference in their results can, I think, be partly explained by Herbert's great horticultural skill and his access to hothouses. Of his many important findings, I'll give just one as an example: "every ovule in a pod of Crinum capense fertilized by C. revolutum produced a plant, which I never saw to occur in a case of its natural fertilization." So here we have perfect fertility -- even greater than normal fertility -- in a first cross between two distinct species.
This Crinum case leads me to mention a remarkable fact: individual plants of certain species of Lobelia, Verbascum, and Passiflora can easily be fertilized by pollen from a different species, but not by pollen from the same plant, even though that pollen can be proved perfectly sound by using it to fertilize other plants or species. In the genus Hippeastrum, in Corydalis as shown by Professor Hildebrand, and in various orchids as shown by Mr. Scott and Fritz Muller, all individuals share this peculiar condition. So with some species certain abnormal individuals, and with other species all individuals, can actually be hybridized much more easily than they can be fertilized by pollen from the same individual plant! To give one example: a bulb of Hippeastrum aulicum produced four flowers. Three were fertilized by Herbert with their own pollen, and the fourth was fertilized by pollen from a compound hybrid descended from three distinct species. The result was that "the ovaries of the three first flowers soon ceased to grow, and after a few days perished entirely, whereas the pod fertilized by the pollen of the hybrid made vigorous growth and rapid progress to maturity, and bore good seed, which germinated freely." Herbert tried similar experiments for many years, always with the same result. These cases show what slight and mysterious factors the greater or lesser fertility of a species sometimes depends on.
The practical experiments of horticulturists, though not conducted with scientific precision, deserve some attention. Everyone knows how extensively the species of Pelargonium, Fuchsia, Calceolaria, Petunia, Rhododendron, and others have been crossed, yet many of these hybrids seed freely. For instance, Herbert states that a hybrid from Calceolaria integrifolia and plantaginea, species that differ enormously in general appearance, "reproduces itself as perfectly as if it had been a natural species from the mountains of Chile." I've taken considerable trouble to determine the fertility of some complex rhododendron crosses, and I'm assured that many of them are perfectly fertile. Mr. C. Noble, for instance, tells me that he raises rootstocks for grafting from a hybrid between Rhododendron ponticum and catawbiense, and that this hybrid "seeds as freely as it is possible to imagine." If hybrids, when properly treated, had always declined in fertility in each successive generation, as Gartner believed, the fact would be well known to nurserymen. Horticulturists raise large beds of the same hybrid, and only this treatment is really fair, because insect activity allows the different individuals to cross freely with each other, preventing the harmful effects of close inbreeding. Anyone can easily verify the effectiveness of insect activity by examining the flowers of the more sterile hybrid rhododendrons that produce no pollen: you'll find plenty of pollen on their stigmas, brought from other flowers.
Regarding animals, far fewer careful experiments have been done than with plants. If our classification systems can be trusted -- that is, if animal genera are as distinct from each other as plant genera -- then we can conclude that animals more distantly related on the scale of nature can be crossed more easily than plants. But the hybrids themselves are, I think, more sterile. We should keep in mind, though, that since few animals breed freely in captivity, few experiments have been properly conducted. For instance, the canary has been crossed with nine distinct species of finches, but since none of these breeds freely in captivity, we have no right to expect that the first crosses between them and the canary, or their hybrids, should be perfectly fertile. Also, regarding the fertility of more fertile hybrid animals across successive generations, I know of hardly any case where two families of the same hybrid were raised simultaneously from different parents, avoiding the harmful effects of close inbreeding. On the contrary, brothers and sisters have usually been crossed in each successive generation, against the constantly repeated advice of every breeder. Under these conditions, it's not at all surprising that the inherent sterility of the hybrids kept increasing.
Although I know of hardly any thoroughly well-documented cases of perfectly fertile hybrid animals, I have reason to believe that hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus with P. torquatus, are perfectly fertile. Quatrefages reports that hybrids from two moths (Bombyx cynthia and arrindia) were proved in Paris to be fertile among themselves for eight generations. It has recently been claimed that two species as different as the hare and rabbit, when they can be induced to breed, produce offspring that are highly fertile when crossed back to one of the parent species. The hybrids from common and Chinese geese (A. cygnoides) -- species so different that they're usually placed in separate genera -- have often bred in this country with either pure parent, and in one case they bred among themselves. This was done by Mr. Eyton, who raised two hybrids from the same parents but from different hatches, and from these two birds he raised no fewer than eight hybrids (grandchildren of the pure geese) from a single nest. In India, however, these crossbred geese must be far more fertile, because I'm assured by two highly capable judges -- Mr. Blyth and Captain Hutton -- that whole flocks of these crossed geese are kept in various parts of the country. And since they're kept for profit where neither pure parent species exists, they must certainly be highly or perfectly fertile.
With our domesticated animals, the various breeds when crossed together are fully fertile. Yet in many cases they descend from two or more wild species. From this fact we must conclude either that the original wild parent species produced perfectly fertile hybrids right away, or that the hybrids subsequently became fully fertile under domestication. This second possibility, first proposed by the naturalist Pallas, seems far more probable and can hardly be doubted. It's almost certain, for instance, that our dogs descend from several wild species. Yet, with perhaps the exception of certain indigenous domestic dogs of South America, all are fully fertile together. But by analogy I strongly doubt that the several original wild species would have freely interbred and produced fully fertile hybrids at first. Similarly, I've recently obtained decisive evidence that crossed offspring from Indian humped cattle and common cattle are perfectly fertile among themselves. And based on Rutimeyer's observations of their significant skeletal differences, as well as Mr. Blyth's observations of their differences in habits, voice, constitution, and so on, these two forms must be regarded as genuine distinct species. The same applies to the two main breeds of pig. We must, therefore, either abandon the belief that species are universally sterile when crossed, or we must view this sterility in animals not as a permanent characteristic, but as one that can be eliminated by domestication.
Finally, considering all the established facts about crossbreeding in plants and animals, we can conclude that some degree of sterility, both in first crosses and in hybrids, is an extremely common result -- but that, given our current state of knowledge, it cannot be considered absolutely universal.
Now let's look more closely at the laws governing the sterility of first crosses and of hybrids. Our main goal will be to see whether these laws suggest that species have been specially equipped with sterility to prevent them from crossing and blending together in total confusion. The following conclusions are drawn mainly from Gartner's excellent work on the hybridization of plants. I've taken considerable care to find out how far they apply to animals, and considering how limited our knowledge of hybrid animals is, I've been surprised to find how generally the same rules hold for both kingdoms.
I've already noted that the degree of fertility, both in first crosses and in hybrids, ranges from zero to perfect fertility. The number of ways this gradation can be demonstrated is surprising, but I can only sketch the barest outline here. When pollen from a plant in one family is placed on the stigma of a plant in a completely different family, it has no more effect than a bit of dust. From this absolute zero of fertility, the pollen of different species applied to the stigma of a single species within the same genus produces a perfect gradation in seed numbers, all the way up to nearly complete or even total fertility -- and as we've seen, in certain unusual cases, even to an excess of fertility beyond what the plant's own pollen produces. Similarly with hybrids themselves: some have never produced, and probably never will produce, even with pollen from the pure parents, a single fertile seed. But in some of these cases, a first trace of fertility can be detected because the pollen of one pure parent species causes the hybrid's flower to wither earlier than it otherwise would -- and early withering is well known to be a sign that fertilization has begun. From this extreme sterility, we find self-fertilized hybrids producing more and more seeds, all the way up to perfect fertility.
Hybrids raised from two species that are very difficult to cross and rarely produce any offspring are generally very sterile. But the parallel between the difficulty of making a first cross and the sterility of the resulting hybrids -- two classes of facts that are usually lumped together -- is by no means exact. There are many cases where two pure species, as in the genus Verbascum, can be crossed with unusual ease and produce numerous hybrid offspring, yet these hybrids are remarkably sterile. On the other hand, there are species that can be crossed only rarely or with extreme difficulty, but when hybrids are finally produced, they're quite fertile. Even within a single genus, like Dianthus, both of these opposite outcomes occur.
The fertility of both first crosses and hybrids is more easily affected by unfavorable conditions than is the fertility of pure species. But the fertility of first crosses is also inherently variable: it's not always the same when the same two species are crossed under the same circumstances. It partly depends on the constitution of the particular individuals that happened to be chosen for the experiment. The same is true of hybrids, whose fertility often differs greatly among individuals raised from seed in the same capsule and exposed to the same conditions.
The term "systematic affinity" means the overall resemblance between species in structure and constitution. The fertility of first crosses and of the hybrids they produce is largely governed by their systematic affinity. This is clearly shown by the fact that hybrids have never been raised between species placed by taxonomists in different families, while very closely related species generally unite with ease. But the match between systematic affinity and the ease of crossing is far from exact. Numerous cases could be cited of very closely related species that won't unite at all, or only with extreme difficulty, and of very distinct species that cross with the greatest ease. Within the same family there may be one genus, like Dianthus, where many species can be readily crossed, and another genus, like Silene, where the most persistent efforts have failed to produce a single hybrid even between extremely close species. Even within a single genus we find this same difference. For instance, the many species of Nicotiana have been more extensively crossed than those of almost any other genus, yet Gartner found that N. acuminata -- which isn't an especially distinct species -- stubbornly refused to fertilize, or be fertilized by, no fewer than eight other Nicotiana species. Many similar cases could be cited.
No one has been able to identify what kind or what amount of difference, in any recognizable characteristic, is enough to prevent two species from crossing. It can be shown that plants that differ enormously in growth habit and general appearance, with strongly marked differences in every part of the flower, even in the pollen, fruit, and cotyledons, can still be crossed. Annual and perennial plants, deciduous and evergreen trees, plants that live in different habitats and are adapted to extremely different climates -- all can often be crossed with ease.
By a "reciprocal cross" between two species, I mean, for instance, the case of a female donkey being crossed by a stallion, and then a mare being crossed by a male donkey. These two species can then be said to have been reciprocally crossed. There is often an enormous difference in the ease of making reciprocal crosses. Such cases are highly important, because they prove that the ability of two species to cross is often completely independent of their systematic affinity -- that is, of any difference in their structure or constitution, except in their reproductive systems. This difference in reciprocal crosses was observed long ago by Kolreuter. To give one example: Mirabilis jalapa can easily be fertilized by the pollen of M. longiflora, and the resulting hybrids are reasonably fertile. But Kolreuter tried more than two hundred times over eight consecutive years to reciprocally fertilize M. longiflora with pollen from M. jalapa, and utterly failed. Several other equally striking cases could be given. The marine biologist Thuret observed the same phenomenon in certain seaweeds. Gartner found that this difference in the ease of making reciprocal crosses is extremely common in lesser degrees. He observed it even between closely related forms (like Matthiola annua and glabra) that many botanists rank as mere varieties. It's also a remarkable fact that hybrids raised from reciprocal crosses, though composed of exactly the same two species -- one species used first as the father and then as the mother -- rarely differ in external appearance yet generally differ in fertility, sometimes to a small and occasionally to a large degree.
Several other unusual rules could be drawn from Gartner's work. For instance, some species have a remarkable ability to cross with other species, while other species in the same genus have a remarkable ability to stamp their likeness on their hybrid offspring -- but these two abilities don't necessarily go together. There are certain hybrids that, instead of being intermediate between their two parents as usual, always closely resemble one of them. Such hybrids, though externally so similar to one of their pure parent species, are with rare exceptions extremely sterile. Similarly, among hybrids that are usually intermediate between their parents, exceptional and abnormal individuals sometimes appear that closely resemble one of the pure parents -- and these hybrids are almost always utterly sterile, even when other hybrids raised from seed in the same capsule have considerable fertility. These facts show how completely a hybrid's fertility can be independent of its external resemblance to either pure parent.
Considering all the rules I've now described about the fertility of first crosses and hybrids, we see that when forms distinct enough to be considered good species are united, their fertility ranges from zero to perfect fertility, or even, under certain conditions, to excess. Their fertility, besides being highly sensitive to favorable and unfavorable conditions, is inherently variable. It's by no means always the same in the first cross and in the hybrids produced from it. The fertility of hybrids is not related to how much they resemble either parent in outward appearance. And finally, the ease of making a first cross between any two species is not always governed by their systematic affinity or degree of resemblance. This last point is clearly proved by the difference in reciprocal crosses between the same two species, because depending on which species is used as father or mother, there is generally some difference, and occasionally an enormous difference, in the ease of making the cross. The hybrids produced from reciprocal crosses also often differ in fertility.
Now, do these complex and peculiar rules suggest that species have been endowed with sterility simply to prevent them from blending in nature? I don't think so. For why should the sterility be so enormously different in degree when various species are crossed, if it's equally important to keep all of them from blending? Why should the degree of sterility be inherently variable among individuals of the same species? Why should some species cross easily yet produce very sterile hybrids, while others cross with extreme difficulty yet produce fairly fertile hybrids? Why should there often be such a big difference in reciprocal crosses between the same two species? Why, we might even ask, has the production of hybrids been permitted at all? Granting species the special power to produce hybrids and then stopping their further reproduction through varying degrees of sterility -- not closely related to the ease of the original cross -- seems a strange arrangement.
The rules and facts I've described, on the other hand, clearly indicate to me that the sterility of both first crosses and hybrids is simply incidental on, or caused by, unknown differences in their reproductive systems. These differences are of such a peculiar and limited nature that, in reciprocal crosses between the same two species, the male sexual element of one species will often act freely on the female sexual element of the other, but not the reverse. It will be helpful to explain more fully, with an example, what I mean by sterility being incidental on other differences rather than a specially endowed quality. Since the ability of one plant to be grafted or budded onto another is unimportant for their survival in nature, I expect no one will argue that this ability is a specially endowed quality. Everyone will agree that it's incidental on differences in the growth patterns of the two plants. We can sometimes see the reason why one tree won't graft onto another -- differences in growth rate, wood hardness, the timing or nature of their sap, and so on. But in many cases, we can assign no reason at all. Large differences in size between two plants, one being woody and the other herbaceous, one being evergreen and the other deciduous, and adaptation to wildly different climates -- none of these necessarily prevents grafting. Just as with hybridization, the ability to graft is limited by systematic affinity: no one has been able to graft together trees from completely different families. On the other hand, closely related species and varieties of the same species can usually, though not always, be grafted with ease. But this ability, as with hybridization, is by no means absolutely governed by systematic affinity. Although many distinct genera within the same family have been grafted together, in other cases species of the same genus won't take on each other. The pear can be grafted far more readily on the quince, which is classified as a separate genus, than on the apple, which belongs to the same genus. Even different varieties of the pear graft with different degrees of success on the quince, and different varieties of apricot and peach graft with varying success on certain varieties of plum.
Just as Gartner found that there was sometimes an innate difference between individuals of the same two species in crossing, so the plant physiologist Sagaret believes the same holds for different individuals of the same two species in grafting. Just as the ease of making reciprocal crosses is often far from equal, the same is sometimes true in grafting. The common gooseberry, for instance, cannot be grafted on the currant, yet the currant will take, though with difficulty, on the gooseberry.
We've seen that the sterility of hybrids with imperfect reproductive organs is a different matter from the difficulty of uniting two pure species with perfect reproductive organs. Yet these two distinct classes of cases run largely parallel. Something similar happens in grafting: the botanist Thouin found that three species of Robinia, which seeded freely on their own roots and could be grafted without great difficulty on a fourth species, became barren when grafted. On the other hand, certain species of Sorbus, when grafted on other species, yielded twice as much fruit as when on their own roots. This latter fact reminds us of the extraordinary cases of Hippeastrum, Passiflora, and others, which seed much more freely when fertilized with pollen from a different species than with pollen from the same plant.
So we see that although there's a clear and fundamental difference between the mere adhesion of grafted stocks and the union of male and female elements in reproduction, there is a rough parallel in the results of grafting and of crossing distinct species. And just as we must regard the curious and complex rules governing the ease of grafting as incidental on unknown differences in the plants' vegetative systems, I believe the even more complex rules governing the ease of first crosses are incidental on unknown differences in their reproductive systems. These differences, in both cases, follow systematic affinity to some extent, as you'd expect, since systematic affinity attempts to express every kind of resemblance and difference between organisms. The facts clearly don't indicate that the greater or lesser difficulty of either grafting or crossing has been a special endowment -- even though, in the case of crossing, the difficulty matters greatly for the stability of species, while in the case of grafting, it's unimportant for their survival.
At one time it seemed probable to me, as it has to others, that the sterility of first crosses and hybrids might have been slowly acquired through the natural selection of slightly reduced fertility, which -- like any other variation -- spontaneously appeared in certain individuals of one variety when crossed with another. It would clearly be advantageous for two varieties or incipient species to be kept from blending, on the same principle that a breeder selecting two varieties at the same time needs to keep them separate. But several arguments work against this idea.
First, species living in separate regions are often sterile when crossed. It could clearly have been of no advantage to species already separated by geography to become mutually sterile, and so this could not have happened through natural selection. Someone might argue that if a species became sterile with one neighboring species, sterility with others would follow as a necessary side effect. But second, it's almost as much opposed to the theory of natural selection as to the theory of special creation that, in reciprocal crosses, the male element of one species should be rendered completely impotent on a second species, while the male element of that second species can freely fertilize the first. This peculiar state of the reproductive system could hardly have been advantageous to either species.
The greatest difficulty in considering whether natural selection played a role in making species mutually sterile lies in the existence of many gradual steps, from slightly reduced fertility to absolute sterility. It might be admitted that it would benefit an incipient species to be rendered slightly sterile when crossed with its parent form or with some other variety, since fewer degraded hybrid offspring would be produced to mix their traits with the new species in the process of forming. But anyone who takes the trouble to think through how this first degree of sterility could be increased through natural selection to the high degree common in so many species -- and universal in species that have diverged to the level of distinct genera or families -- will find the subject extraordinarily complex. After careful reflection, it seems to me that this could not have happened through natural selection. Consider any two species that, when crossed, produce few and sterile offspring. What could favor the survival of those individuals that happened to be endowed with a slightly higher degree of mutual infertility, and that thus moved one small step closer to absolute sterility? Yet such an advance, if natural selection were at work, must have happened constantly in many species, since a multitude are completely sterile with each other. With sterile worker insects, we have reason to think that changes in structure and fertility were slowly accumulated through natural selection because of the indirect advantage given to the community. But an individual animal that doesn't belong to a social community, if rendered slightly sterile when crossed with another variety, would gain no advantage for itself, nor would it indirectly benefit other individuals of the same variety in a way that would lead to their preservation.
But it would be unnecessary to discuss this in detail, because with plants we have conclusive evidence that the sterility of crossed species must be due to some principle entirely independent of natural selection. Both Gartner and Kolreuter proved that in genera containing many species, you can arrange a series from species that when crossed yield fewer and fewer seeds, down to species that never produce a single seed but are still affected by the pollen of other species -- their ovary swells even though no seeds form. It's obviously impossible to select the more sterile individuals when they've already stopped producing seeds. So this peak of sterility, where only the ovary is affected, cannot have been produced through selection. And since the laws governing the various grades of sterility are so uniform throughout the animal and plant kingdoms, we can infer that the cause, whatever it may be, is the same or nearly the same in all cases.
Let's now look more closely at the probable nature of the differences between species that cause sterility in first crosses and in hybrids. For first crosses, the greater or lesser difficulty of achieving a union and obtaining offspring apparently depends on several distinct causes. Sometimes there must be a physical impossibility of the male element reaching the egg, as would happen with a plant whose pistil is too long for the pollen tubes to reach the ovary. It has also been observed that when pollen from one species is placed on the stigma of a distantly related species, although the pollen tubes extend, they don't penetrate the stigmatic surface. Again, the male element may reach the female element but be unable to cause an embryo to develop, as seems to have been the case in some of Thuret's experiments on seaweed. No explanation can be given for these facts, any more than we can explain why certain trees can't be grafted onto others. Finally, an embryo may begin to develop and then die at an early stage. This last possibility hasn't received enough attention, but I believe -- based on observations shared with me by Mr. Hewitt, who has extensive experience hybridizing pheasants and chickens -- that the early death of the embryo is a very frequent cause of sterility in first crosses. Mr. Salter has recently published the results of examining about five hundred eggs produced from various crosses between three species of Gallus and their hybrids. The majority of these eggs had been fertilized, and in most of the fertilized eggs, the embryos had either partially developed and then died, or had developed nearly to maturity but the chicks were unable to break through the shell. Of the chicks that did hatch, more than four-fifths died within the first few days or at most weeks, "without any obvious cause, apparently from mere inability to live." So out of five hundred eggs, only twelve chicks were successfully reared. With plants, hybridized embryos probably often die in a similar way. We know that hybrids raised from very different species are sometimes weak and stunted and die young -- Max Wichura has recently reported some striking cases of this with hybrid willows. It may be worth noting that in some cases of parthenogenesis, embryos within unfertilized silk moth eggs pass through their early developmental stages and then die, just like the embryos produced by a cross between distinct species. Until I learned these facts, I was reluctant to believe in the frequent early death of hybrid embryos, because hybrids, once born, are generally healthy and long-lived -- as we see with the common mule. But hybrids face very different circumstances before and after birth. When born and living in a country where both parents live, they're generally in suitable conditions. But a hybrid has only half the nature and constitution of its mother, so before birth -- while nourished in the mother's womb, or within the egg or seed produced by the mother -- it may be exposed to partly unsuitable conditions, making it liable to die early, especially since all very young organisms are highly sensitive to harmful or unnatural conditions. But ultimately, the cause more likely lies in some imperfection in the original act of fertilization, causing the embryo to develop imperfectly, rather than in the conditions it encounters afterward.
Regarding the sterility of hybrids, where the sexual elements are imperfectly developed, the situation is somewhat different. I've more than once pointed to a large body of evidence showing that when animals and plants are removed from their natural conditions, they are extremely prone to having their reproductive systems seriously affected. This is, in fact, the great barrier to domesticating animals. Between this induced sterility and that of hybrids, there are many similarities. In both cases, the sterility is independent of general health and is often accompanied by larger size or great vigor. In both cases, the sterility comes in various degrees. In both, the male element is most often affected, though sometimes the female more than the male. In both, the tendency follows systematic affinity to some extent: whole groups of animals and plants are rendered infertile by the same unnatural conditions, and whole groups of species tend to produce sterile hybrids. On the other hand, one species in a group will sometimes resist major changes in conditions with unimpaired fertility, and certain species will produce unusually fertile hybrids. No one can predict whether any particular animal will breed in captivity, or whether any exotic plant will seed freely under cultivation. Nor can anyone predict whether any two species of a genus will produce more or less sterile hybrids. Finally, when organisms are placed for several generations under unnatural conditions, they are extremely prone to vary -- which seems partly due to their reproductive systems being specifically affected, though in a lesser degree than when sterility occurs. So it is with hybrids, for their offspring in successive generations are remarkably prone to vary, as every experimenter has observed.
So we see that when organisms are placed under new and unnatural conditions, and when hybrids are produced by the unnatural crossing of two species, the reproductive system -- independently of overall health -- is affected in a very similar way. In the first case, the conditions of life have been disrupted, though often so slightly as to be undetectable by us. In the second case, that of hybrids, the external conditions have remained the same, but the organism has been disrupted by two distinct structures and constitutions -- including, of course, the reproductive systems -- being blended into one. It's hardly possible for two organisms to be compounded into one without some disruption to the development, timing, or mutual relationships of their various parts and organs, or to their interaction with the conditions of life. When hybrids are able to breed among themselves, they pass on this compounded organization to their offspring from generation to generation. So we shouldn't be surprised that their sterility, though somewhat variable, doesn't diminish. In fact, it tends to increase -- generally the result, as I explained earlier, of too much inbreeding. This view, that the sterility of hybrids is caused by two constitutions being combined into one, has been strongly supported by Max Wichura.
I must admit, however, that we can't fully explain, on this or any other view, several facts about the sterility of hybrids. For instance, the unequal fertility of hybrids from reciprocal crosses, or the increased sterility in those hybrids that occasionally and exceptionally bear a close resemblance to one pure parent. Nor do I claim that these remarks get to the root of the matter: no explanation is offered for why an organism, placed under unnatural conditions, becomes sterile. All I've tried to show is that in two somewhat related cases, sterility is the common result -- in one case from the conditions of life being disturbed, in the other from the organism being disturbed by two organisms being compounded into one.
A similar parallel holds for an allied but very different class of facts. There's an old and nearly universal belief, supported by considerable evidence that I've presented elsewhere, that slight changes in the conditions of life benefit all living things. We see farmers and gardeners acting on this by frequently exchanging seed, tubers, and so on from one soil or climate to another, and back again. Convalescing animals benefit greatly from almost any change in their routine. With both plants and animals, there's the clearest evidence that a cross between individuals of the same species that differ to some extent gives vigor and fertility to the offspring, and that close inbreeding continued over several generations between the nearest relatives, if they're kept under the same conditions, almost always leads to decreased size, weakness, or sterility.
So it seems that slight changes in conditions of life benefit all organisms, and slight crosses -- that is, crosses between males and females of the same species that have been exposed to slightly different conditions or that have slightly varied -- give vigor and fertility to the offspring. But as we've seen, organisms long accustomed to particular uniform conditions in nature, when subjected to a significant change in their conditions (as in captivity), are very often rendered more or less sterile. And we know that a cross between two forms that have become widely or specifically different produces hybrids that are almost always somewhat sterile. I'm fully convinced that this double parallel is not an accident or an illusion. Whoever can explain why the elephant and a multitude of other animals are unable to breed when kept under only partial confinement in their native country will be able to explain the primary cause of hybrids being so generally sterile. That person will also be able to explain how the breeds of some of our domesticated animals, which have often been subjected to new and variable conditions, are fully fertile together, even though they descended from distinct species that would probably have been sterile if crossed originally. These two parallel series of facts seem to be connected by some common but unknown bond that is fundamentally related to the principle of life itself. According to Herbert Spencer, this principle is that life depends on, or consists of, the constant action and reaction of various forces which, as throughout nature, are always tending toward equilibrium -- and when this tendency is slightly disturbed by any change, the vital forces gain in power.
This subject can be discussed briefly here, and it will shed some light on hybridism. Several plants belonging to different orders come in two forms, existing in roughly equal numbers, that differ only in their reproductive organs. One form has a long pistil with short stamens, the other a short pistil with long stamens, and the two have differently sized pollen grains. In trimorphic plants, there are three forms that likewise differ in pistil and stamen length, in the size and color of pollen grains, and in other respects. Since each of the three forms has two sets of stamens, the three forms together have six sets of stamens and three kinds of pistils. These organs are proportioned so that half the stamens in two of the forms are at the same height as the stigma of the third form. I've shown, and other observers have confirmed, that for full fertility in these plants, the stigma of one form must be fertilized by pollen from stamens of the corresponding height in another form. So with dimorphic species, two unions -- which we can call "legitimate" -- are fully fertile, and two "illegitimate" unions are more or less infertile. With trimorphic species, six unions are legitimate (fully fertile) and twelve are illegitimate (more or less infertile).
The infertility seen in dimorphic and trimorphic plants when they're illegitimately fertilized -- that is, by pollen from stamens that don't correspond in height with the pistil -- varies greatly in degree, all the way up to absolute sterility, just as it does in crossing distinct species. And just as the degree of sterility in species crosses depends heavily on whether conditions are more or less favorable, I've found the same is true with illegitimate unions. It's well known that if pollen from a different species is placed on a flower's stigma and the flower's own pollen is placed on the same stigma afterward, even after a considerable delay, the plant's own pollen is so strongly prepotent that it generally overwhelms the effect of the foreign pollen. The same holds for the pollen of different forms of the same species: legitimate pollen is strongly prepotent over illegitimate pollen when both are placed on the same stigma. I confirmed this by fertilizing several flowers first illegitimately, and twenty-four hours later legitimately with pollen from a distinctively colored variety. All the seedlings were colored like that variety, proving that the legitimate pollen, though applied a full day later, had completely destroyed or prevented the action of the previously applied illegitimate pollen. Again, just as reciprocal crosses between the same two species sometimes produce very different results, the same thing happens with trimorphic plants. For instance, the mid-styled form of Lythrum salicaria was illegitimately fertilized with the greatest ease by pollen from the longer stamens of the short-styled form, and yielded many seeds. But the short-styled form didn't yield a single seed when fertilized by the longer stamens of the mid-styled form.
In all these respects, and in others I could add, the forms of the same undoubted species, when illegitimately united, behave exactly like two distinct species when crossed. This led me to carefully observe, over four years, many seedlings raised from several illegitimate unions. The main finding is that these "illegitimate" plants are not fully fertile. It's possible to raise from dimorphic species both long-styled and short-styled illegitimate plants, and from trimorphic plants all three illegitimate forms. These can then be properly united in a legitimate manner. When this is done, there seems no reason why they shouldn't produce as many seeds as their parents did when legitimately fertilized. But that's not the case. They're all infertile in various degrees, some so utterly and incurably sterile that they didn't produce during four seasons a single seed or even a seed capsule. The sterility of these illegitimate plants, when united with each other legitimately, can be directly compared to the sterility of hybrids when crossed among themselves. If, on the other hand, a hybrid is crossed with either pure parent species, the sterility is usually much reduced -- and the same is true when an illegitimate plant is fertilized by a legitimate plant. Just as the sterility of hybrids doesn't always parallel the difficulty of making the first cross between the parent species, so the sterility of certain illegitimate plants was unusually great, while the sterility of the union from which they came was not especially great. Just as hybrids raised from the same seed capsule show inherently variable degrees of sterility, the same is true, in a marked way, for illegitimate plants. Finally, many hybrids are abundant and persistent flowerers, while other more sterile hybrids produce few flowers and are weak, miserable dwarfs -- and exactly the same occurs with the illegitimate offspring of various dimorphic and trimorphic plants.
Taken together, there is the closest similarity in character and behavior between illegitimate plants and hybrids. It's hardly an exaggeration to say that illegitimate plants are hybrids produced within the boundaries of the same species through the improper union of certain forms, while ordinary hybrids are produced through the improper union of so-called distinct species. We've also already seen that there is the closest similarity in all respects between first illegitimate unions and first crosses between distinct species. This will perhaps be made clearer with an illustration. Suppose a botanist found two well-marked varieties (and such exist) of the long-styled form of the trimorphic Lythrum salicaria, and decided to test by crossing whether they were specifically distinct. He would find that they yielded only about one-fifth of the proper number of seeds, and that they behaved in all the other ways I've described as if they were two distinct species. But to make sure, he would raise plants from his supposed hybrid seed, and he would find that the seedlings were miserably dwarfed and utterly sterile, behaving in all other respects like ordinary hybrids. He might then insist that he had proved, in line with the standard view, that his two varieties were as distinct species as any in the world -- but he would be completely wrong.
The facts I've now presented on dimorphic and trimorphic plants are important for three reasons. First, they show that the physiological test of reduced fertility, both in first crosses and in hybrids, is not a reliable criterion for distinguishing species. Second, we can conclude that some unknown bond connects the infertility of illegitimate unions with the infertility of their illegitimate offspring, and we're led to extend this same view to first crosses and hybrids. Third, and this seems to me especially important, we find that two or three forms of the same species may exist and differ in no way whatsoever -- either in structure or in constitution relative to external conditions -- yet be sterile when united in certain ways. We must remember that it's the union of sexual elements from individuals of the same form (for instance, two long-styled forms) that results in sterility, while the union of sexual elements belonging to two different forms is fertile. So the case appears at first sight to be exactly the reverse of what happens in ordinary unions within the same species and in crosses between distinct species. It's doubtful, however, whether this is really so, but I won't go further into this obscure subject.
We may, however, infer from the study of dimorphic and trimorphic plants that the sterility of distinct species when crossed, and of their hybrid progeny, depends exclusively on the nature of their sexual elements, and not on any difference in their structure or general constitution. We're led to this same conclusion by considering reciprocal crosses, in which the male of one species cannot be united, or can be united only with great difficulty, with the female of a second species, while the reverse cross works perfectly well. That excellent observer Gartner likewise concluded that species when crossed are sterile due to differences confined to their reproductive systems.
It might be argued as an overwhelming objection that there must be some essential distinction between species and varieties, since varieties -- however much they differ in external appearance -- cross with perfect ease and produce perfectly fertile offspring. With some exceptions that I'll describe shortly, I fully accept that this is the general rule. But the subject is surrounded by difficulties, because if we look at varieties produced in nature and find that two forms previously classified as varieties are sterile with each other to any degree, most naturalists immediately reclassify them as species. For instance, the blue and red pimpernel, which most botanists consider varieties, were said by Gartner to be completely sterile when crossed, and he consequently classified them as undoubted species. If we argue in this circle, the fertility of all varieties produced in nature will of course have to be taken for granted.
If we turn to varieties produced, or thought to have been produced, under domestication, we're still dealing with some uncertainty. When it's stated, for instance, that certain South American indigenous domestic dogs don't readily mate with European dogs, the most obvious explanation -- and probably the correct one -- is that they descend from originally distinct species. Nevertheless, the perfect fertility of so many domestic breeds, which differ enormously from each other in appearance -- like pigeon breeds, or cabbage varieties -- is a remarkable fact, especially when we consider how many species there are that closely resemble each other yet are completely sterile when crossed. Several considerations, however, make the fertility of domestic varieties less remarkable. First, the amount of external difference between two species is no sure guide to their degree of mutual sterility, so similar differences between varieties would be no sure guide either. With species, the cause of sterility lies exclusively in differences in their sexual constitution. The varying conditions that domesticated animals and cultivated plants have been subjected to have had so little tendency to modify the reproductive system in a way that leads to mutual sterility that we have good grounds for accepting the directly opposite doctrine of the naturalist Pallas: that such conditions generally eliminate this tendency. So the domesticated descendants of species that would probably have been somewhat sterile when crossed in their natural state become perfectly fertile together. With plants, cultivation is so far from creating a tendency toward sterility between distinct species that, in several well-documented cases I've already mentioned, certain plants have been affected in the opposite way: they've become self-impotent while still able to fertilize and be fertilized by other species. If we accept the Pallasian doctrine that long-continued domestication eliminates sterility -- and it can hardly be rejected -- then it becomes extremely unlikely that the same conditions would also induce sterility, though in certain cases, with species of a peculiar constitution, it might occasionally happen. This is why, I believe, domestic animal varieties have not become mutually sterile, and why only a few such cases, which I'm about to describe, have been observed in plants.
The real difficulty, as I see it, is not why domestic varieties haven't become mutually infertile when crossed, but why this has so commonly happened with natural varieties as soon as they've been permanently modified enough to be ranked as species. We're far from knowing the precise cause -- and this isn't surprising, given how profoundly ignorant we are of the normal and abnormal workings of the reproductive system. But we can see that species, because of their struggle for existence with numerous competitors, will have been exposed for long periods to more uniform conditions than domestic varieties have experienced, and this may make a big difference in the outcome. We know how commonly wild animals and plants become sterile when taken from their natural conditions and placed in captivity. The reproductive functions of organisms that have always lived under natural conditions would probably be similarly sensitive to the influence of an unnatural cross. Domesticated organisms, on the other hand, which -- as the very fact of their domestication shows -- weren't originally highly sensitive to changes in their conditions, and which can now generally endure repeated changes with undiminished fertility, might be expected to produce varieties that are less likely to have their reproductive powers harmed by crossing with other varieties that arose in a similar way.
So far I've been speaking as if varieties of the same species were always fertile when crossed. But the evidence for at least some sterility in the following cases is impossible to ignore, and I'll briefly summarize them. The evidence is at least as good as that on which we accept the sterility of many species. It also comes from hostile witnesses who, in all other cases, consider fertility and sterility as reliable marks of species status. Gartner kept a dwarf kind of maize with yellow seeds and a tall variety with red seeds growing near each other in his garden for several years. Although maize plants have separate male and female flowers, they never naturally crossed. He then fertilized thirteen flowers of one kind with pollen of the other, but only a single ear produced any seed, and that one ear produced only five grains. Manipulation couldn't have been harmful in this case, since the plants have separate sexes. No one, I believe, has suspected that these varieties of maize are distinct species. And it's important to note that the hybrid plants raised from these crosses were themselves perfectly fertile -- so even Gartner didn't dare classify the two varieties as specifically distinct.
Girou de Buzareingues crossed three varieties of gourd, which, like maize, has separate sexes, and he reports that the ease of cross-fertilization decreases as the varieties become more different. I'm not sure how trustworthy these experiments are, but the forms he used are classified as varieties by both Sagaret (who bases his classification largely on the fertility test) and Naudin.
The following case is far more remarkable, and seems at first unbelievable. But it's the result of an extraordinary number of experiments conducted over many years on nine species of Verbascum, by a researcher as meticulous and critical as Gartner: namely, that yellow and white varieties when crossed produce fewer seeds than when similarly colored varieties of the same species are crossed. Moreover, he reports that when yellow and white varieties of one species are crossed with yellow and white varieties of a different species, more seed is produced by crosses between similarly colored flowers than between differently colored ones. Mr. Scott also experimented on Verbascum species and varieties, and although he couldn't confirm Gartner's results on crossing distinct species, he finds that differently colored varieties of the same species yield fewer seeds, in the proportion of eighty-six to one hundred, than similarly colored varieties. Yet these varieties differ in nothing but flower color, and one variety can sometimes be raised from the seed of another.
Kolreuter, whose accuracy every subsequent observer has confirmed, proved the remarkable fact that one particular variety of common tobacco was more fertile than the other varieties when crossed with a very different species. He experimented on five forms commonly considered varieties, and he tested them by the most rigorous method -- reciprocal crosses -- finding their hybrid offspring perfectly fertile. But one of these five varieties, when used as either father or mother and crossed with Nicotiana glutinosa, always yielded hybrids that were less sterile than those produced from the four other varieties when crossed with N. glutinosa. So the reproductive system of this one variety must have been modified in some way and to some degree.
From these facts, it can no longer be maintained that varieties when crossed are always completely fertile. Given the great difficulty of detecting infertility in varieties in nature -- since a supposed variety, if proved infertile to any degree, would almost universally be reclassified as a species -- and given that breeders attend only to external traits in their domestic varieties, and that such varieties haven't been exposed for very long periods to uniform conditions of life -- from all these considerations, we can conclude that fertility does not constitute a fundamental distinction between varieties and species when crossed. The general sterility of crossed species can safely be regarded not as a special acquisition or endowment, but as incidental on changes of an unknown nature in their sexual elements.
Setting aside the question of fertility, the offspring of species and of varieties when crossed can be compared in several other respects. Gartner, who badly wanted to draw a clear line between species and varieties, could find very few differences between the hybrid offspring of species and the crossbred offspring of varieties -- and these differences, as far as I can see, are quite unimportant. On the other hand, hybrids and crossbreeds agree very closely in many important respects.
I'll discuss this subject very briefly. The most important distinction is that, in the first generation, crossbreeds are more variable than hybrids. But Gartner admits that hybrids from species that have long been cultivated are often variable in the first generation, and I've seen striking examples of this myself. Gartner further admits that hybrids between very closely related species are more variable than those from very distinct species, and this shows that the difference in variability is a matter of degree, not kind. When crossbreeds and the more fertile hybrids are propagated for several generations, the extreme variability of the offspring in both cases is well known. There are a few examples of both hybrids and crossbreeds maintaining a uniform character over many generations. The variability in successive generations of crossbreeds is perhaps somewhat greater than in hybrids.
This greater variability of crossbreeds compared to hybrids isn't surprising. The parents of crossbreeds are varieties, mostly domestic varieties (very few experiments have been done with natural varieties), and this implies recent variability that would often continue and add to the variability arising from crossing. The slight variability of hybrids in the first generation, contrasting with their greater variability in later generations, is a curious fact worth noting. It relates to my view on one of the causes of ordinary variability: namely, that the reproductive system, being highly sensitive to changed conditions of life, fails under such circumstances to produce offspring closely resembling the parent form in all respects. Now, hybrids in the first generation descend from species (excluding those long cultivated) that haven't had their reproductive systems affected in any way, so they're not variable. But hybrids themselves have seriously affected reproductive systems, and their descendants are highly variable.
But to return to our comparison: Gartner states that crossbreeds are more prone than hybrids to revert to either parent form. But this, if true, is certainly only a difference of degree. Moreover, Gartner expressly states that hybrids from long-cultivated plants are more subject to reversion than hybrids from species in their natural state, which probably explains the conflicting results obtained by different observers. Max Wichura doubts whether hybrids ever revert to their parent forms -- and he experimented on uncultivated species of willows. Naudin, on the other hand, insists in the strongest terms on the nearly universal tendency toward reversion in hybrids -- and he experimented mainly on cultivated plants. Gartner further states that when any two species, even very closely related ones, are crossed with a third species, the hybrids are very different from each other, whereas if two very distinct varieties of one species are crossed with another species, the hybrids don't differ much. But this conclusion, as far as I can tell, is based on a single experiment and seems directly opposed to the results of several experiments by Kolreuter.
These are the only differences -- and they're unimportant ones -- that Gartner was able to identify between hybrid and crossbred plants. On the other hand, the degrees and kinds of resemblance to their respective parents, especially in hybrids from closely related species, follow the same laws in both crossbreeds and hybrids, according to Gartner. When two species are crossed, one sometimes has a dominant power to stamp its likeness on the hybrid. I believe this is true with plant varieties too, and with animals, one variety certainly often has this dominant power over another. Hybrid plants from a reciprocal cross generally resemble each other closely, and so do crossbred plants from a reciprocal cross. Both hybrids and crossbreeds can be reduced to either pure parent form by repeated backcrossing over successive generations.
These observations apparently apply to animals as well, but the subject is more complicated here, partly because of secondary sexual characteristics, and especially because dominance in transmitting likeness is often stronger in one sex than the other, both when crossing species and when crossing varieties. For instance, I agree with those authors who maintain that the donkey is dominant over the horse: both the mule and the hinny resemble the donkey more closely than the horse. But the dominance is stronger in the male donkey, so the mule (offspring of a male donkey and a mare) is more donkey-like than the hinny (offspring of a female donkey and a stallion).
Some authors have placed great emphasis on the supposed fact that only with crossbreeds, not hybrids, do the offspring closely resemble one parent rather than being intermediate. But this does sometimes happen with hybrids, though I grant it's less common than with crossbreeds. Looking at the cases I've collected of crossbred animals closely resembling one parent, the resemblances seem mainly confined to characters that are almost monstrous in nature and that appeared suddenly -- such as albinism, melanism, absence of tail or horns, or extra fingers and toes -- rather than characters slowly acquired through selection. A tendency to suddenly revert to the full character of either parent would also be much more likely in crossbreeds, which descend from varieties that were often produced suddenly and are semi-monstrous in character, than in hybrids, which descend from species that were produced slowly and naturally. On the whole, I entirely agree with Dr. Prosper Lucas, who, after compiling an enormous body of facts about animals, concludes that the laws of resemblance between child and parents are the same whether the two parents differ little or much from each other -- that is, whether they're individuals of the same variety, of different varieties, or of distinct species.
Setting aside fertility and sterility, in all other respects the offspring of crossed species and crossed varieties show a general and close similarity. If we view species as having been specially created and varieties as having been produced by secondary laws, this similarity would be astonishing. But it fits perfectly with the view that there is no essential distinction between species and varieties.
First crosses between forms distinct enough to be ranked as species, and their hybrids, are very generally -- but not universally -- sterile. The sterility comes in all degrees, and is often so slight that the most careful experimenters have arrived at completely opposite conclusions when trying to classify forms by this test. The sterility is inherently variable among individuals of the same species and is highly sensitive to favorable and unfavorable conditions. The degree of sterility doesn't strictly follow systematic affinity, but is governed by several curious and complex laws. It is generally different, and sometimes wildly different, in reciprocal crosses between the same two species. It is not always equal in the first cross and in the hybrids produced from that cross.
Just as with grafting trees, where the ability of one species or variety to take on another is incidental on differences -- generally unknown -- in their vegetative systems, so in crossing, the greater or lesser ease with which one species can unite with another is incidental on unknown differences in their reproductive systems. There is no more reason to think that species have been specially equipped with various degrees of sterility to prevent them from crossing and blending in nature, than to think that trees have been specially equipped with various and somewhat analogous degrees of difficulty in being grafted together to prevent them from fusing in our forests.
The sterility of first crosses and of their hybrid offspring has not been acquired through natural selection. In the case of first crosses, it seems to depend on several factors, in some cases mainly on the early death of the embryo. In the case of hybrids, it apparently depends on their entire organization being disrupted by being compounded from two distinct forms. This sterility is closely related to what so frequently affects pure species when exposed to new and unnatural conditions of life. Whoever can explain these latter cases will be able to explain the sterility of hybrids. This view is strongly supported by a parallel of another kind: first, slight changes in living conditions boost the vigor and fertility of all organisms; and second, crossing forms that have been exposed to slightly different conditions, or that have slightly varied, promotes the size, vigor, and fertility of their offspring. The facts on the sterility of illegitimate unions in dimorphic and trimorphic plants, and of their illegitimate progeny, perhaps make it probable that some unknown bond connects the degree of fertility of first unions with that of their offspring in all cases. The study of these dimorphic facts, as well as the results of reciprocal crosses, clearly points to the conclusion that the primary cause of the sterility of crossed species is confined to differences in their sexual elements. But why, in the case of distinct species, the sexual elements should so generally have become more or less modified, leading to their mutual infertility, we don't know -- though it seems closely related to species having been exposed for long periods to nearly uniform conditions of life.
It's not surprising that the difficulty of crossing any two species and the sterility of their hybrid offspring should generally correspond, even though these are due to distinct causes, because both depend on the amount of difference between the species being crossed. Nor is it surprising that the ease of making a first cross, the fertility of the resulting hybrids, and the ability to graft together -- though this last clearly depends on very different factors -- should all run somewhat parallel with the systematic affinity of the forms involved, since systematic affinity captures resemblances of every kind.
First crosses between forms known to be varieties, or similar enough to be considered varieties, and their crossbred offspring, are very generally -- but not, as is so often claimed, always -- fertile. This nearly universal and perfect fertility isn't surprising when we remember how prone we are to argue in a circle about varieties in nature, and when we remember that most varieties have been produced under domestication by selecting for external differences alone and haven't been exposed for very long periods to uniform conditions. We should also bear in mind that long-continued domestication tends to eliminate sterility and is therefore unlikely to induce it. Setting aside the question of fertility, in all other respects there is the closest general resemblance between hybrids and crossbreeds: in their variability, in their capacity to be absorbed into either parent form through repeated backcrossing, and in their inheritance of traits from both parents. Finally, then, although we're as ignorant of the precise cause of the sterility of first crosses and hybrids as we are of why animals and plants removed from their natural conditions become sterile, the facts presented in this chapter don't seem to me to argue against the belief that species originally existed as varieties.
In Chapter VI, I listed the main objections that could fairly be raised against the views I've been defending in this book. Most of them have now been addressed. One, however -- the distinctness of species and the absence of countless transitional forms blending them together -- is a very obvious difficulty. I gave reasons why such links don't commonly exist today, even under the conditions that seem most favorable for them: namely, across an extensive, continuous area with gradually changing physical conditions. I tried to show that each species' survival depends more on the other organisms already living around it than on climate alone, and therefore that the conditions truly governing life don't shade into one another as smoothly as temperature or moisture do. I also tried to show that intermediate varieties, because they exist in smaller numbers than the forms they connect, will generally be outcompeted and driven to extinction as those forms continue to evolve. But the main reason we don't find countless intermediate links everywhere in nature is the very process of natural selection itself, through which new varieties continually replace and supplant their parent forms. Yet in proportion to the enormous scale on which this process of extinction has operated, the number of intermediate varieties that must have formerly existed is truly staggering. So why isn't every geological formation and every rock layer packed with such intermediate links? The fossil record certainly doesn't reveal any such finely graded chain of life, and this is perhaps the most obvious and serious objection that can be raised against my theory. The explanation, I believe, lies in the extreme imperfection of the geological record.
First, we should always keep in mind what kind of intermediate forms must have existed according to the theory. When looking at any two species, I've found it hard to avoid picturing forms directly halfway between them. But this is completely the wrong way to think about it. We should always look for forms intermediate between each species and a common but unknown ancestor -- and that ancestor will generally have differed in some ways from all of its modified descendants. Here's a simple illustration: the fantail and pouter pigeons are both descended from the rock pigeon. If we had every intermediate variety that ever existed, we'd have an extremely detailed series connecting each breed back to the rock pigeon. But we would not have any varieties directly intermediate between the fantail and the pouter -- none, for instance, combining a somewhat expanded tail with a somewhat enlarged crop, the distinctive features of these two breeds. What's more, these two breeds have changed so much that if we had no historical or indirect evidence about their origin, it would have been impossible to determine just from comparing their anatomy with the rock pigeon, Columba livia, whether they descended from this species or from some closely related species like C. oenas.
The same applies to natural species. If we look at forms as different as the horse and the tapir, we have no reason to suppose that links directly intermediate between them ever existed. Instead, both descended from an unknown common ancestor. That common ancestor would have had a general overall resemblance to both the tapir and the horse, but in certain features it may have differed considerably from both -- perhaps even more than they differ from each other. So in all such cases, we wouldn't be able to recognize the ancestral form of any two or more species, even if we carefully compared its anatomy with that of its modified descendants, unless we also had a nearly complete chain of intermediate links.
It is theoretically possible that one of two living forms descended from the other -- for instance, a horse from a tapir. In that case, direct intermediate links would have existed between them. But this would mean that one form had remained unchanged for a very long time while its descendants underwent enormous change, and the principle of competition between organisms -- between parent and offspring -- makes this an extremely rare event. In all cases, new and improved forms of life tend to replace the old, unimproved ones.
According to the theory of natural selection, all living species have been connected to the parent species of each genus by differences no greater than we see between the natural and domestic varieties of the same species today. And those parent species, now generally extinct, were in their turn similarly connected to more ancient forms -- and so on backward, always converging toward the common ancestor of each great group. So the number of intermediate and transitional links between all living and extinct species must have been inconceivably vast. But if this theory is true, such forms certainly lived upon the earth.
Apart from our failure to find fossil remains of these infinitely numerous connecting links, someone might object that there simply hasn't been enough time for so much evolutionary change, given that all changes happen slowly. It's nearly impossible for me to convey to a reader who isn't a practicing geologist the facts that lead the mind to grasp -- even faintly -- the immensity of geological time. Anyone who can read Sir Charles Lyell's great work, Principles of Geology, which future historians will recognize as having produced a revolution in natural science, and still not accept how vast past periods of time have been, may as well close this book right now. But it isn't enough just to study the Principles of Geology, or to read specialized papers by different researchers on individual formations, noting how each author tries to give some inadequate idea of how long each formation -- or even each individual layer -- took to form. The best way to grasp past time is to understand the forces at work: to learn how deeply the land surface has been worn away, and how much sediment has been deposited. As Lyell pointed out well, the extent and thickness of our sedimentary formations are both the result and the measure of the erosion that the earth's crust has undergone elsewhere. So a person should examine for themselves the great piles of stacked rock layers, and watch the streams carrying down mud, and the waves wearing away sea cliffs, in order to begin to comprehend something about the duration of past time -- the monuments of which surround us on every side.
It's worth walking along the coast, where it's formed of moderately hard rock, and observing the process of erosion. In most places, the tides reach the cliffs only briefly, twice a day, and the waves eat into them only when loaded with sand or pebbles -- there's good evidence that pure water does nothing to wear away rock. Eventually the base of the cliff is undermined, huge fragments fall, and these, lying where they fell, have to be worn down atom by atom until they're small enough to be rolled by the waves, at which point they're more quickly ground into pebbles, sand, or mud. But how often do we see, along the bases of retreating cliffs, rounded boulders thickly covered with marine life, showing how little they've been worn down and how seldom they're rolled about! Moreover, if we follow any line of eroding rocky cliff for a few miles, we find that it's only here and there -- along a short stretch or around a headland -- that the cliffs are actively wearing away right now. Elsewhere, the surface appearance and vegetation show that years have passed since the waves last reached their base.
We've recently learned, however, from the observations of Ramsay -- at the forefront of many excellent researchers including Jukes, Geikie, Croll, and others -- that erosion by the atmosphere is a far more important force than wave action along the coast. The entire land surface is exposed to the chemical action of air and rainwater with its dissolved carbonic acid, and in colder regions to frost. The broken-down material is carried down even gentle slopes during heavy rains, and to a greater extent than you might think, especially in dry regions, by the wind. It's then transported by streams and rivers, which, when flowing fast, deepen their channels and grind the fragments to pieces. On a rainy day, even in gently rolling countryside, you can see the effects of atmospheric erosion in the muddy streams flowing down every slope. Ramsay and Whitaker showed -- and it's a truly striking observation -- that the great lines of escarpment in the Wealden district and those running across England, which were formerly thought to be ancient coastlines, can't have formed that way. Each line is composed of one and the same rock formation, while our actual sea cliffs are everywhere formed by the cutting through of multiple different formations. Given this, we're forced to conclude that these escarpments owe their existence mainly to the fact that the rocks composing them have resisted atmospheric erosion better than the surrounding surface, which has gradually been lowered, leaving the harder rock standing out as ridges. Nothing impresses the mind with the vast duration of time more forcibly than the realization that atmospheric forces -- which seem so weak and work so slowly -- have produced such enormous results.
With this impression of how slowly the land is worn down through atmospheric and coastal erosion, it helps, in order to appreciate the duration of past time, to consider two things: on one hand, the masses of rock that have been removed from many large areas, and on the other, the thickness of our sedimentary formations. I remember being struck when viewing volcanic islands that had been worn by the waves and pared all around into vertical cliffs a thousand or two thousand feet high. The gentle slope of the lava flows, reflecting their formerly liquid state, showed at a glance how far the hard, rocky beds had once extended out into the open ocean. The same story is told even more clearly by geological faults -- those great cracks along which rock layers have been pushed up on one side, or dropped down on the other, by thousands of feet. Since the crust cracked -- and it doesn't much matter whether the movement was sudden or, as most geologists now believe, slow and accomplished in many stages -- the land surface has been so completely smoothed down that no trace of these enormous dislocations is visible from above. The Craven fault, for instance, extends for more than thirty miles, and along this line the vertical displacement of the rock layers ranges from 600 to 3,000 feet. Professor Ramsay has described a downthrow in Anglesey of 2,300 feet, and he tells me he fully believes there's one in Merionethshire of 12,000 feet. Yet in these cases there's nothing on the surface to show such enormous movements. The pile of rocks on either side of the crack has been smoothly swept away.
On the other hand, in all parts of the world, the piles of sedimentary rock layers are wonderfully thick. In the Andes, I estimated one mass of conglomerate at ten thousand feet. Although conglomerates probably accumulated faster than finer sediments, the fact that they're made of worn and rounded pebbles -- each one bearing the stamp of time -- helps show how slowly the mass must have been built up. Professor Ramsay has given me the maximum thickness, measured directly in most cases, of the successive formations in different parts of Great Britain. Here are the results:
Paleozoic strata (not including volcanic beds): 57,154 feet Secondary strata: 13,190 feet Tertiary strata: 2,240 feet
That makes a total of 72,584 feet -- very nearly thirteen and three-quarter miles. Some of these formations, which are represented in England by thin beds, are thousands of feet thick on the Continent. Moreover, between each successive formation there are, in the opinion of most geologists, blank periods of enormous length. So the lofty pile of sedimentary rocks in Britain gives only an inadequate idea of the time that elapsed during their accumulation. Thinking about all these facts impresses the mind in almost the same way as the futile attempt to grasp the idea of eternity.
Yet this impression is partly misleading. Mr. Croll, in a thought-provoking paper, argues that we don't err "in forming too great a conception of the length of geological periods," but rather in trying to estimate them in years. When geologists look at large and complex geological phenomena, and then at figures representing several million years, the two produce completely different effects on the mind, and the numbers are immediately pronounced too small. Regarding atmospheric erosion, Croll shows -- by calculating the known amount of sediment carried down annually by certain rivers, relative to their drainage areas -- that 1,000 feet of solid rock, as it gradually disintegrated, would be removed from the average level of the whole area in the course of six million years. This seems astonishing, and some factors suggest the figure may be too large. But even halved or quartered, it's still remarkable. Few of us really grasp what a million means. Croll offers this illustration: take a narrow strip of paper, eighty-three feet four inches long, and stretch it along the wall of a large hall. Then mark off one-tenth of an inch at one end. That tenth of an inch represents one hundred years, and the entire strip represents a million years. But keep in mind, in relation to the subject of this book, what a hundred years actually implies, represented as it is by a measurement utterly insignificant in a hall of those dimensions. Several leading breeders, within a single lifetime, have so dramatically modified some of the larger animals -- which reproduce much more slowly than most smaller ones -- that they've created what fully deserves to be called a new sub-breed. Few people have devoted careful attention to any one lineage for more than half a century, so a hundred years represents the work of two breeders in succession. We shouldn't suppose that species in the wild ever change as quickly as domestic animals under deliberate, methodical selective breeding. The fairer comparison would be with the effects of unconscious selection -- that is, the preservation of the most useful or beautiful animals with no intention of altering the breed. Yet even through this unconscious process, various breeds have noticeably changed over the course of two or three centuries.
Species, however, probably change much more slowly, and within any given region only a few change at the same time. This slowness follows from the fact that all the inhabitants of the same area are already so well adapted to one another that openings in the natural order don't appear until after long intervals, triggered by physical changes of some kind or by the immigration of new forms. Moreover, the right kind of variations -- those that might help some inhabitants better fit their new circumstances -- wouldn't always appear right away. Unfortunately, we have no way to determine, in terms of years, how long it takes to modify a species. But I'll return to the subject of time.
Now let's turn to our richest museums, and what a paltry display they offer! Everyone admits that our collections are incomplete. The remark of that admirable paleontologist Edward Forbes should never be forgotten: that very many fossil species are known and named from single and often broken specimens, or from just a few specimens collected at one location. Only a small portion of the earth's surface has been geologically explored, and no part with enough care, as the important discoveries made every year in Europe prove. No entirely soft-bodied organism can be preserved. Shells and bones decay and disappear when left on the seabed where sediment isn't accumulating. We probably hold a completely mistaken view when we assume that sediment is being deposited over nearly the entire ocean floor at a rate fast enough to bury and preserve fossil remains. Throughout an enormously large proportion of the ocean, the bright blue tint of the water speaks to its purity. The many recorded cases of one formation being conformably covered, after an immense gap in time, by another and later formation -- without the underlying bed showing any signs of erosion in the interval -- seem explicable only if the ocean floor sometimes lies undisturbed for ages. The remains that do get buried in sand or gravel will, when those beds are eventually lifted above sea level, generally be dissolved by rainwater charged with carbonic acid seeping through. Some of the many kinds of animals that live on the beach between high and low tide marks are rarely preserved. For instance, the several species of Chthamalinae (a subfamily of barnacles) coat rocks all over the world in enormous numbers. They're all strictly coastal, with the exception of a single Mediterranean species that lives in deep water -- and this one has been found as a fossil in Sicily, while not one other species has ever been found in any tertiary formation. Yet we know the genus Chthamalus existed during the Cretaceous period. Finally, many great deposits requiring vast amounts of time for their accumulation are completely devoid of fossils, and we can't explain why. One of the most striking examples is the Flysch formation, which consists of shale and sandstone several thousand -- occasionally even six thousand -- feet thick, extending for at least 300 miles from Vienna to Switzerland. Despite the most careful searching of this great mass, no fossils have been found except a few plant remains.
As for the land-dwelling organisms that lived during the Secondary and Paleozoic periods, it's unnecessary to point out that our evidence is fragmentary in the extreme. For instance, until recently not a single land snail was known from either of these vast periods, with the exception of one species discovered by Lyell and Dr. Dawson in the Carboniferous rocks of North America. But now land shells have been found in the Lias. Regarding mammal fossils, a glance at the historical table published in Lyell's Manual will bring home the truth -- how accidental and rare their preservation is -- far better than pages of detail could. Their rarity shouldn't surprise us when we remember how large a proportion of Tertiary mammal bones have been discovered either in caves or in lake-bed deposits, and that not a single cave or true lake-bed formation is known from the age of our Secondary or Paleozoic formations.
But the imperfection of the geological record results mainly from another, more important cause than any of the above: namely, the wide gaps in time between successive formations. This principle has been emphatically accepted by many geologists and paleontologists who, like E. Forbes, completely reject the idea that species change. When we see the formations listed in textbooks, or when we follow them in the field, it's hard to avoid thinking they follow one right after another. But we know, for instance, from Sir R. Murchison's great work on Russia, that wide gaps exist between the stacked formations in that country. The same is true in North America and many other parts of the world. The most skilled geologist, if their attention had been confined exclusively to these large territories, would never have suspected that during the periods blank and barren in their own country, great piles of sediment packed with new and distinctive life forms had been accumulating elsewhere. And if, in any single region, we can barely estimate the length of time that passed between consecutive formations, we can conclude that this could never be accurately determined anywhere. The frequent and dramatic changes in the mineral composition of consecutive formations, generally implying major changes in the geography of the surrounding lands from which the sediment came, fit well with the idea that vast intervals of time separated each formation.
We can, I think, see why the geological formations of each region are almost always intermittent -- that is, they haven't followed one another in an unbroken sequence. Few facts struck me more when examining many hundreds of miles of the South American coast, which has been uplifted several hundred feet in geologically recent times, than the absence of any recent deposits extensive enough to survive even a short geological period. Along the entire west coast, which is home to a distinctive marine fauna, tertiary beds are so poorly developed that no record of its successive distinct marine faunas will probably be preserved for some distant future age. A little reflection explains why. Along the rising coast of western South America, no extensive formations containing recent or Tertiary remains can be found anywhere, even though the supply of sediment must have been enormous for ages, given the tremendous erosion of the coastal rocks and the muddy rivers entering the sea. The explanation is that the coastal and near-shore deposits are continually worn away as soon as the slow, gradual uplift of the land brings them within reach of the grinding action of the waves.
We can, I think, conclude that sediment must accumulate in extremely thick, solid, or extensive masses to withstand the relentless action of the waves when first uplifted and during later shifts in level, as well as subsequent atmospheric erosion. Such thick and extensive accumulations of sediment can form in two ways. First, in the deep ocean -- but in that case the bottom won't be home to as many or as varied organisms as shallower seas, and the mass, when uplifted, will give only an imperfect record of the creatures that lived in the area during its accumulation. Second, sediment may be deposited to any thickness and extent over a shallow bottom, if that bottom continues slowly sinking. In this latter case, as long as the rate of sinking and the supply of sediment roughly balance each other, the sea will remain shallow and favorable for many diverse forms of life. This can produce a fossil-rich formation thick enough, when uplifted, to resist significant erosion.
I'm convinced that nearly all our ancient formations, which are rich in fossils throughout the greater part of their thickness, were formed during periods of subsidence. Since I first published this view in 1845, I've followed the progress of geology and have been surprised to see how author after author, when studying this or that major formation, has reached the same conclusion -- that it accumulated during subsidence. I might add that the only ancient Tertiary formation on the west coast of South America that has been bulky enough to resist the erosion it's so far endured -- though it will hardly survive to a distant geological age -- was deposited during a downward shift in level, and so gained considerable thickness.
All geological evidence tells us plainly that each area has undergone numerous slow shifts in level, and these shifts seem to have affected wide areas. So formations rich in fossils and thick enough to resist later erosion will have formed over wide areas during periods of sinking, but only where the supply of sediment was enough to keep the sea shallow and to bury and preserve remains before they had time to decay. On the other hand, as long as the seabed remained stationary, thick deposits couldn't have accumulated in the shallow parts most favorable to life. Even less could this have happened during the alternating periods of uplift -- or, more accurately, the beds that accumulated during those periods will generally have been destroyed by being lifted up into the zone of wave action.
These observations apply mainly to coastal and near-shore deposits. In the case of an extensive, shallow sea like the one covering a large part of the Malay Archipelago, where the depth ranges from thirty or forty to sixty fathoms, a widely spread formation might form during a period of uplift without suffering too badly from erosion during its slow rise. But the formation couldn't be very thick, because the rising movement would make it thinner than the depth of water in which it formed. Nor would the deposit be well consolidated, or capped by later formations, so it would likely be worn away by atmospheric weathering and wave action during later changes in level. However, Mr. Hopkins has suggested that if one part of the area sank after rising and before being eroded, the deposit formed during the rising phase, though thin, might be protected by fresh accumulations and so be preserved for a long time.
Mr. Hopkins also believes that sedimentary beds of considerable horizontal extent have rarely been completely destroyed. But all geologists -- except the few who think our present metamorphic schists and igneous rocks once formed the original nucleus of the globe -- will agree that these latter rocks have been stripped of their covering to an enormous extent. It's hardly possible that such rocks could have solidified and crystallized while exposed at the surface. But if the metamorphic action occurred at great depths in the ocean, the original protective layer of rock above them may not have been very thick. Granting, then, that gneiss, mica-schist, granite, diorite, and so on were once necessarily buried, how can we account for the vast exposed areas of such rocks in many parts of the world, except by concluding that they've been completely stripped of all the layers that once covered them? That such extensive areas exist is beyond doubt. The granitic region of Parime is described by Humboldt as being at least nineteen times the size of Switzerland. South of the Amazon, Boue maps out an area of these rocks equal to Spain, France, Italy, part of Germany, and the British Isles all combined. This region hasn't been carefully explored, but from the consistent testimony of travelers, the granitic area is very large. Von Eschwege describes a detailed cross-section of these rocks stretching 260 geographical miles inland from Rio de Janeiro in a straight line, and I traveled 150 miles in another direction and saw nothing but granitic rocks. Numerous specimens I collected along the entire coast from near Rio de Janeiro to the mouth of the Plata -- a distance of 1,100 geographical miles -- all belonged to this class. Inland, along the whole northern bank of the Plata, I saw, besides modern Tertiary beds, only one small patch of slightly metamorphosed rock, which alone could have been part of the original covering over the granitic series. Turning to a well-known region, the United States and Canada, as shown in Professor H. D. Rogers' beautiful map -- I estimated the areas by cutting out and weighing the paper -- I find that the metamorphic (excluding the "semi-metamorphic") and granite rocks exceed the whole of the newer Paleozoic formations in the ratio of 19 to 12.5. In many regions, the metamorphic and granite rocks would turn out to be far more extensive than they appear if all the sedimentary beds resting unconformably on them were removed -- beds that couldn't have been part of the original covering under which they crystallized. So it's likely that in some parts of the world, entire formations have been completely eroded away, without a trace left behind.
One observation here is worth a passing note. During periods of uplift, the area of dry land and the adjoining shallow seas will increase, and new habitats will often form -- all conditions favorable, as I've explained before, for the origin of new varieties and species. But during such periods there will generally be a blank in the geological record. Conversely, during periods of sinking, the inhabited area and the number of inhabitants will decrease (except along the shores of a continent when it first breaks up into an archipelago). Consequently, during subsidence, though there will be much extinction, few new varieties or species will arise. And it's during these very periods of subsidence that the fossil-richest deposits have accumulated.
From all these considerations, there's no doubt that the geological record, viewed as a whole, is extremely incomplete. But if we focus our attention on any one formation, it becomes much harder to understand why we don't find closely graded varieties between the related species that lived at its beginning and at its end. There are several recorded cases of the same species showing different varieties in the upper and lower parts of the same formation. Trautschold, for example, gives a number of instances with ammonites, and Hilgendorf has described a remarkable case of ten gradually changing forms of Planorbis multiformis in the successive beds of a freshwater formation in Switzerland. Although each formation undeniably took a vast number of years to be deposited, several reasons can explain why it shouldn't normally include a complete graded series of links between the species living at its start and finish. I can't assign precise weight to each of the following considerations, but here they are.
Although each formation may represent a very long stretch of time, each is probably short compared with the period needed to transform one species into another. I'm aware that two paleontologists whose opinions deserve great respect -- Bronn and Woodward -- have concluded that the average duration of each formation is two or three times longer than the average lifespan of individual species. But it seems to me that insurmountable difficulties prevent us from reaching any solid conclusion here. When we see a species first appearing in the middle of a formation, it would be extremely rash to conclude that it hadn't previously existed somewhere else. Likewise, when we find a species disappearing before the last layers were deposited, it would be equally rash to assume it went extinct at that point. We forget how small Europe is compared with the rest of the world, and the various stages of the same formation across Europe haven't been correlated with perfect accuracy.
We can safely conclude that marine animals of all kinds have undergone extensive migration due to climate changes and other factors. When we see a species first appearing in a formation, the most likely explanation is that it had just immigrated into that area. It's well known, for instance, that several species appear somewhat earlier in the Paleozoic beds of North America than in those of Europe -- apparently because time was needed for their migration from American to European seas. In examining the most recent deposits in various parts of the world, it's been noted everywhere that some species still living today are common in the deposit but have gone extinct in the immediately surrounding sea -- or conversely, that some are now abundant in the nearby sea but rare or absent in that particular deposit. It's an excellent lesson to reflect on the confirmed amount of migration by European species during the Ice Age, which forms only part of one geological period -- and also to consider the changes in land level, the extreme climate shifts, and the vast span of time all contained within that same glacial period. Yet it's doubtful whether, in any part of the world, sedimentary deposits containing fossils were continuously accumulating in the same area throughout that entire period. It's unlikely, for instance, that sediment was being deposited throughout the whole Ice Age near the mouth of the Mississippi, within the depth range where marine animals thrive best, because we know that major geographic changes occurred in other parts of America during that time. When such beds as were deposited in shallow water near the mouth of the Mississippi during some part of the Ice Age are eventually uplifted, fossils will probably first appear and disappear at different levels, owing to species migration and geographic changes. And in the distant future, a geologist examining these beds would be tempted to conclude that the average lifespan of the embedded fossils was shorter than the Ice Age itself -- when in reality it was far longer, extending from before the glacial epoch to the present day.
To get a perfect gradation between two forms in the upper and lower parts of the same formation, the deposit must have accumulated continuously over a long period -- long enough for the slow process of modification. So the deposit must be very thick, and the species undergoing change must have lived in the same area the whole time. But as we've seen, a thick formation rich in fossils throughout can only accumulate during a period of subsidence. And to keep the depth roughly the same -- which is necessary for the same marine species to keep living in the same spot -- the supply of sediment must nearly match the rate of sinking. But this same sinking will tend to submerge the land from which the sediment comes, reducing the supply even as the downward movement continues. In fact, this nearly exact balance between sediment supply and subsidence is probably a rare occurrence. More than one paleontologist has observed that very thick deposits are usually devoid of fossils except near their upper or lower edges.
Each individual formation, like the whole pile of formations in any country, seems to have generally accumulated intermittently. When we see a formation made up of beds with very different mineral compositions, we can reasonably suspect that deposition was interrupted from time to time. Nor will the closest inspection of a formation tell us how long its deposition actually took. Many examples could be given of beds only a few feet thick that represent formations elsewhere thousands of feet thick, which must have taken an enormous period to accumulate -- yet no one unaware of this would have suspected the vast span of time represented by the thinner formation. Many cases could be given of the lower beds of a formation having been uplifted, eroded, submerged, and then re-covered by the upper beds of the same formation -- showing what wide, easily overlooked gaps occurred during its accumulation. In other cases, we have the clearest evidence in great fossilized trees, still standing upright as they grew, of many long intervals of time and changes in level during the process of deposition -- intervals that would never have been suspected had the trees not been preserved. Lyell and Dr. Dawson, for example, found Carboniferous beds 1,400 feet thick in Nova Scotia, with ancient root-bearing layers stacked one above another at no fewer than sixty-eight different levels. So when the same species occurs at the bottom, middle, and top of a formation, the probability is that it didn't live at that spot during the entire period of deposition. Instead, it disappeared and reappeared, perhaps many times, during the same geological period. Consequently, if it underwent a significant amount of change during the deposition of any one formation, a cross-section wouldn't include all the fine intermediate gradations that must, according to my theory, have existed -- only abrupt, though perhaps slight, changes of form.
It's critically important to remember that naturalists have no golden rule for distinguishing species from varieties. They allow each species a little variability, but when they encounter a somewhat larger difference between two forms, they rank both as separate species -- unless they can connect them through a complete series of intermediate gradations. And for the reasons just given, we can rarely hope to do this in any one geological section. Suppose B and C are two species, and a third, A, is found in an older, underlying bed. Even if A were exactly intermediate between B and C, it would simply be classified as a third, distinct species -- unless it could also be closely connected by intermediate varieties to one or both forms. Nor should we forget, as I've explained before, that A might actually be the ancestor of B and C, and yet wouldn't necessarily be intermediate between them in all respects. So we might find the parent species and its several modified descendants in the lower and upper beds of the same formation, yet without numerous transitional gradations we wouldn't recognize their blood relationship, and would classify them as distinct species.
It's well known that many paleontologists have founded their species on extremely slight differences, and they do this even more readily when the specimens come from different sub-stages of the same formation. Some experienced shell experts are now reclassifying many of the very finely distinguished species of d'Orbigny and others as mere varieties. On this view, we do find the kind of evidence of change that the theory predicts. Look again at the later Tertiary deposits, which contain many shells believed by most naturalists to be identical with living species. Yet some excellent naturalists, like Agassiz and Pictet, insist that all these Tertiary species are specifically distinct, even though the difference is admitted to be very slight. So here, unless we believe these eminent naturalists have been misled by their imaginations, and that these late Tertiary species really show no difference whatsoever from their living counterparts -- or unless we accept, against the judgment of most naturalists, that these Tertiary species are all truly distinct from modern ones -- we have evidence of the frequent occurrence of exactly the kind of slight modifications the theory requires. If we look at somewhat wider intervals of time -- at distinct but consecutive stages of the same major formation -- we find that the embedded fossils, though universally ranked as specifically different, are far more closely related to each other than species found in more widely separated formations. So here again we have clear evidence of change in the direction the theory predicts. But I'll return to this subject in the following chapter.
With animals and plants that reproduce rapidly and don't travel far, there's reason to think, as we've seen before, that their varieties are generally local at first, and that such local varieties don't spread widely and replace their parent form until they've been modified and perfected to a considerable degree. Under this view, the chance of discovering all the early stages of transition between two forms in any one formation in any one country is small, since the successive changes are thought to have been local, confined to one spot. Most marine animals have a wide range, and we've seen that among plants, it's those with the widest range that most often produce varieties. So with shells and other marine animals, it's likely that those with the widest ranges -- far exceeding the limits of the known geological formations in Europe -- have most often given rise to local varieties and eventually to new species. This would again greatly reduce our chances of tracing the stages of transition in any single geological formation.
There's a more important consideration, leading to the same conclusion, recently emphasized by Dr. Falconer: the period during which each species was actively changing, though long in terms of years, was probably short compared with the period during which it remained unchanged.
We also shouldn't forget that even today, with perfect specimens available for examination, two forms can rarely be connected by intermediate varieties and thus proven to be the same species until many specimens have been collected from many places. With fossil species, this can rarely be done. We can perhaps best appreciate how unlikely it is that we'll be able to connect species through numerous, fine, intermediate fossil links by asking ourselves this: Will geologists of some future period be able to prove that our different breeds of cattle, sheep, horses, and dogs descended from a single stock or from several original stocks? Or again, will they be able to determine whether certain seashells along the shores of North America -- ranked by some experts as distinct species from their European counterparts, and by others as mere varieties -- are really varieties, or are truly distinct species? This could be settled by those future geologists only by discovering numerous intermediate gradations in the fossil record -- and such success is improbable in the highest degree.
It has been asserted over and over again by writers who believe in the immutability of species that geology provides no linking forms. As we'll see in the next chapter, this claim is certainly wrong. As Sir J. Lubbock has pointed out, "Every species is a link between other related forms." If we take a genus containing, say, twenty species, recent and extinct, and destroy four-fifths of them, no one doubts that the remaining ones would appear much more distinct from each other. If the most extreme forms in the genus happened to be destroyed, the genus itself would appear more distinct from related genera. What geological research has not revealed is the former existence of infinitely numerous gradations, as fine as those between existing varieties, connecting nearly all existing and extinct species. But this shouldn't be expected -- yet this objection has been repeatedly advanced as a most serious argument against my views.
It may be worth summing up these remarks on the causes of the geological record's imperfection with an imaginary illustration. The Malay Archipelago is roughly the size of Europe from the North Cape to the Mediterranean, and from Britain to Russia -- and therefore equal in area to all the geological formations that have been examined with any accuracy, except those of the United States of America. I fully agree with Mr. Godwin-Austen that the present condition of the Malay Archipelago, with its many large islands separated by wide, shallow seas, probably represents the former state of Europe while most of our formations were being deposited. The Malay Archipelago is one of the richest regions on earth for living things. Yet if every species that ever lived there were collected, how imperfectly would they represent the natural history of the world!
But we have every reason to believe that the land-dwelling organisms of the archipelago would be preserved in an extremely imperfect manner in the formations we suppose to be accumulating there. Not many of the strictly coastal animals, or those living on bare underwater rocks, would be buried in sediment. And those buried in gravel or sand wouldn't survive to a distant age. Wherever sediment wasn't accumulating on the seabed, or wasn't accumulating fast enough to protect organic remains from decay, no fossils could be preserved.
Formations rich in many kinds of fossils, and thick enough to survive into an age as distant in the future as the Secondary formations are in the past, would generally form in the archipelago only during periods of subsidence. These periods of subsidence would be separated from each other by immense intervals of time, during which the area would be either stationary or rising. While rising, the fossiliferous formations on the steeper shores would be destroyed almost as soon as they accumulated, by the relentless wave action, as we now see along the coasts of South America. Even throughout the extensive, shallow seas within the archipelago, sedimentary beds could hardly accumulate to any great thickness during periods of uplift, or become capped and protected by later deposits, and so would have little chance of surviving to a very distant future. During periods of subsidence, there would probably be much extinction. During periods of uplift, there would be much variation, but the geological record would then be less complete.
It's doubtful whether the duration of any one great period of subsidence over the whole or part of the archipelago, together with the simultaneous accumulation of sediment, would exceed the average lifespan of individual species -- and both conditions are essential for preserving all the transitional gradations between any two or more species. If those gradations weren't fully preserved, transitional varieties would simply appear as so many new, though closely related, species. It's also likely that each great period of subsidence would be interrupted by shifts in level, and that slight climate changes would occur during such long periods. In those cases, the inhabitants of the archipelago would migrate, and no closely consecutive record of their modifications could be preserved in any one formation.
Very many of the marine species in the archipelago now range thousands of miles beyond its boundaries. Analogy plainly suggests that it would be mainly these far-ranging species -- though only some of them -- that would most often produce new varieties. Those varieties would at first be local, confined to one place, but if they possessed any clear advantage, or once they were further modified and improved, they would slowly spread and replace their parent forms. When such varieties returned to their original homes, they would differ from their former state in a nearly uniform though perhaps extremely slight degree, and since they would be found in slightly different sub-stages of the same formation, they would -- following the principles used by many paleontologists -- be classified as new and distinct species.
If, then, there's some truth in these observations, we have no right to expect to find in our geological formations an infinite number of the fine transitional forms that, according to the theory, connected all past and present species of the same group into one long, branching chain of life. We should expect to find only a few links -- and we certainly do find some, related to one another more distantly or more closely. And these links, however close they may be, if found in different stages of the same formation, would be ranked by many paleontologists as distinct species. But I don't pretend that I would ever have suspected how poor the record is, even in the best-preserved geological sections, had not the absence of countless transitional links between the species living at the start and end of each formation weighed so heavily on my theory.
The abrupt way in which whole groups of species suddenly appear in certain formations has been pointed to by several paleontologists -- for instance, by Agassiz, Pictet, and Sedgwick -- as a fatal objection to the belief in the transmutation of species. If numerous species belonging to the same genera or families really sprang into life all at once, that fact would indeed be fatal to the theory of evolution through natural selection. The development by natural selection of a group of forms, all descended from a single ancestor, must have been an extremely slow process, and the ancestors must have lived long before their modified descendants. But we continually overestimate the completeness of the geological record, and falsely conclude that because certain genera or families haven't been found below a certain stage, they didn't exist before it. In all cases, positive fossil evidence can be trusted implicitly; negative evidence is worthless, as experience has shown again and again. We continually forget how large the world is compared with the area over which our geological formations have been carefully examined. We forget that groups of species may have existed and slowly multiplied elsewhere for a long time before they invaded the ancient archipelagoes of Europe and the United States. We don't allow enough for the enormous intervals of time between our consecutive formations -- intervals perhaps longer, in many cases, than the time it took for each formation to accumulate. These intervals would have provided time for species to multiply from a single ancestor, and in the succeeding formation, such groups of species would appear as if suddenly created.
Let me recall a point I made earlier: it might take a long succession of ages to adapt an organism to some entirely new way of life -- for instance, to fly through the air. The transitional forms would therefore likely remain confined to one region for a long time. But once the adaptation had been achieved and a few species had gained a great advantage over other organisms, a comparatively short time would be needed to produce many divergent forms that would spread rapidly and widely around the world. Professor Pictet, in his excellent review of this work, comments on early transitional forms and, using birds as an example, can't see how the successive modifications of the forelimbs of a supposed ancestral form could possibly have been advantageous. But look at the penguins of the Southern Ocean. Don't these birds have their forelimbs in exactly this intermediate state -- "neither true arms nor true wings"? Yet penguins hold their own victoriously in the battle for life, existing in enormous numbers and in many species. I don't claim that we see in penguins the actual transitional stages through which bird wings evolved. But what special difficulty is there in believing that it might benefit the modified descendants of the penguin to first become able to flap along the ocean surface like the loggerhead duck, and eventually to rise from the surface and glide through the air?
Let me give a few examples to illustrate these points and to show how easy it is to make the mistake of thinking that whole groups of species appeared all at once. Even in the short interval between the first and second editions of Pictet's great work on paleontology, published in 1844-46 and 1853-57, the conclusions about the first appearance and disappearance of several animal groups were considerably revised -- and a third edition would require still more changes. I can recall the well-known fact that in geology textbooks published not many years ago, mammals were always said to have appeared abruptly at the beginning of the Tertiary era. Yet now one of the richest known accumulations of fossil mammals belongs to the middle of the Secondary era, and true mammals have been discovered in the New Red Sandstone near the very beginning of that great series. Cuvier used to argue that no monkey occurred in any Tertiary stratum, but now extinct primate species have been discovered in India, South America, and in Europe, as far back as the Miocene. If not for the rare accident of footprints being preserved in the New Red Sandstone of the United States, who would have dared to suppose that at least thirty different bird-like animals, some of them gigantic, existed during that period? Not a fragment of bone has been discovered in those beds. Not long ago, paleontologists maintained that the entire class of birds appeared suddenly during the Eocene period. But now we know, on the authority of Professor Owen, that a bird certainly lived during the deposition of the Upper Greensand. And still more recently, that strange bird the Archaeopteryx -- with its long, lizard-like tail bearing a pair of feathers on each joint, and wings furnished with two free claws -- was discovered in the Jurassic limestone slates of Solenhofen. Hardly any recent discovery shows more powerfully how little we yet know about the former inhabitants of the world.
Let me give another example that particularly struck me, since it passed under my own eyes. In a paper on fossil sessile barnacles, I argued that given the large number of existing and extinct Tertiary species; the extraordinary abundance of individuals of many species all over the world, from the Arctic to the equator, living at various depths from the upper tidal limit to fifty fathoms; the perfect manner in which specimens are preserved in the oldest Tertiary beds; the ease with which even a fragment of a shell plate can be identified -- given all this, I concluded that if sessile barnacles had existed during the Secondary periods, they would certainly have been preserved and discovered. And since not a single species had been found in beds of that age, I concluded that this great group had suddenly developed at the start of the Tertiary era. This troubled me greatly, adding, as I then thought, yet another instance of the sudden appearance of a major group of species. But my work had hardly been published when a skilled paleontologist, M. Bosquet, sent me a drawing of a perfect specimen of an unmistakable sessile barnacle that he himself had extracted from the Chalk of Belgium. And, as if to make the case as striking as possible, this barnacle was a Chthamalus -- a very common, large, and widespread genus, of which not one species has yet been found even in any Tertiary stratum. Still more recently, a Pyrgoma, a member of a different subfamily of sessile barnacles, was discovered by Mr. Woodward in the Upper Chalk. So we now have abundant evidence that this group of animals existed during the Secondary period.
The case most often cited by paleontologists as an example of the apparently sudden appearance of a whole group is the bony fishes (teleosteans), which according to Agassiz appeared low in the Cretaceous period. This group includes the vast majority of living fish species. But certain Jurassic and Triassic forms are now generally accepted as teleosteans, and even some Paleozoic forms have been classified that way by one leading authority. If teleostean fish had truly appeared suddenly in the northern hemisphere at the start of the Cretaceous, that would have been remarkable -- but it wouldn't have been an insurmountable difficulty unless it could also be shown that these species appeared suddenly and simultaneously in other parts of the world. It's almost unnecessary to point out that hardly any fossil fish are known from south of the equator; and by scanning Pictet's Paleontology, you'll see that very few species are known from several formations in Europe. Some fish families today have a restricted range; the teleosteans may formerly have had a similarly restricted range, and after developing extensively in one sea, may have spread widely. Nor do we have any right to assume that the world's seas have always been as freely connected from south to north as they are today. Even now, if the Malay Archipelago were converted to dry land, the tropical parts of the Indian Ocean would form a large, perfectly enclosed basin in which any great group of marine animals might multiply. There they would remain confined until some species adapted to cooler waters and managed to round the southern capes of Africa or Australia, reaching other, distant seas.
Given all these considerations -- given our ignorance of the geology of countries beyond Europe and the United States, and given the revolution in our paleontological knowledge brought about by the discoveries of the last dozen years -- it seems to me about as rash to make dogmatic pronouncements about the succession of life forms throughout the world as it would be for a naturalist to land for five minutes on a barren point in Australia and then presume to discuss the number and range of its species.
There is another related difficulty, and it's much more serious. I'm referring to the way species belonging to several major divisions of the animal kingdom appear suddenly in the lowest known fossil-bearing rocks. Most of the arguments that have convinced me that all existing species of the same group descended from a single ancestor apply with equal force to the earliest known species. For instance, there's no doubt that all the Cambrian and Silurian trilobites descended from some one crustacean, which must have lived long before the Cambrian age and probably differed greatly from any known animal. Some of the most ancient animals, such as the Nautilus and Lingula, don't differ much from living species. And according to the theory, it can't be supposed that these ancient species were the ancestors of all the species in their groups that appeared later, because they aren't intermediate in character in any way.
Consequently, if the theory is true, it's undeniable that before the lowest Cambrian layer was deposited, long periods elapsed -- as long as, or probably far longer than, the entire interval from the Cambrian age to the present day. And during those vast periods, the world swarmed with living creatures. Here we face a formidable objection: it seems doubtful whether the earth, in a condition fit for life, has existed long enough. Sir W. Thompson concludes that the consolidation of the crust can hardly have occurred less than twenty or more than four hundred million years ago, but probably not less than ninety-eight or more than two hundred million years. These very wide limits show how uncertain the data are, and other factors may yet need to be added to the calculation. Mr. Croll estimates that about sixty million years have elapsed since the Cambrian period, but this -- judging from the small amount of evolutionary change since the start of the Ice Age -- seems a very short time for the many and great changes in life that have certainly occurred since the Cambrian. And the previous one hundred and forty million years can hardly be considered sufficient for the development of the varied forms of life that already existed during the Cambrian period. It's probable, however, as Sir William Thompson insists, that the world at a very early stage was subject to more rapid and violent physical changes than those occurring now, and such changes would have tended to drive evolutionary changes at a correspondingly faster rate.
To the question of why we don't find rich fossil-bearing deposits belonging to these assumed earliest periods before the Cambrian system, I can give no satisfactory answer. Several eminent geologists, led by Sir R. Murchison, were until recently convinced that the fossils in the lowest Silurian layer represented the first dawn of life. Other highly qualified judges, such as Lyell and E. Forbes, disputed this conclusion. We shouldn't forget that only a small portion of the world is well known geologically. Not long ago, Barrande added another, lower stage -- rich in new and distinctive species -- beneath the then-known Silurian system. And now, still lower down in the Lower Cambrian formation, Mr. Hicks has found beds in South Wales rich in trilobites and containing various mollusks and segmented worms. The presence of phosphatic nodules and organic matter, even in some of the lowest apparently lifeless rocks, probably indicates life at those periods. And the existence of the Eozoon in the Laurentian formation of Canada is generally accepted. There are three great series of rock layers beneath the Silurian system in Canada, and in the lowest of these the Eozoon is found. Sir W. Logan states that their "combined thickness may possibly far surpass that of all the succeeding rocks, from the base of the Paleozoic series to the present time. We are thus carried back to a period so remote that the appearance of the so-called primordial fauna (of Barrande) may by some be considered a comparatively modern event." The Eozoon belongs to the most simply organized of all animal classes, but is highly organized for its class. It existed in countless numbers and, as Dr. Dawson has remarked, certainly preyed on other tiny organisms, which must have lived in great numbers. So the words I wrote in 1859 about the existence of living beings long before the Cambrian period -- which are almost the same as those later used by Sir W. Logan -- have proved true. Nevertheless, the difficulty of explaining why there aren't vast piles of fossil-rich rock layers beneath the Cambrian system remains very great. It doesn't seem likely that the most ancient beds have been entirely worn away by erosion, or that their fossils have been completely destroyed by metamorphic processes, because if that were the case, we'd find only small remnants of the formations immediately above them, and those would always appear in a partially metamorphosed condition. But the descriptions we have of the Silurian deposits across immense territories in Russia and North America don't support the idea that the older a formation is, the more consistently it has suffered extreme erosion and metamorphism.
The problem must remain unexplained for now, and it can fairly be raised as a valid argument against the views I'm defending here. But to show that an explanation may eventually come, I'll offer the following hypothesis. From the nature of the fossils found in the various formations of Europe and the United States -- which don't appear to have lived in very deep water -- and from the miles-thick sediment these formations are composed of, we can conclude that from first to last, large islands or tracts of land, from which the sediment was derived, existed near the present continents of Europe and North America. This same view has since been supported by Agassiz and others. But we don't know what conditions existed during the intervals between the successive formations -- whether Europe and the United States were dry land during those gaps, or a shallow underwater surface near land where sediment wasn't being deposited, or the floor of a deep, open ocean.
Looking at the existing oceans, which are three times as extensive as the land, we see them dotted with many islands. But hardly any truly oceanic island (with the possible exception of New Zealand, if it can be called truly oceanic) has yet been found to preserve even a remnant of any Paleozoic or Secondary formation. So we can perhaps conclude that during the Paleozoic and Secondary periods, neither continents nor large continental islands existed where our oceans now extend -- because if they had, Paleozoic and Secondary formations would almost certainly have accumulated from sediment worn from their shores, and would have been at least partially uplifted by the shifts in level that must have occurred during those enormously long periods. If we can draw any conclusions from these facts, it would be that where our oceans now extend, oceans have extended from the remotest period on record. And on the other hand, where continents now exist, large tracts of land have existed -- subject, no doubt, to great shifts in level -- since the Cambrian period. The colored map attached to my volume on Coral Reefs led me to conclude that the great oceans are still mainly areas of subsidence, the great archipelagoes still areas of oscillating levels, and the continents areas of uplift. But we have no reason to assume this has always been so from the beginning of time. Our continents seem to have been built by a predominance, across many oscillations, of the force of uplift. But might the areas of predominant movement have shifted over the ages? In periods long before the Cambrian epoch, continents may have existed where oceans now spread, and clear, open oceans may have existed where our continents now stand. Nor would we be justified in assuming that if, say, the bed of the Pacific Ocean were now converted to dry land, we would find sedimentary formations older than the Cambrian in recognizable condition there -- even supposing they had once been deposited. It might well be that layers that had sunk several miles closer to the earth's center, under the enormous weight of the water above, would have undergone far more metamorphic change than layers that had always remained nearer the surface. The immense areas in some parts of the world -- in South America, for example -- of exposed metamorphic rocks, which must have been heated under great pressure, have always seemed to me to require some special explanation. Perhaps what we see in these vast areas are the formations from long before the Cambrian epoch, in a completely metamorphosed and denuded state.
The several difficulties discussed in this chapter -- that although we find many links between existing and formerly existing species in our geological formations, we don't find infinitely numerous fine transitional forms closely joining them all together; that whole groups of species appear suddenly in our European formations; and that formations rich in fossils are almost entirely absent, as far as we currently know, beneath the Cambrian rocks -- are all undoubtedly very serious. We can see this in the fact that the most eminent paleontologists -- Cuvier, Agassiz, Barrande, Pictet, Falconer, E. Forbes, and others -- and all our greatest geologists -- Lyell, Murchison, Sedgwick, and the rest -- have unanimously, and often passionately, maintained the immutability of species. But Sir Charles Lyell now lends the weight of his great authority to the opposite side, and most geologists and paleontologists have been greatly shaken in their former belief. Those who consider the geological record to be in any degree complete will undoubtedly reject my theory at once. For my part, following Lyell's metaphor, I look at the geological record as a history of the world imperfectly kept and written in a changing dialect. Of this history we possess the last volume alone, covering only two or three countries. Of this volume, only here and there a short chapter has been preserved, and of each page, only here and there a few lines. Each word of the slowly changing language, more or less different in the successive chapters, may represent the forms of life entombed in our consecutive formations, which falsely appear to have been abruptly introduced. On this view, the difficulties discussed above are greatly diminished, or even disappear.
Now let's see whether the various facts and patterns in the geological succession of living things fit better with the traditional view that species never change, or with the idea that they slowly and gradually evolve through variation and natural selection.
New species have appeared very slowly, one after another, both on land and in the water. Lyell has shown that the evidence for this in the various Tertiary stages is nearly impossible to resist. Every year, new discoveries fill in the gaps between those stages and make the transition between extinct and surviving forms look more gradual. In some of the most recent geological beds -- undoubtedly ancient when measured in years -- only one or two species have gone extinct, and only one or two are new, appearing there for the first time, either locally or, as far as we know, anywhere on Earth. The Secondary formations are more fragmented, but as Bronn has pointed out, the species embedded in each formation didn't all appear or disappear at the same time.
Species belonging to different genera and classes haven't changed at the same rate or to the same degree. In the older Tertiary beds, a few shells from still-living species can be found among a multitude of extinct forms. The paleontologist Falconer gave a striking example of this: a living crocodile species is found alongside many extinct mammals and reptiles in the sub-Himalayan deposits. The Silurian Lingula has barely changed from the living species of that genus, while most other Silurian mollusks and all the crustaceans have changed enormously. Land organisms seem to have changed faster than marine ones -- a striking example of this has been observed in Switzerland. There's some reason to think that organisms higher on the scale of complexity change more quickly than simpler ones, though there are exceptions. The amount of evolutionary change, as Pictet has noted, isn't the same in each successive formation. Yet if we compare any formations that aren't extremely close together in time, we'll find that all the species have undergone some change. Once a species has disappeared from the face of the Earth, we have no reason to think the exact same form ever reappears. The strongest apparent exception to this rule is Barrande's so-called "colonies," which intrude for a time into the middle of an older formation and then let the pre-existing fauna reappear. But Lyell's explanation -- that this is simply a case of temporary migration from a different geographical region -- seems to settle the matter.
All of these facts fit well with our theory, which includes no fixed law of development forcing all the inhabitants of an area to change abruptly, simultaneously, or to the same degree. The process of evolution must be slow and will generally affect only a few species at any given time, because each species' variability is independent of every other's. Whether the variations that arise will be accumulated through natural selection to a greater or lesser degree -- producing more or less permanent change -- depends on many factors: whether the variations are beneficial, how freely crossbreeding occurs, the slowly changing physical conditions of the region, the immigration of new species, and the nature of the other organisms competing with the varying species. So it's not at all surprising that one species keeps the same form much longer than others, or that when it does change, it changes less. We see similar patterns among the living inhabitants of different countries. For instance, the land snails and beetles of Madeira have come to differ considerably from their closest relatives on mainland Europe, while the marine shells and birds have stayed the same. We can probably explain the apparently faster rate of change in land-dwelling and more complex organisms compared to marine and simpler ones by the more intricate relationships that higher organisms have with their living and nonliving surroundings, as I explained in an earlier chapter. When many inhabitants of any area have evolved and improved, we can understand -- based on the principle of competition and the all-important relationships between organisms in the struggle for life -- that any form that didn't become at least somewhat modified and improved would be likely to go extinct. This is why all the species in a region eventually change, if we look at long enough intervals of time. Otherwise, they'd simply die out.
Within the same class, the average amount of change over long and equal periods of time may be roughly the same. But since lasting formations rich in fossils depend on large masses of sediment being deposited in sinking areas, our formations have almost inevitably accumulated at wide and irregular intervals. As a result, the amount of evolutionary change shown by fossils in consecutive formations isn't uniform. Each formation, on this view, doesn't mark a new and complete act of creation. It's just an occasional snapshot, taken almost at random, from an ever-slowly-changing drama.
We can clearly understand why a species, once lost, should never reappear, even if the exact same conditions of life returned. Although the offspring of one species might adapt to fill the role of another species in the natural world -- and no doubt this has happened countless times -- the two forms, old and new, would never be identical. Both would almost certainly inherit different traits from their distinct ancestors, and organisms that already differ would vary in different ways. For example, it's possible that if all our fantail pigeons were destroyed, breeders could create a new breed hardly distinguishable from the current one. But if the parent rock pigeon were also destroyed -- and in nature we have every reason to think that parent forms are generally replaced and driven to extinction by their improved offspring -- it would be incredible that a fantail identical to the existing breed could be produced from any other species of pigeon, or even from any other well-established domestic variety. The successive variations would almost certainly differ to some degree, and the newly formed variety would probably inherit some distinctive differences from its ancestor.
Groups of species -- that is, genera and families -- follow the same general rules in their appearance and disappearance as individual species do, changing more or less quickly and to a greater or lesser degree. Once a group disappears, it never comes back; its existence, as long as it lasts, is continuous. I'm aware that there seem to be some exceptions to this rule, but they are surprisingly few. So few, in fact, that E. Forbes, Pictet, and Woodward -- all strongly opposed to views like mine -- accept that it holds true. And the rule fits perfectly with the theory. All the species in the same group, however long the group has lasted, are modified descendants of one another and ultimately of a common ancestor. In the genus Lingula, for instance, the species that have appeared in succession through all geological ages must have been connected by an unbroken chain of generations, from the lowest Silurian rocks to the present day.
We saw in the last chapter that whole groups of species sometimes appear to have developed abruptly, and I tried to explain this fact, which would be fatal to my theory if true. But such cases are certainly exceptional. The general rule is a gradual increase in numbers until the group reaches its peak, followed sooner or later by a gradual decline. If you represented the number of species in a genus, or the number of genera in a family, as a vertical line of varying thickness rising through successive geological formations, the line would sometimes seem to begin not at a sharp point but abruptly -- though this is misleading. It then gradually thickens upward, often maintaining a steady width for a stretch, and finally thins out in the upper beds, marking the group's decline and eventual extinction. This gradual increase in the number of species within a group fits perfectly with the theory. Species of the same genus, and genera of the same family, can only increase slowly and progressively. The process of modification and the production of many related forms is necessarily slow and gradual: one species first gives rise to two or three varieties, which slowly become species, which in turn produce other varieties and species by equally slow steps, and so on -- like the branching of a great tree from a single trunk -- until the group becomes large.
So far I've only mentioned the disappearance of species and groups of species in passing. Under the theory of natural selection, the extinction of old forms and the production of new, improved ones are intimately connected. The old idea that all life on Earth was periodically swept away by catastrophes has been largely abandoned, even by geologists like Elie de Beaumont, Murchison, and Barrande, whose general views might naturally lead them to that conclusion. On the contrary, the study of the Tertiary formations gives us every reason to believe that species and groups of species disappear gradually, one after another -- first from one location, then from another, and finally from the world altogether. In some cases, though -- like when an isthmus breaks apart and floods of new inhabitants pour into an adjoining sea, or when an island finally sinks beneath the waves -- extinction may have been rapid. Both individual species and whole groups of species last for very unequal periods. Some groups, as we've seen, have endured from the earliest known dawn of life to the present day. Others disappeared before the end of the Paleozoic era. No fixed law seems to determine how long any single species or genus lasts. There's reason to think that the extinction of a whole group is generally slower than its initial rise. If we represent a group's appearance and disappearance as a vertical line of varying thickness, the line tapers more gradually at its upper end (marking the progress of extinction) than at its lower end (marking the group's first appearance and early growth). In some cases, however, the extinction of entire groups -- like the ammonites near the end of the Secondary period -- has been astonishingly sudden.
Extinction has been wrapped in gratuitous mystery. Some authors have even supposed that species, like individuals, have a fixed lifespan. No one has been more astonished than I at the extinction of species. When I found in La Plata the tooth of a horse embedded alongside the remains of Mastodon, Megatherium, Toxodon, and other extinct giants -- all of which coexisted with still-living shellfish at a very late geological period -- I was filled with amazement. After all, since the horse was reintroduced to South America by the Spanish, it has run wild across the entire continent and multiplied at an unparalleled rate. I asked myself: what could so recently have wiped out the ancient horse under conditions that seemed so favorable? But my astonishment was unfounded. Professor Owen quickly realized that the tooth, though very similar to a modern horse's, belonged to an extinct species. If that horse were still alive but somewhat rare, no naturalist would have been the least bit surprised by its rarity, since rarity is a feature of a huge number of species in all classes, in all countries. If we ask ourselves why this or that species is rare, we answer that something in its conditions of life is unfavorable -- but what that something is, we can almost never say. Supposing that fossil horse still existed as a rare species, we could have been certain -- from what we know of all other mammals, even the slow-breeding elephant, and from the history of the domestic horse's introduction to South America -- that under more favorable conditions it would have stocked the whole continent within a very few years. But we couldn't have identified the unfavorable conditions that kept its numbers down -- whether it was one factor or several, at what stage of the horse's life they acted, or how severely each one contributed. If conditions had been getting worse, however slowly, we certainly wouldn't have noticed it. Yet the fossil horse would have become rarer and rarer, and finally gone extinct -- its place seized by some more successful competitor.
It's extremely hard to always keep in mind that the growth of every living creature is constantly being held in check by unseen hostile forces, and that these same unseen forces are more than enough to cause rarity and ultimately extinction. This is so poorly understood that I've repeatedly heard people express surprise that such enormous creatures as the Mastodon and the even more ancient dinosaurs went extinct -- as if sheer size guaranteed victory in the battle of life. On the contrary, great size would in some cases speed up extinction, as Owen has pointed out, because of the larger amount of food required. Before humans inhabited India or Africa, something must have been keeping the elephant population in check. The highly capable Dr. Falconer believed that it was mainly insects, constantly harassing and weakening the elephants in India, that held back their numbers. Bruce reached the same conclusion about the African elephant in Abyssinia. And it's certain that insects and blood-sucking bats determine whether large introduced mammals can survive in several parts of South America.
In many cases in the more recent Tertiary formations, we can see that rarity comes before extinction. And we know this has been the pattern for animals that have been wiped out, either locally or entirely, by human activity. I'll repeat what I published in 1845: to accept that species generally become rare before they become extinct -- to feel no surprise at a species being rare, yet to marvel when it finally vanishes -- is much the same as accepting that sickness is the forerunner of death, feeling no surprise at sickness, but then wondering and suspecting foul play when the sick person dies.
The theory of natural selection is built on the idea that each new variety, and ultimately each new species, is produced and maintained by having some advantage over its competitors. The extinction of less successful forms almost inevitably follows. It works the same way with our domesticated breeds: when a new, slightly improved variety is raised, it first replaces the less improved varieties in its own neighborhood. When much improved, it gets transported far and wide -- like our Shorthorn cattle -- and takes the place of other breeds in other countries. The appearance of new forms and the disappearance of old ones, whether natural or artificial, are bound together. In thriving groups, the number of new species produced within a given time has at some periods probably exceeded the number of old species that went extinct. But we know that species haven't been increasing indefinitely, at least during the later geological periods. So looking at more recent times, we can conclude that the production of new forms has caused the extinction of roughly the same number of old ones.
Competition is generally most severe, as I've explained and illustrated with examples, between forms that are most similar to each other. So the improved, modified descendants of a species will generally drive its parent species to extinction. And if many new forms have developed from a single species, the closest relatives of that species -- the other species in the same genus -- will be the most vulnerable. In this way, I believe, a cluster of new species descended from one ancestor -- a new genus -- comes to replace an old genus belonging to the same family. But it must often have happened that a new species from one group seized the place occupied by a species from a completely different group, causing its extinction. If many related forms develop from the successful invader, many existing species will have to give way, and it will generally be related forms that suffer, since they share some inherited weakness. But whether it's species from the same class or a different one that give way to modified, improved species, a few of the losers may often survive for a long time by being adapted to some specialized way of life or by inhabiting some distant, isolated location where they escape severe competition. For instance, some species of Trigonia -- a large genus of shells in the Secondary formations -- survive in Australian seas. And a few members of the once-great, now nearly extinct group of ganoid fishes still inhabit our freshwater. So the complete extinction of a group is generally, as we've seen, a slower process than its rise.
As for the apparently sudden extinction of whole families or orders -- like trilobites at the end of the Paleozoic era and ammonites at the end of the Secondary period -- we should remember what I said earlier about the probably vast intervals of time between our consecutive formations. During those intervals, there may have been much slow extinction going on. Furthermore, when many species of a new group rapidly take over an area -- whether through sudden immigration or unusually fast diversification -- many older species will have been driven to extinction correspondingly fast. And the forms that give way will typically be related to one another, because they share the same inherited weaknesses.
So it seems to me that the way both individual species and whole groups go extinct fits well with the theory of natural selection. We don't need to be mystified by extinction. If we're going to be mystified by anything, let it be by our own presumption in imagining, even for a moment, that we understand the many complex factors on which each species' existence depends. If we forget for an instant that every species tends to increase enormously, and that some check is always at work -- though we rarely notice it -- the whole workings of nature will be completely obscured. When we can say precisely why this species is more abundant than that one, or why this species and not another can establish itself in a given country -- then, and not until then, should we feel surprised that we can't explain the extinction of any particular species or group.
Few paleontological discoveries are more striking than the fact that life forms change almost simultaneously around the world. Our European Chalk formation, for instance, can be recognized in many distant regions under the most different climates, even where not a fragment of actual chalk rock is found -- in North America, equatorial South America, Tierra del Fuego, the Cape of Good Hope, and the Indian peninsula. At all these far-flung locations, the fossils in certain beds bear an unmistakable resemblance to those of the European Chalk. It's not that the exact same species appear everywhere -- in some cases, not a single species is identical -- but they belong to the same families, genera, and subdivisions of genera, and sometimes share even trivial features like surface ornamentation. Moreover, forms not found in the European Chalk but present in the formations just above or below it appear in the same order at these distant points around the world. In the successive Paleozoic formations of Russia, Western Europe, and North America, several authors have observed a similar parallelism in life forms. The same holds true, according to Lyell, for the European and North American Tertiary deposits. Even if we completely set aside the few fossil species that are common to both the Old and New Worlds, the general parallelism in the succession of life forms through the Paleozoic and Tertiary stages would still be obvious, and the various formations could easily be matched up.
These observations, however, apply to the marine life of the world. We don't have enough data to judge whether land and freshwater organisms at distant locations changed in the same parallel way. We can doubt whether they did. If the Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to Europe from La Plata without any information about their geological age, no one would have guessed that they lived alongside seashells that are still alive today. But since these bizarre giants coexisted with the Mastodon and Horse, one could at least have inferred that they lived during one of the later Tertiary stages.
When I say that marine life forms changed simultaneously throughout the world, I don't mean within the same year, or even the same century. The expression doesn't even have a very precise geological meaning. If all the marine animals now living in Europe, and all those that lived there during the Pleistocene period -- a very remote time as measured in years, encompassing the entire Ice Age -- were compared with those now living in South America or Australia, even the most skilled naturalist would have difficulty saying whether the present or the Pleistocene inhabitants of Europe more closely resembled those of the southern hemisphere. Similarly, several highly competent observers maintain that the living organisms of the United States are more closely related to those that lived in Europe during certain late Tertiary stages than to the present inhabitants of Europe. If this is true, then fossil-bearing beds now being deposited on the shores of North America could eventually be classified with somewhat older European beds. Nevertheless, looking to the far future, there can be little doubt that all the more recent marine formations -- the upper Pliocene, Pleistocene, and truly modern beds of Europe, North and South America, and Australia -- would be correctly ranked as geologically simultaneous. They contain fossils that are somewhat related to one another and don't include forms found only in the older deposits beneath them.
The fact that life forms change more or less simultaneously, in this broad sense, at distant parts of the world, has greatly impressed those excellent observers de Verneuil and d'Archiac. After noting the parallelism of Paleozoic life forms across various parts of Europe, they wrote: "If, struck by this strange sequence, we turn our attention to North America and discover an analogous series of phenomena, it will appear certain that all these changes in species -- their extinction and the introduction of new ones -- cannot be due to mere changes in ocean currents or other causes that are more or less local and temporary, but must depend on general laws governing the entire animal kingdom." Barrande has made equally forceful remarks to the same effect. It is indeed futile to look to changes in currents, climate, or other physical conditions as the cause of these great transformations in life forms across the world, under the most different climates. We must, as Barrande has said, look to some special law. We'll see this more clearly when we examine the present distribution of organisms and find how slight the connection is between the physical conditions of various countries and the nature of their inhabitants.
This great fact -- the parallel succession of life forms throughout the world -- is fully explained by the theory of natural selection. New species are formed by having some advantage over older forms. The forms that are already dominant, or have some edge over other species in their own region, give rise to the greatest number of new varieties or emerging species. We have clear evidence of this: the plants that are dominant -- the most common and widely distributed -- produce the greatest number of new varieties. It's also natural that dominant, variable, and widely spreading species, which have already invaded the territories of other species to some extent, would have the best chance of spreading still further and giving rise to new varieties and species in new regions. The process of spreading would often be very slow, depending on climate changes, geographical shifts, chance events, and the gradual acclimatization of new species to various climates they must pass through. But over time, dominant forms would generally manage to spread and would ultimately prevail. The spread would probably be slower for the land-dwelling inhabitants of separate continents than for marine inhabitants of the connected seas. So we'd expect to find, as we do, a less strict parallelism in the succession of land organisms than in marine ones.
Thus, it seems to me, the parallel and -- in a broad sense -- simultaneous succession of the same forms of life throughout the world fits well with the principle that new species form when dominant species spread widely and vary. These new species, being themselves dominant thanks to having some advantage over their already-dominant parents as well as other species, spread in turn, vary, and produce new forms. The old forms that are outcompeted and give up their places to the new and victorious ones will generally be related to one another, having inherited some shared weakness. Therefore, as new and improved groups spread around the world, old groups disappear, and the succession of forms everywhere tends to match up, both in their first appearance and final extinction.
There's one more point worth making on this subject. I've given my reasons for believing that most of our great fossil-rich formations were deposited during periods of subsidence, and that vast blank intervals, as far as fossils are concerned, occurred during periods when the sea floor was either stationary or rising, or when sediment wasn't being deposited fast enough to bury and preserve organisms. During these long, blank intervals, I believe the inhabitants of each region underwent considerable modification and extinction, and that there was much migration from other parts of the world. Since large areas tend to be affected by the same geological movements, it's likely that strictly contemporaneous formations have often accumulated over very wide areas in the same part of the globe. But we're far from justified in concluding that this was always the case. When two formations were deposited in two regions during nearly, but not exactly, the same period, we should find in both -- for the reasons I've explained -- the same general succession of life forms. But the species wouldn't match exactly, because there would have been a little more time in one region than the other for modification, extinction, and immigration.
I suspect that cases like this occur in Europe. Mr. Prestwich, in his excellent studies of the Eocene deposits of England and France, draws a close general parallel between the successive stages in the two countries. But when he compares certain stages in England with those in France, he finds that although the numbers of species belonging to the same genera show a curious agreement, the species themselves differ in ways that are very hard to explain given how close the two areas are -- unless an isthmus separated two seas inhabited by distinct but contemporaneous faunas. Lyell has made similar observations about some of the later Tertiary formations. Barrande also shows that there's a striking general parallelism in the successive Silurian deposits of Bohemia and Scandinavia, yet he finds a surprising amount of difference in the species. If the formations in these regions weren't deposited during exactly the same periods -- with a formation in one region often corresponding to a blank interval in the other -- and if in both regions species went on slowly changing during the accumulation of each formation and during the long intervals between them, then the formations in the two regions could be arranged in the same order based on the general succession of life forms. The order would appear to be strictly parallel, but the species wouldn't all be the same in the apparently corresponding stages.
Now let's look at the relationships between extinct and living species. They all fall into a few grand classes, and this fact is immediately explained by the principle of descent. As a general rule, the more ancient a form is, the more it differs from living forms. But as Buckland pointed out long ago, every extinct species can be classified either within a still-existing group or between existing groups. The fact that extinct forms help fill in the gaps between living genera, families, and orders is certainly true, but since this has often been ignored or even denied, it's worth making some remarks on the subject and giving some examples. If we look only at the living species of a class, or only at the extinct ones, the series is much less complete than if we combine both into one general system. In the writings of Professor Owen, we constantly encounter the term "generalized forms" applied to extinct animals. In the writings of Agassiz, we find "prophetic" or "synthetic types." These terms all mean that such forms are, in fact, intermediate or connecting links. Another distinguished paleontologist, M. Gaudry, has shown in the most striking way that many of the fossil mammals he discovered in Attica serve to bridge the gaps between living genera. Cuvier classified the ruminants and pachyderms as two of the most distinct orders of mammals, but so many fossil links have been unearthed that Owen had to completely revise the classification, placing certain pachyderms in the same suborder as ruminants. For example, he traces a gradual series of forms that dissolves the apparently wide gap between the pig and the camel. The hoofed mammals are now divided into even-toed and odd-toed groups, but the Macrauchenia of South America connects these two grand divisions to some extent. No one would deny that Hipparion is intermediate between the modern horse and certain other hoofed forms. What a remarkable connecting link in the chain of mammals is the Typotherium from South America -- as the name Professor Gervais gave it suggests -- which can't be placed in any existing order. The sea cows form a very distinct group of mammals, and one of the most remarkable features of the living dugong and manatee is the complete absence of hind limbs, without even a trace remaining. But the extinct Halitherium had, according to Professor Flower, a bony thighbone "attached to a well-defined hip socket," and it thus makes some approach to ordinary hoofed mammals, to which the sea cows are related in other ways. The whales are wildly different from all other mammals, but the Tertiary Zeuglodon and Squalodon -- placed by some naturalists in an order by themselves -- are considered by Professor Huxley to be undoubtedly whales, and "to constitute connecting links with the aquatic carnivores."
Even the wide gap between birds and reptiles has been shown by Huxley to be partially bridged in the most unexpected way: on one side by the ostrich and the extinct Archaeopteryx, and on the other by Compsognathus, one of the dinosaurs -- that group which includes the most gigantic of all land reptiles. Turning to the invertebrates, Barrande -- and no higher authority could be named -- states that he learns more every day that, although Paleozoic animals can certainly be classified within existing groups, at that ancient time the groups were not as sharply separated from each other as they are now.
Some writers have objected to any extinct species or group being considered intermediate between two living species or groups. If what they mean is that an extinct form must be directly intermediate in all its features between two living forms, the objection is probably valid. But in a natural classification, many fossil species certainly stand between living species, and some extinct genera stand between living genera -- even between genera belonging to different families. The most common pattern, especially with very distinct groups like fish and reptiles, seems to be this: if the living groups are distinguished today by, say, twenty characteristics, the ancient members were separated by somewhat fewer. So the two groups formerly made a somewhat closer approach to each other than they do now.
It's a common belief that the more ancient a form is, the more it tends to connect groups that are now widely separated. This observation must be limited to groups that have undergone substantial change over geological time, and it would be hard to prove in every case -- since now and then a living animal like the Lepidosiren is discovered with affinities pointing toward very different groups. Yet if we compare the older reptiles and amphibians, the older fish, the older cephalopods, and the Eocene mammals with their modern counterparts, we must admit there's truth in the observation.
Let's see how well these various facts fit the theory of descent with modification. Since the subject is somewhat complex, I need to ask the reader to turn to the diagram in Chapter IV. We can suppose that the numbered italic letters represent genera and the dotted lines diverging from them represent species within each genus. The diagram is much too simple -- too few genera and too few species are shown -- but that doesn't matter for our purposes. The horizontal lines can represent successive geological formations, and all the forms below the uppermost line can be considered extinct. The three living genera, a14, q14, and p14, form a small family; b14 and f14 form a closely related family or subfamily; and o14, i14, and m14 form a third family. These three families, together with the many extinct genera on the various lines of descent branching from the parent form (A), will make up an order, because all will have inherited something in common from their ancient ancestor. Following the principle of the continued tendency to divergence of character, which I illustrated using this same diagram, the more recent any form is, the more it will generally differ from its ancient ancestor. This explains the rule that the most ancient fossils differ the most from living forms. We shouldn't assume, however, that divergence of character is inevitable. It depends entirely on whether a species' descendants can seize on many different niches in the natural world. So it's entirely possible, as we've seen with some Silurian forms, that a species might keep changing slightly in response to slightly altered conditions yet retain its same general features through a vast stretch of time. This is represented in the diagram by the letter F14.
All the many forms, extinct and living, descended from (A) make up one order, as I noted before. Through the continued effects of extinction and divergence of character, this order has become divided into several subfamilies and families, some of which died out at different times and some of which have survived to the present.
By looking at the diagram, we can see that if many of the extinct forms embedded in successive formations were discovered at several points low in the series, the three living families on the uppermost line would become less distinct from each other. If, for instance, the genera a1, a5, a10, f8, m3, m6, and m9 were unearthed, the three families would be so closely linked together that they'd probably have to be merged into one great family -- much as has happened with ruminants and certain pachyderms. Yet someone who objected to calling these extinct genera "intermediate" -- the ones that link the living genera of three families -- would be partly justified, since they are intermediate not directly but only through a long and roundabout path through many very different forms. If many extinct forms were discovered above one of the middle horizontal lines (say, above No. VI) but none from below it, then only two of the families (those on the left, a14 etc. and b14 etc.) would have to be merged. The remaining two families would be less distinct from each other than before the fossils were found, but they'd still be separate. Similarly, if the three families made up of eight genera (a14 to m14) on the uppermost line differ from each other by half a dozen important features, then the families that existed at the period marked VI would certainly have differed by fewer features, since at that earlier stage of descent they'd have diverged less from their common ancestor. This is how ancient, extinct genera often end up being more or less intermediate in character between their modified descendants, or between their collateral relatives.
In nature, the process would be far more complicated than the diagram shows. The groups would have been more numerous, they would have lasted for extremely unequal lengths of time, and they would have been modified in varying degrees. Since we possess only the last volume of the geological record, and that in very fragmentary condition, we have no right to expect -- except in rare cases -- to fill in the wide gaps in the natural classification and thereby unite distinct families or orders. All we can reasonably expect is that groups which have undergone substantial modification within known geological periods should, in older formations, make some slight approach toward each other. The older members should differ less from each other in some of their features than the living members of the same groups do. And according to the combined evidence of our best paleontologists, this is frequently the case.
Thus, on the theory of descent with modification, the main facts about the relationships of extinct forms to each other and to living forms are explained in a satisfying way. And they are completely inexplicable on any other view.
On this same theory, it's clear that the fauna during any one great period of Earth's history will be intermediate in general character between what preceded it and what followed it. The species that lived at the sixth great stage of descent in the diagram are the modified offspring of those at the fifth stage and the parents of those that became still more modified at the seventh. So they could hardly fail to be roughly intermediate in character between the forms above and below them. We must, however, allow for the complete extinction of some earlier forms, for immigration of new forms from other regions, and for a large amount of modification during the long blank intervals between successive formations. With those allowances, the fauna of each geological period is undoubtedly intermediate in character between the preceding and following faunas. I need give only one example: when the Devonian system was first discovered, paleontologists immediately recognized its fossils as intermediate in character between those of the overlying Carboniferous and underlying Silurian systems. But each fauna isn't necessarily exactly intermediate, since unequal amounts of time have elapsed between consecutive formations.
It's no real objection to this statement -- that each period's fauna as a whole is roughly intermediate between the preceding and following ones -- that certain genera are exceptions to the rule. For instance, the species of mastodons and elephants, when arranged by Dr. Falconer in two ways -- first by their mutual affinities and second by their periods of existence -- don't match up. The most extreme species in character aren't the oldest or the most recent, and those that are intermediate in character aren't intermediate in age. But even in this and similar cases, suppose for a moment that the record of each species' first appearance and disappearance were complete (which is far from the case): we'd still have no reason to expect that forms produced in succession would necessarily last for corresponding lengths of time. A very ancient form might occasionally have survived much longer than a form produced later elsewhere, especially for land-dwelling organisms in separate regions. To compare small things with great: if the main living and extinct varieties of the domestic pigeon were arranged by their physical similarities, this arrangement wouldn't closely match the order in which they were created, and even less the order in which they disappeared. The parent rock pigeon still lives, while many varieties intermediate between the rock pigeon and the carrier pigeon have gone extinct. Carriers, which are extreme in the important trait of beak length, originated earlier than short-beaked tumblers, which are at the opposite end of the series in this respect.
Closely connected with the idea that fossils from an intermediate formation are intermediate in character is the fact, stressed by all paleontologists, that fossils from two consecutive formations are far more closely related to each other than fossils from two widely separated formations. Pictet gives a well-known example: the general resemblance of the fossils from the various stages of the Chalk formation, even though the species are distinct at each stage. This fact alone, from its generality, seems to have shaken Professor Pictet's belief in the immutability of species. Anyone familiar with the distribution of living species around the globe won't try to explain the close resemblance of distinct species in consecutive formations by claiming the physical conditions of the ancient areas stayed nearly the same. Remember that marine life forms have changed almost simultaneously throughout the world, and therefore under the most different climates and conditions. Consider the tremendous upheavals in climate during the Pleistocene period, which includes the entire Ice Age, and notice how little the marine species were affected.
On the theory of descent, it makes perfect sense that fossils from closely consecutive formations should be closely related even though they're classified as distinct species. Since the accumulation of each formation was often interrupted, and since long blank intervals separated successive formations, we shouldn't expect to find -- as I tried to show in the last chapter -- all the intermediate varieties between the species at the beginning and end of these periods in any one or two formations. But after intervals that are very long in years, though only moderately long in geological terms, we should find closely related forms -- what some authors call "representative species." And that's exactly what we do find. In short, we find just the kind of evidence of slow, barely perceptible changes in species that we have every right to expect.
We saw in Chapter IV that the best standard yet proposed for measuring an organism's degree of perfection or "highness" is the degree of differentiation and specialization of its parts at maturity. We also saw that since specialization benefits each organism, natural selection will tend to make each being's organization more specialized and perfect -- and in this sense, higher. This doesn't mean that natural selection can't leave many creatures with simple, unimproved structures suited to simple conditions of life. In some cases, it may even simplify an organism's structure, though leaving these simplified beings better adapted to their new way of life. In another and more general sense, new species become superior to their predecessors because they have to outcompete all the older forms they encounter in the struggle for life. We can therefore conclude that if the Eocene inhabitants of the world could be brought back and put into competition with today's inhabitants under a similar climate, the ancient forms would be beaten and driven to extinction by the modern ones. The same would happen to the Secondary forms if pitted against Eocene ones, and to Paleozoic forms against Secondary ones. So by this fundamental test of victory in the battle for life, as well as by the standard of organ specialization, modern forms ought -- according to the theory of natural selection -- to stand higher than ancient ones. Is this actually the case? A large majority of paleontologists would say yes, and their answer seems correct, though it's difficult to prove.
It's no valid objection to this conclusion that certain brachiopods have barely changed since an extremely remote geological period, or that certain land and freshwater shells have remained nearly the same since they first appeared. Nor is it an insurmountable difficulty that the foraminifera, as Dr. Carpenter has emphasized, haven't advanced in organization since the Laurentian epoch. Some organisms would have to remain suited to simple conditions of life, and what could be better adapted to that role than these lowly organized protozoans? Such objections would be fatal to my theory if it required advancement in organization as a necessary outcome. They would also be fatal if these foraminifera, for instance, could be shown to have first appeared during the Laurentian epoch, or the brachiopods during the Cambrian -- because then there wouldn't have been enough time for these organisms to have developed to the level they'd already reached. Once an organism has advanced to any given point, there's no requirement under natural selection for it to keep progressing, though it will need to be slightly modified in each successive age to hold its place as conditions change slightly. The objections I've mentioned all hinge on how old the world really is and when the various forms of life first appeared -- and that's very much open to debate.
The question of whether organization as a whole has advanced is extraordinarily complex. The geological record, always imperfect, doesn't extend far enough back to show with unmistakable clarity that organization has greatly advanced within the known history of the world. Even today, looking at members of the same class, naturalists don't agree on which forms should be ranked highest. Some consider sharks the highest fish because of their structural similarities to reptiles in certain important respects. Others rank the bony fishes (teleosteans) as highest. The ganoid fishes stand between sharks and teleosteans. Today, teleosteans are overwhelmingly dominant in numbers, but formerly only sharks and ganoids existed. So depending on which standard of "highness" you choose, you could say that fish have either advanced or declined in organization. Trying to compare members of completely different groups on a scale of highness seems hopeless. Who will decide whether a cuttlefish is higher than a bee -- that insect which the great Von Baer believed to be "in fact more highly organized than a fish, though built on a completely different plan"? In the complex struggle for life, it's entirely plausible that crustaceans not very high in their own class might outcompete cephalopods, the most advanced mollusks. Such crustaceans, though not highly developed, would rank very high among invertebrates if judged by the most decisive test of all: the law of battle. Beyond these inherent difficulties in deciding which forms are most advanced, we shouldn't just compare the highest members of a class at two different periods -- though that's undoubtedly an important factor. We should compare all members, high and low, at the two periods. At an ancient time, the highest and lowest mollusk-like animals -- cephalopods and brachiopods -- swarmed in enormous numbers. Today, both groups are greatly reduced, while other groups intermediate in organization have greatly expanded. Consequently, some naturalists maintain that mollusks were formerly more highly developed than now. But a stronger case can be made the other way, by considering the vast reduction in brachiopods and the fact that our living cephalopods, though few in number, are more highly organized than their ancient counterparts. We should also compare the relative proportions of higher and lower classes throughout the world at any two periods. If, for instance, fifty thousand kinds of vertebrate animals exist today, and we knew that only ten thousand kinds existed at some former time, that increase in numbers in the highest class -- implying a great displacement of lower forms -- would represent a clear advance in the organization of the world. We can see how hopelessly difficult it is to make a perfectly fair comparison, under such enormously complex conditions, of the organizational standards of imperfectly known faunas from successive periods.
We'll appreciate this difficulty more clearly by looking at certain present-day faunas and floras. From the extraordinary way that European organisms have recently spread over New Zealand and seized places that must previously have been occupied by native species, we must believe that if all the animals and plants of Great Britain were set free in New Zealand, a multitude of British forms would eventually become thoroughly established there and would drive many of the natives to extinction. On the other hand, since hardly a single species from the southern hemisphere has gone wild anywhere in Europe, we may well doubt whether, if all the organisms of New Zealand were released in Great Britain, any significant number could seize the places now occupied by our native plants and animals. In this sense, the organisms of Great Britain stand much higher on the scale than those of New Zealand. Yet the most skilled naturalist, just from examining the species of both countries, couldn't have predicted this result.
Agassiz and several other highly qualified judges insist that ancient animals resemble, to some extent, the embryos of modern animals in the same classes, and that the geological succession of extinct forms roughly parallels the embryological development of living forms. This view fits beautifully with our theory. In a later chapter, I'll try to show that adults differ from their embryos because variations arise at later stages of life and are inherited at corresponding ages. This process, while leaving the embryo almost unchanged, continually adds more and more differences to the adult over successive generations. The embryo thus becomes a kind of portrait, preserved by nature, of the species' earlier and less modified condition. This view may be true and yet may never be fully provable. Seeing, for instance, that the oldest known mammals, reptiles, and fish unquestionably belong to their proper classes -- though some of these ancient forms are slightly less distinct from each other than the typical members of the same groups are today -- it would be pointless to look for animals with the common embryological character of the vertebrates until fossil-rich beds are discovered far below the lowest Cambrian rocks. And the chances of that discovery are slim.
Mr. Clift showed many years ago that the fossil mammals from Australian caves were closely related to the living marsupials of that continent. In South America, a similar relationship is obvious even to an untrained eye, in the gigantic pieces of armor -- resembling those of the armadillo -- found in several parts of La Plata. Professor Owen has shown in the most striking way that most of the fossil mammals buried there in such abundance are related to South American types. This relationship is even more clearly seen in the wonderful collection of fossil bones assembled by Lund and Clausen from the caves of Brazil. I was so impressed by these facts that I strongly argued, in 1839 and 1845, for this "law of the succession of types" -- this "wonderful relationship in the same continent between the dead and the living." Professor Owen later extended the same generalization to the mammals of the Old World. We see the same law in his reconstructions of the extinct giant birds of New Zealand, and in the birds from the caves of Brazil. Mr. Woodward has shown that the same law holds for seashells, though because most mollusks are so widely distributed, it's not as clearly displayed by them. Other cases could be added, such as the relationship between the extinct and living land snails of Madeira, and between the extinct and living brackish-water shells of the Aralo-Caspian Sea.
Now, what does this remarkable law of the succession of the same types within the same areas mean? You'd have to be very bold to try to account for the difference between the inhabitants of Australia and South America (at the same latitude) by pointing to their different physical conditions -- and then turn around and explain the continuity of the same types within each continent during the later Tertiary periods by pointing to similar conditions. Nor can anyone claim that marsupials had to be produced chiefly or solely in Australia, or that edentates and other American types had to be produced solely in South America. We know that Europe was once home to numerous marsupials. And I've shown in the publications I mentioned that the distribution of land mammals in the Americas was formerly different from what it is now. North America once had a much stronger resemblance to the present character of the southern half of the continent, and the southern half was formerly more closely allied to the northern half than it is today. Similarly, we know from the discoveries of Falconer and Cautley that Northern India was once more closely related in its mammals to Africa than it is now. Analogous facts could be given for the distribution of marine animals.
On the theory of descent with modification, the great law of the long-enduring but not immutable succession of the same types within the same areas is immediately explained. The inhabitants of any part of the world will obviously tend to leave in that region, during the next period of time, closely related though somewhat modified descendants. If the inhabitants of one continent formerly differed greatly from those of another, their modified descendants will still differ in nearly the same way and to nearly the same degree. But after very long intervals of time and great geographical changes allowing much migration, the weaker forms will yield to the more dominant ones, and there will be nothing permanent about the distribution of living things.
Someone might ask mockingly whether I suppose that the Megatherium and other enormous creatures that once lived in South America left behind the sloth, armadillo, and anteater as their degenerate descendants. This can't be accepted for a moment. Those huge animals went completely extinct and left no offspring. But in the caves of Brazil there are many extinct species that are closely similar in size and in all other features to species still living in South America, and some of these fossils may have been the actual ancestors of living species. We must remember that, according to our theory, all species in the same genus descend from a single species. So if six genera, each with eight species, are found in one geological formation, and in the next formation there are six new related or representative genera each with the same number of species, then generally only one species from each old genus left modified descendants making up the new genera. The other seven species in each old genus died out without leaving any offspring. Or -- and this is the far more common case -- two or three species in only two or three of the six older genera were the parents of the new genera, while the remaining species and the remaining old genera went completely extinct. In declining groups, with genera and species shrinking in numbers (as with the edentates of South America), even fewer genera and species will leave modified descendants.
I've tried to show that the geological record is extremely imperfect; that only a small portion of the globe has been carefully explored geologically; that only certain classes of organisms have been widely preserved as fossils; that the number of both specimens and species in our museums is absolutely nothing compared to the number of generations that must have passed even during a single formation; that owing to subsidence being almost necessary for the accumulation of fossil-rich deposits thick enough to survive future erosion, great intervals of time must have elapsed between most of our successive formations; that there was probably more extinction during periods of subsidence and more variation during periods of elevation, and during the latter the record was least well kept; that each single formation was not deposited continuously; that the duration of each formation is probably short compared to the average lifespan of a species; that migration played an important role in the first appearance of new forms in any given area and formation; that widely ranging species are the ones that have varied most often and have most frequently given rise to new species; that varieties were at first local; and lastly, that although every species must have passed through numerous transitional stages, the periods during which each underwent modification -- though many and long in years -- were probably short compared to the periods during which each remained unchanged. These causes, taken together, will largely explain why, though we do find many links, we don't find endless varieties connecting all extinct and living forms by the finest gradual steps. It should also be kept in mind that any linking variety found between two forms would be classified, unless the whole chain could be perfectly reconstructed, as a new and distinct species -- since we have no reliable way to distinguish species from varieties.
Anyone who rejects this view of the geological record's imperfection will rightly reject the whole theory. Such a person might ask in vain: where are the countless transitional links that must once have connected the closely related or representative species found in successive stages of the same great formation? They might refuse to accept the immense intervals of time between our consecutive formations. They might overlook how important a role migration has played, when the formations of a great region like Europe are considered. They might point to the apparent -- but often falsely apparent -- sudden appearance of whole groups of species. They might ask: where are the remains of those infinitely numerous organisms that must have existed long before the Cambrian system was laid down? We now know that at least one animal did exist then. But I can answer this last question only by supposing that where our oceans now extend, they have extended for an enormous span of time, and where our continents now stand, they have stood since the beginning of the Cambrian. But long before that, the world looked very different. The older continents, made of formations more ancient than any we know, now exist only as remnants in a metamorphosed condition, or lie still buried under the ocean.
Setting aside these difficulties, the other great leading facts of paleontology fit beautifully with the theory of descent with modification through variation and natural selection. We can understand how new species come in slowly and successively; how species of different classes don't necessarily change together, at the same rate, or to the same degree, yet in the long run all undergo some modification. The extinction of old forms is the almost inevitable consequence of the production of new ones. We can understand why a species, once gone, never reappears. Groups of species increase in number slowly and persist for unequal periods of time, because the process of modification is necessarily slow and depends on many complex factors. The dominant species of large, dominant groups tend to leave many modified descendants, which form new subgroups and groups. As these form, the species of weaker groups, with their inherited disadvantages from a common ancestor, tend to go extinct together, leaving no modified offspring on the face of the Earth. But the complete extinction of a whole group has sometimes been a slow process, thanks to a few descendants lingering on in protected, isolated locations. Once a group has wholly disappeared, it doesn't come back, because the chain of generations has been broken.
We can understand how dominant forms that spread widely and produce the most varieties tend to populate the world with related but modified descendants, and how these generally succeed in displacing the groups that are weaker in the struggle for existence. After long intervals of time, then, life on Earth appears to have changed simultaneously.
We can understand how all forms of life, ancient and modern, fall into a few grand classes. We can understand, from the continued tendency toward divergence of character, why the more ancient a form is, the more it generally differs from those now living -- why ancient and extinct forms often fill gaps between existing forms, sometimes merging two groups previously classed as distinct, but more commonly just bringing them a little closer together. The more ancient a form is, the more often it stands in some degree intermediate between groups that are now distinct, because the more ancient it is, the more closely it will be related to, and consequently resemble, the common ancestor of groups that have since diverged widely. Extinct forms are seldom directly intermediate between living forms; they are intermediate only through a long and roundabout path through other extinct and different forms. We can clearly see why fossils from closely consecutive formations are closely related: they are closely linked by generation. We can clearly see why the remains from an intermediate formation are intermediate in character.
The inhabitants of the world at each successive period have beaten their predecessors in the race for life and are, to that extent, higher on the scale. Their structure has generally become more specialized, and this may explain the common belief among paleontologists that organization on the whole has progressed. Extinct and ancient animals resemble, to some degree, the embryos of more recent animals in the same classes, and this wonderful fact receives a simple explanation on our theory. The succession of the same types of structure within the same areas during the later geological periods stops being mysterious and becomes understandable on the principle of inheritance.
If, then, the geological record is as imperfect as many believe -- and at the very least, it can be said that the record can't be proven to be much more complete -- the main objections to the theory of natural selection are greatly weakened or disappear entirely. On the other hand, all the chief laws of paleontology plainly proclaim, as it seems to me, that species have been produced by ordinary generation: old forms supplanted by new and improved forms of life, the products of variation and the survival of the fittest.
When we look at how living things are distributed across the globe, the first great fact that jumps out is this: we can't explain the similarities or differences between the inhabitants of various regions simply by climate and other physical conditions. Nearly every author who has studied this subject recently has come to the same conclusion. The case of the Americas alone would almost be enough to prove it. If we set aside the arctic and northern temperate zones, all authors agree that one of the most fundamental dividing lines in geographical distribution runs between the New World and the Old. Yet if we travel across the vast American continent, from the central United States to its southern tip, we encounter the most wildly diverse conditions -- humid regions, arid deserts, towering mountains, grassy plains, forests, marshes, lakes, and great rivers, under almost every temperature. There is hardly a climate or condition in the Old World that can't be matched in the New -- at least closely enough for the same species to get by. Granted, you can point to small areas in the Old World that are hotter than anything in the New World, but these aren't home to a fauna different from the surrounding regions. It's rare to find a group of organisms confined to a small area whose conditions are only slightly unusual. Despite this general parallel between the conditions of the Old and New Worlds, their living inhabitants are strikingly different!
In the southern hemisphere, if we compare large stretches of land in Australia, South Africa, and western South America between latitudes 25 and 35 degrees, we find areas that are extremely similar in every condition. Yet it would be impossible to point out three faunas and floras more completely different from one another. Or consider this: we can compare the organisms of South America south of latitude 35 degrees with those north of 25 degrees -- regions separated by a span of ten degrees of latitude and exposed to considerably different conditions. Yet these organisms are incomparably more closely related to each other than they are to those of Australia or Africa under nearly the same climate. Similar facts could be given for sea life as well.
A second great fact that strikes us in our general survey is that barriers of any kind -- obstacles to free migration -- are closely and importantly connected to the differences between the organisms of various regions. We see this in the great differences among nearly all the land-based organisms of the New and Old Worlds, except in the northern parts, where the land almost connects. There, under a slightly different climate, there could have been free migration for northern temperate species, as there now is for strictly arctic ones. We see the same pattern in the enormous differences between the inhabitants of Australia, Africa, and South America at the same latitude -- countries that are about as isolated from each other as possible. On each continent, too, we see the same thing: on opposite sides of high, continuous mountain ranges, across great deserts, and even across large rivers, we find different organisms. But since mountain chains, deserts, and the like aren't as impassable, or as enduring, as the oceans separating continents, these differences are far less dramatic than those between distinct continents.
Turning to the sea, we find the same pattern. The marine inhabitants of the eastern and western shores of South America are very different, with extremely few shells, crustaceans, or echinoderms in common. But Dr. Gunther has recently shown that about thirty percent of the fish are the same on opposite sides of the Isthmus of Panama -- a fact that has led naturalists to believe the isthmus was once open water. West of the American shores, a wide stretch of open ocean extends without a single island as a stopping point for emigrants. Here we have a barrier of a different kind, and as soon as we cross it, we meet in the eastern Pacific islands with an entirely distinct fauna. So three marine faunas run north to south in parallel lines not far from each other, under similar climates, but because they're separated by impassable barriers -- either land or open sea -- they're almost completely distinct. On the other hand, traveling still further west from the eastern tropical Pacific islands, we encounter no impassable barriers and find countless islands as stepping stones, or continuous coasts, until after crossing a hemisphere we reach the shores of Africa. Over this vast distance, we meet with no well-defined, distinct marine faunas. Although very few marine animals are common to the three neighboring faunas of eastern and western America and the eastern Pacific islands, many fish range from the Pacific into the Indian Ocean, and many shells are shared between the eastern Pacific islands and the eastern shores of Africa -- on almost exactly opposite sides of the globe.
A third great fact, partly included in what I've just said, is the relatedness of organisms across the same continent or sea, even though the species themselves are different at different locations. This is a law of the widest generality, and every continent offers countless examples. Still, the naturalist traveling from north to south never fails to be struck by how successive groups of organisms -- specifically distinct but closely related -- replace each other. He hears closely related but distinct kinds of birds singing nearly similar songs and sees their nests built in much the same way, but not quite alike, with eggs colored in nearly the same pattern. The plains near the Straits of Magellan are home to one species of rhea (the American ostrich), while northward the plains of La Plata have another species of the same genus -- not a true ostrich or emu, like those living in Africa and Australia at the same latitude. On these same plains of La Plata we see the agouti and vizcacha, animals with nearly the same habits as our hares and rabbits, belonging to the same order of rodents, but clearly showing an American body plan. We climb the lofty peaks of the Cordillera and find an alpine species of vizcacha. We look to the waters and find not the beaver or muskrat, but the coypu and capybara -- rodents of the South American type. Countless other examples could be given. If we look to the islands off the American coast, however much they differ in geological structure, the inhabitants are essentially American, even though they may all be unique species. We can look back to past ages, as shown in the last chapter, and find American types prevailing on the American continent and in American seas. In these facts we see some deep organic connection, running through space and time, across the same areas of land and water, independent of physical conditions. Any naturalist who isn't led to ask what this connection is would have to be pretty dull.
The connection is simply inheritance -- that cause which, as far as we actually know, is the only thing that produces organisms quite like each other, or, as we see with varieties, nearly alike. The differences between inhabitants of different regions can be attributed to modification through variation and natural selection, and probably to a lesser degree to the direct influence of different physical conditions. How different they are will depend on how effectively migration of dominant life forms from one region to another has been blocked, and at how remote a period -- on the nature and number of the original immigrants -- and on how the inhabitants have interacted with each other, leading to the preservation of different modifications. The relationship of organism to organism in the struggle for life is, as I've often pointed out, the most important of all relationships. This is why barriers matter so much -- they check migration. And time matters too, for the slow process of modification through natural selection. Wide-ranging species, abundant in numbers, that have already triumphed over many competitors in their own extensive territories, will have the best chance of seizing new places when they spread into new countries. In their new homes they'll be exposed to new conditions and will frequently undergo further modification and improvement. They'll become still more successful and will produce groups of modified descendants. By this principle of inheritance with modification, we can understand why sections of genera, whole genera, and even entire families are confined to the same areas -- as is so commonly and conspicuously the case.
There is no evidence, as I noted in the last chapter, of any law of inevitable development. Since the variability of each species is an independent property, and natural selection will take advantage of it only insofar as it benefits each individual in its complex struggle for life, the amount of modification in different species won't be uniform. If a number of species, after having long competed with each other in their old homeland, were to migrate together into a new and subsequently isolated country, they would be unlikely to change much. Neither migration nor isolation by themselves do anything. These forces come into play only by bringing organisms into new relationships with each other, and to a lesser degree with the surrounding physical conditions. As we saw in the last chapter, some forms have kept nearly the same character from enormously remote geological periods. In the same way, certain species have migrated over vast distances and haven't changed much or at all.
Under this view, it's clear that the several species of the same genus, though living in the most distant corners of the world, must originally have come from the same source, since they all descended from the same ancestor. For species that have undergone little modification through whole geological periods, it's not hard to believe they migrated from the same region. During the vast geographical and climate changes that have occurred since ancient times, almost any amount of migration is possible. But for many other cases where we have reason to think the species of a genus evolved within comparatively recent times, there's a real difficulty here. It's also obvious that the individuals of the same species, though now living in distant and isolated regions, must have come from one spot where their parents first arose. As I've already explained, it's incredible that identical individuals could have been produced from specifically distinct parents.
This brings us to a question that naturalists have debated extensively: whether species were created at one point on the earth's surface, or at multiple points. There are undoubtedly many cases where it's extremely hard to understand how the same species could have migrated from a single point to the various distant and isolated places where it's now found. Still, the simplicity of the view that each species first appeared within a single region is compelling. Anyone who rejects it rejects the straightforward explanation of ordinary reproduction followed by migration, and instead invokes a miracle. It's universally accepted that in most cases, a species' range is continuous. When a plant or animal lives at two points so far apart, or separated by such an obstacle, that the gap couldn't easily have been crossed by migration, the fact is treated as something remarkable and exceptional. The inability to cross a wide sea is clearer for land mammals than perhaps for any other organisms. Accordingly, we find no unexplainable cases of the same mammals living at distant points of the world. No geologist has any trouble with Great Britain having the same four-legged animals as the rest of Europe, since the two were no doubt once connected. But if the same species can arise at two separate points, why don't we find a single mammal common to both Europe and Australia, or Europe and South America? The living conditions are nearly the same, as shown by the fact that a multitude of European animals and plants have become established in America and Australia. And some of the native plants are identical at these distant points of the northern and southern hemispheres. The answer, I believe, is that mammals haven't been able to migrate, while some plants, with their varied means of dispersal, have crossed the wide and broken gaps between continents. The great and striking influence of barriers makes sense only if the great majority of species arose on one side and haven't been able to cross to the other. Some few families, many subfamilies, very many genera, and a still greater number of sections of genera are confined to a single region. Several naturalists have observed that the most natural genera -- those in which the species are most closely related -- are generally confined to the same country, or if they have a wide range, it's continuous. What a strange anomaly it would be if the directly opposite rule held when we go one step lower, to the individuals of the same species, and these had not been -- at least initially -- confined to some one region!
It seems to me, as it has to many other naturalists, that the most probable view is this: each species was produced in one area alone, and subsequently migrated from that area as far as its powers of migration and ability to survive under past and present conditions allowed. There are undoubtedly many cases where we can't explain how the same species could have gotten from one point to another. But the geographical and climate changes that have certainly occurred within recent geological times must have broken up the formerly continuous ranges of many species. So the question becomes: are the exceptions to continuous ranges so numerous and so serious that we should abandon the belief -- made probable by general considerations -- that each species arose in one area and migrated from there as far as it could? It would be hopelessly tedious to discuss every exceptional case of the same species now living at distant and separated points, and I don't pretend that an explanation could be offered for many of them. But after some preliminary remarks, I'll discuss a few of the most striking classes of facts: first, the existence of the same species on the summits of distant mountain ranges, and at distant points in the arctic and antarctic regions; and second (in the following chapter), the wide distribution of freshwater organisms; and third, the occurrence of the same land species on islands and on the nearest mainland, though separated by hundreds of miles of open sea. If the presence of the same species at distant and isolated points on earth can in many cases be explained by each species having migrated from a single birthplace, then -- considering our ignorance about former climate and geography changes and about the various occasional means of transport -- the belief that a single birthplace is the rule seems to me incomparably the safest.
In discussing this subject we'll also be able to consider an equally important point: whether the several species of a genus, which must under my theory all descend from a common ancestor, can have migrated from some one area, undergoing modification along the way. If it can be shown that migration from one region to another probably occurred at some former period -- in cases where most species in one region are different from but closely related to those in another -- then our general view will be much strengthened. The explanation is straightforward under the principle of descent with modification. A volcanic island, for example, thrust up a few hundred miles from a continent, would probably receive a few colonists over time. Their descendants, though modified, would still be related by inheritance to the inhabitants of that continent. Cases like this are common and, as we'll see later, inexplicable under the theory of independent creation. This view of how species in one region relate to those in another doesn't differ much from that put forward by Mr. Wallace, who concluded that "every species has come into existence coincident both in space and time with a preexisting closely allied species." And it's now well known that he attributes this coincidence to descent with modification.
The question of single or multiple centers of creation is different from a related question: whether all the individuals of the same species descend from a single pair, or a single hermaphrodite, or whether -- as some authors suppose -- from many individuals created simultaneously. For organisms that never crossbreed (if any such exist), each species must descend from a succession of modified varieties that replaced each other but never blended with other individuals or varieties of the same species. So at each successive stage of modification, all the individuals of the same form would descend from a single parent. But in the great majority of cases -- namely, all organisms that normally reproduce sexually or occasionally crossbreed -- the individuals of the same species living in the same area will be kept nearly uniform by interbreeding. Many individuals will change simultaneously, and the total modification at each stage won't be due to descent from any single parent. To illustrate what I mean: our English racehorses differ from every other breed, but they don't owe their difference and superiority to descent from any single pair. They owe it to the continued care of selecting and training many individuals in each generation.
Before discussing the three classes of facts that present the greatest difficulty for the theory of "single centers of creation," I need to say a few words about the means of dispersal.
Sir Charles Lyell and other authors have covered this subject thoroughly. I can give here only the briefest summary of the most important facts. Changes in climate must have had a powerful influence on migration. A region now impassable for certain organisms because of its climate might have been a highway for migration when the climate was different. I'll discuss this in some detail shortly. Changes in land elevation must also have been highly influential: a narrow isthmus now separates two marine faunas; submerge it, or imagine it was once submerged, and the two faunas would blend together. Where the sea now extends, land may once have connected islands or even continents, allowing land organisms to pass from one to another. No geologist disputes that great changes in elevation have occurred within the time of existing organisms. Edward Forbes argued that all the islands in the Atlantic must have been recently connected with Europe or Africa, and Europe likewise with America. Other authors have hypothetically bridged every ocean and linked almost every island to some mainland. If Forbes's arguments are to be trusted, it must be admitted that scarcely a single island exists that hasn't recently been joined to some continent. This view cuts through the whole puzzle of how the same species reached the most distant points and removes many difficulties. But in my judgment, we aren't justified in accepting such enormous geographical changes within the period of existing species. It seems to me that we have plenty of evidence for great fluctuations in land and sea level, but not for such vast changes in the position and extent of our continents as would have connected them to each other and to the various intervening oceanic islands in recent times. I freely admit that many islands, now beneath the sea, formerly existed and may have served as stepping stones for plants and many animals during migration. In the coral-producing oceans, such sunken islands are now marked by rings of coral or atolls standing above them. When it is fully accepted -- as it will be someday -- that each species came from a single birthplace, and when we know something definite about the means of distribution, we'll be able to speculate confidently about former extensions of the land. But I don't believe it will ever be proved that within recent times most of our continents, which now stand quite separate, have been continuously or nearly continuously connected to each other and to the many existing oceanic islands. Several facts about distribution point against such enormous geographical upheavals in recent times: the great difference in marine faunas on opposite sides of almost every continent; the close relationship between the Tertiary inhabitants of various lands and seas and their present inhabitants; the degree of relatedness between island mammals and those of the nearest continent, partly determined (as we'll see later) by the depth of the intervening ocean. These and other facts argue against the enormous geographical revolutions that Forbes's view requires. The nature and proportions of oceanic island inhabitants also argue against former continental connections. And the almost universally volcanic composition of such islands doesn't support the idea that they're the remnants of sunken continents. If they had originally been continental mountain ranges, at least some of the islands would have been made of granite, metamorphic schists, old fossiliferous rocks, and other materials, instead of consisting of mere piles of volcanic matter.
I need to say a few words now about what are called accidental means of distribution -- though "occasional means" would be more accurate. I'll confine myself to plants here. In botanical works, this or that plant is often said to be poorly suited for wide dispersal, but how easily seeds can actually be transported across the sea is almost entirely unknown. Until I tried some experiments, with the help of Mr. Berkeley, it wasn't even known how long seeds could survive in seawater. To my surprise, I found that out of eighty-seven kinds, sixty-four germinated after being soaked for twenty-eight days, and a few survived 137 days of immersion. It's worth noting that certain plant orders were far more damaged than others: nine legumes were tried, and with one exception they resisted the saltwater poorly; seven species of the related orders Hydrophyllaceae and Polemoniaceae were all killed by a month's immersion. For convenience, I mostly tested small seeds without their capsules or fruit. Since all of these sank within a few days, they couldn't have floated across wide stretches of sea, whether or not they were damaged by saltwater. I then tried some larger fruits and capsules, and some of these floated for a long time. It's well known how different the buoyancy of green and seasoned timber is, and it occurred to me that floods would often wash dried plants or branches with seed capsules or fruit attached into the sea. So I dried the stems and branches of ninety-four plants with ripe fruit and placed them in seawater. Most sank quickly, but some that had floated only briefly when green floated much longer when dried. For example, ripe hazelnuts sank immediately, but when dried they floated for ninety days and afterward germinated when planted. An asparagus plant with ripe berries floated for twenty-three days; when dried it floated for eighty-five days, and the seeds then germinated. The ripe seeds of Helosciadium sank in two days; when dried they floated for over ninety days and then germinated. Altogether, out of the ninety-four dried plants, eighteen floated for more than twenty-eight days, and some of those eighteen floated for much longer. So since 64 out of 87 kinds of seeds germinated after twenty-eight days' immersion, and since 18 out of 94 distinct species with ripe fruit (though not all the same species as in the first experiment) floated for more than twenty-eight days after drying, we can tentatively conclude that the seeds of about 14 out of 100 kinds of plants from any country might be floated by sea currents for twenty-eight days and still retain their ability to germinate. In Johnston's Physical Atlas, the average speed of the Atlantic currents is thirty-three miles per day (some currents run at sixty miles per day). At this average rate, the seeds of 14 out of 100 plants from one country could be floated 924 miles across the sea to another country. If stranded there and blown inland by a gale to a favorable spot, they would germinate.
After my experiments, M. Martens tried similar ones in a better way: he placed the seeds in a box in the actual sea, so they were alternately wet and exposed to air, like truly floating plants. He tested ninety-eight seeds, mostly different from mine, but he chose many large fruits and seeds from plants that grow near the sea -- which would have favored both longer flotation and better resistance to saltwater. On the other hand, he didn't first dry the plants or branches with fruit, which, as we've seen, would have made some of them float much longer. His result was that 18 out of 98 of his different seed types floated for forty-two days and were then able to germinate. But I have no doubt that plants exposed to actual waves would float for less time than those protected from violent movement in our experiments. So it would perhaps be safer to assume that the seeds of about 10 out of 100 plants in a flora, after being dried, could float across 900 miles of sea and then germinate. The fact that larger fruits often float longer than small ones is interesting, since plants with large seeds or fruit -- which, as Alph. de Candolle has shown, generally have restricted ranges -- could hardly be transported by any other means.
Seeds can occasionally be transported in another way. Driftwood washes up on most islands, even those in the middle of the widest oceans. The natives of coral islands in the Pacific get stones for their tools solely from the roots of drifted trees -- these stones being a valuable royal tax. I've found that when irregularly shaped stones are embedded in tree roots, small pockets of soil are very frequently enclosed in the gaps between and behind them, packed so tightly that not a particle could be washed away during the longest voyage. From one small pocket of soil completely enclosed by the roots of an oak about fifty years old, three flowering plants germinated -- I'm certain of the accuracy of this observation. I can also show that the carcasses of birds floating on the sea sometimes escape being immediately eaten. Many kinds of seeds in the crops of floating birds retain their ability to grow for a long time. Peas and vetches, for instance, are killed by even a few days in seawater, but some taken from the crop of a pigeon that had floated on artificial seawater for thirty days nearly all germinated, to my surprise.
Living birds can hardly fail to be highly effective seed transporters. I could give many facts showing how frequently birds of all kinds are blown by storms vast distances across the ocean. We can safely assume that under such circumstances their flight speed would often be thirty-five miles an hour, and some authors have given much higher estimates. I've never seen nutritious seeds pass through a bird's intestines intact, but hard fruit seeds pass unharmed through even the digestive system of a turkey. Over the course of two months, I picked up twelve kinds of seeds from the droppings of small birds in my garden. These looked perfect, and some that were tested germinated. But here's a more important fact: the crops of birds don't secrete gastric juice and don't -- as I know from testing -- harm the germination of seeds at all. After a bird has found and eaten a large supply of food, it's well established that the grains don't all pass into the gizzard for twelve or even eighteen hours. A bird could easily be blown five hundred miles in that time. Hawks are known to hunt for tired birds, and the contents of their torn crops could easily get scattered. Some hawks and owls swallow their prey whole and after twelve to twenty hours regurgitate pellets that, as I know from experiments at the Zoological Gardens, contain seeds capable of germinating. Seeds of oat, wheat, millet, canary grass, hemp, clover, and beet germinated after spending twelve to twenty-one hours in the stomachs of different birds of prey. Two beet seeds even grew after being retained for two days and fourteen hours. Freshwater fish, I've found, eat seeds of many land and water plants. Fish are frequently eaten by birds, and so the seeds could be transported from place to place. I pushed many kinds of seeds into the stomachs of dead fish and then gave the bodies to fishing eagles, storks, and pelicans. After many hours, these birds either rejected the seeds in pellets or passed them in their droppings, and several of these seeds retained their ability to germinate. Some seeds, however, were always killed by this process.
Locusts are sometimes blown great distances from land. I personally caught one 370 miles from the coast of Africa, and I've heard of others caught at greater distances. The Rev. R. T. Lowe told Sir Charles Lyell that in November 1844, swarms of locusts visited the island of Madeira. They were in countless numbers, as thick as snowflakes in the heaviest blizzard, and extended upward as far as could be seen with a telescope. For two or three days they slowly circled round and round in an immense ellipse, at least five or six miles across, and at night landed on the taller trees, which were completely coated with them. Then they disappeared over the sea as suddenly as they had appeared and haven't visited the island since. Now, in parts of Natal, some farmers believe (though on insufficient evidence) that harmful seeds are introduced into their grasslands in the dung left by the great flights of locusts that often visit the region. Because of this belief, Mr. Weale sent me in a letter a small packet of dried pellets, from which I extracted several seeds under the microscope and raised seven grass plants from them, belonging to two species of two genera. So a swarm of locusts like the one that visited Madeira could easily introduce several kinds of plants to an island lying far from the mainland.
Although the beaks and feet of birds are generally clean, soil sometimes sticks to them. In one case I removed sixty-one grains, and in another twenty-two grains, of dry clay from the foot of a partridge, and in the clay there was a pebble as large as a vetch seed. Here's a better case: a friend sent me the leg of a woodcock with a little cake of dry soil stuck to the shank, weighing only nine grains. This contained a seed of the toad-rush (Juncus bufonius), which germinated and flowered. Mr. Swaysland of Brighton, who for the past forty years has closely watched our migratory birds, tells me he has often shot wagtails, wheatears, and whinchats on their first arrival at our shores, before they had even landed. He has several times noticed little cakes of soil stuck to their feet. Many facts could be given showing how generally soil is packed with seeds. For instance, Professor Newton sent me the leg of a red-legged partridge (Caccabis rufa) that had been wounded and couldn't fly, with a ball of hard earth stuck to it weighing six and a half ounces. The soil had been kept for three years, but when broken up, watered, and placed under a bell glass, no fewer than eighty-two plants sprang from it. These consisted of twelve monocotyledons, including the common oat and at least one kind of grass, and seventy dicotyledons, which -- judging from the young leaves -- included at least three distinct species. With such facts before us, can we doubt that the many birds that are annually blown by storms across great stretches of ocean, and that annually migrate -- for instance, the millions of quails crossing the Mediterranean -- must occasionally transport a few seeds embedded in dirt on their feet or beaks? I'll return to this subject later.
Since icebergs are known to sometimes carry soil and stones, and have even carried brushwood, bones, and the nest of a land bird, they must occasionally have transported seeds from one part to another of the arctic and antarctic regions, as Lyell suggested. During the Glacial period, they would have carried seeds from one part of the now-temperate regions to another. In the Azores, because of the large number of plants shared with Europe compared with other Atlantic islands that stand nearer the mainland, and because of their somewhat northern character compared with the latitude (as noted by Mr. H. C. Watson), I suspected that these islands had been partly stocked by ice-borne seeds during the Glacial epoch. At my request, Sir Charles Lyell wrote to M. Hartung to ask whether he had observed erratic boulders on these islands, and he replied that he had found large fragments of granite and other rocks that don't occur in the archipelago. We can therefore safely conclude that icebergs once deposited their rocky loads on the shores of these mid-ocean islands, and it's at least possible that they also brought the seeds of northern plants.
Considering that these several means of transport -- and other means that undoubtedly remain to be discovered -- have been operating year after year for tens of thousands of years, it would be truly remarkable if many plants had not been widely transported. These means of transport are sometimes called accidental, but this isn't strictly correct: ocean currents aren't accidental, nor is the prevailing direction of gales. It should be noted that almost no means of transport would carry seeds for very great distances. Seeds don't keep their vitality when exposed to seawater for a long time, nor could they be carried long in the crops or intestines of birds. These means would, however, be enough for occasional transport across a few hundred miles of sea, or from island to island, or from a continent to a neighboring island -- but not from one distant continent to another. The floras of distant continents wouldn't become mixed by such means; they would remain as distinct as they are now. Ocean currents, given their direction, would never bring seeds from North America to Britain, though they might -- and do -- bring seeds from the West Indies to our western shores. But even those seeds, if not killed by their long immersion in saltwater, couldn't survive our climate. Almost every year, one or two land birds are blown across the whole Atlantic from North America to the western shores of Ireland and England. But seeds could be transported by these rare wanderers only by one means -- dirt clinging to their feet or beaks -- which is itself a rare accident. Even then, how small the chance that a seed would land on favorable soil and grow to maturity! But it would be a great mistake to argue that because a well-stocked island like Great Britain hasn't -- as far as we know (and it would be very hard to prove this) -- received immigrants from Europe or any other continent through occasional transport in the last few centuries, a poorly stocked island, even one farther from the mainland, wouldn't receive colonists by similar means. Out of a hundred kinds of seeds or animals transported to an island, even one far less well stocked than Britain, perhaps no more than one would be so well suited to its new home as to become established. But this is no valid argument against what could be accomplished by occasional transport over the long span of geological time, while an island was being uplifted and before it had become fully stocked with inhabitants. On almost bare land, with few or no destructive insects or birds, nearly every seed that happened to arrive, if suited to the climate, would germinate and survive.
The presence of many identical plants and animals on mountain summits separated by hundreds of miles of lowlands -- where alpine species couldn't possibly survive -- is one of the most striking cases of the same species living at distant points without any apparent way of having migrated between them. It's remarkable enough to see so many of the same plant species living on the snowy Alps or Pyrenees and in the far north of Europe. But it's far more remarkable that the plants on the White Mountains of the United States are all the same as those of Labrador, and nearly all the same -- as the botanist Asa Gray tells us -- as those on the highest mountains of Europe. As long ago as 1747, such facts led the naturalist Gmelin to conclude that the same species must have been independently created at many separate points. We might have stuck with that belief if Agassiz and others hadn't called vivid attention to the Glacial period, which, as we'll now see, provides a simple explanation. We have evidence of almost every conceivable kind, both biological and geological, that within a very recent geological period, central Europe and North America experienced an arctic climate. The ruins of a house destroyed by fire don't tell their story more plainly than the mountains of Scotland and Wales, with their scored flanks, polished surfaces, and perched boulders, tell of the icy rivers that lately filled their valleys. So drastically has Europe's climate changed that in northern Italy, gigantic moraines left by ancient glaciers are now draped with vineyards and cornfields. Throughout a large part of the United States, erratic boulders and scored rocks plainly reveal a former cold period.
The former influence of the glacial climate on the distribution of Europe's inhabitants, as explained by Edward Forbes, is essentially this. But we can follow the changes more easily by imagining a new glacial period slowly arriving and then passing away, as formerly occurred. As the cold set in, and each more southern zone became suitable for the inhabitants of the north, these northern species would take the places of the former temperate-zone inhabitants. The latter would at the same time travel further and further south -- unless they were stopped by barriers, in which case they would die out. The mountains would become covered with snow and ice, and their former alpine inhabitants would descend to the plains. By the time the cold reached its peak, we'd have an arctic fauna and flora covering central Europe as far south as the Alps and Pyrenees, even stretching into Spain. The temperate regions of the United States would likewise be covered by arctic plants and animals, and these would be nearly the same as those of Europe, since the present circumpolar inhabitants -- which we're imagining traveling south everywhere -- are remarkably uniform around the world.
As the warmth returned, the arctic forms would retreat northward, closely followed by the organisms of the more temperate regions. And as the snow melted from the bases of the mountains, the arctic forms would colonize the cleared and thawed ground, always climbing higher as the warmth increased and the snow retreated further, while their relatives continued their northward journey. So when the warmth had fully returned, the same species that had recently lived together on the European and North American lowlands would once again be found in the arctic regions of both the Old and New Worlds, and on many isolated mountain summits far distant from each other.
This explains the identity of many plants at points as immensely remote as the mountains of the United States and those of Europe. It also explains why the alpine plants of each mountain range are especially related to the arctic forms living due north -- or nearly due north -- of them. The first migration when the cold arrived, and the return migration when warmth came back, would generally have been due south and north. The alpine plants of Scotland, as noted by Mr. H. C. Watson, and those of the Pyrenees, as noted by Ramond, are especially related to the plants of northern Scandinavia. Those of the United States are related to Labrador's flora. Those of the Siberian mountains are related to the arctic regions of that country. These views, based on the well-established fact of a former Glacial period, explain the present distribution of alpine and arctic organisms in Europe and America so convincingly that when we find the same species on distant mountain summits in other regions, we can almost conclude -- without further evidence -- that a colder climate once allowed them to migrate across the intervening lowlands, which are now too warm for them to survive.
As the arctic forms moved first south and then back north with the changing climate, they wouldn't have been exposed to any great range of temperatures during their long migrations. And since they all migrated together as a group, their relationships with each other wouldn't have been much disturbed. Following the principles I've laid out in this book, these forms wouldn't have been very likely to change much. But the alpine species, left isolated the moment warmth returned -- first at the bases and ultimately on the summits of mountains -- would have experienced something different. It's unlikely that all the same arctic species would have been stranded on mountain ranges far apart and survived there ever since. They would also, in all probability, have mixed with ancient alpine species that existed on the mountains before the Glacial epoch and were temporarily driven down to the plains during the coldest period. They would also have been exposed to somewhat different climate conditions afterward. Their relationships with each other would thus have been disturbed to some degree, making them more likely to change. And they have changed: if we compare the present alpine plants and animals of the several great European mountain ranges with one another, we find that while many species remain identical, some exist as varieties, some as doubtful forms or subspecies, and some as distinct yet closely related species representing each other on the different ranges.
In the illustration above, I assumed that at the start of our imaginary Glacial period, the arctic organisms were as uniform around the polar regions as they are today. But we also need to assume that many subarctic and some temperate forms were the same around the world, since some species now living on the lower mountain slopes and plains of North America and Europe are identical. How do I account for this uniformity of subarctic and temperate forms around the world at the start of the actual Glacial period? Today, the subarctic and northern temperate organisms of the Old and New Worlds are separated by the entire Atlantic Ocean and by the northern Pacific. During the Glacial period, when the inhabitants of both worlds lived further south than now, they must have been even more completely separated by wider stretches of ocean. So one might well ask how the same species could then -- or previously -- have entered both continents. The explanation, I believe, lies in the climate before the Glacial period began. During the later Pliocene period, the majority of the world's inhabitants were the same species as now, and we have good reason to think the climate was warmer than today. So we can suppose that organisms now living at latitude 60 degrees lived during the Pliocene further north, under the Polar Circle at latitude 66-67 degrees, and that present arctic species then lived on the broken land still nearer to the pole. Now, if we look at a globe, we see that under the Polar Circle there is almost continuous land from western Europe through Siberia to eastern America. This continuity of circumpolar land, together with the freedom for movement that a warmer climate would have allowed, explains the supposed uniformity of subarctic and temperate organisms in the Old and New Worlds before the Glacial epoch.
For reasons I've already mentioned, I believe our continents have long remained in roughly the same relative positions, though subjected to great fluctuations in elevation. I'm strongly inclined to extend the view above and infer that during some earlier and still warmer period, such as the older Pliocene, a large number of the same plants and animals inhabited the almost continuous circumpolar land. These plants and animals, in both the Old and New Worlds, began slowly migrating south as the climate cooled, long before the Glacial period began. We now see their descendants, mostly in modified form, in the central parts of Europe and the United States. This view explains the relationship -- with very little actual identity -- between the organisms of North America and Europe, a relationship that is truly remarkable considering the distance between the two areas and their separation by the whole Atlantic Ocean. It further explains the curious fact, noted by several observers, that the organisms of Europe and America during the later Tertiary stages were more closely related than they are now. During those warmer periods, the northern parts of the Old and New Worlds would have been almost continuously connected by land, serving as a bridge -- since made impassable by cold -- for the exchange of inhabitants.
During the slowly declining warmth of the Pliocene, as soon as the species common to the New and Old Worlds migrated south of the Polar Circle, they would have been completely cut off from each other. For the more temperate organisms, this separation must have happened ages ago. As plants and animals migrated south, they would have mixed in one great region with native American species and had to compete with them; in the other great region, with those of the Old World. Here we have everything favorable for extensive modification -- far more than for the alpine species left isolated within a much more recent period on the mountain ranges and arctic lands of Europe and North America. This is why, when we compare the present living organisms of the temperate regions in the New and Old Worlds, we find very few identical species (though Asa Gray has recently shown that more plants are identical than was formerly supposed). Instead, we find in every major group many forms that some naturalists rank as geographical races and others as distinct species, along with a host of closely related or representative forms that all naturalists consider specifically distinct.
As on land, so in the sea: a slow southward migration of a marine fauna that was nearly uniform along the continuous shores of the Polar Circle during the Pliocene or even earlier would explain -- under the theory of modification -- the many closely related forms now living in completely separated marine areas. This is how I think we can understand the presence of closely related living and extinct Tertiary forms on the eastern and western shores of temperate North America. It also explains the still more striking fact that many closely related crustaceans (as described in Dana's admirable work), some fish, and other marine animals inhabit both the Mediterranean and the seas of Japan -- areas now completely separated by the width of a whole continent and by wide stretches of ocean.
These cases of close relationship -- species either now or formerly inhabiting the seas on the eastern and western shores of North America, the Mediterranean and Japan, and the temperate lands of North America and Europe -- are inexplicable under the theory of creation. We can't claim that such species were created alike to match the nearly similar physical conditions of those areas. If we compare certain parts of South America with parts of South Africa or Australia, for example, we see countries closely similar in all their physical conditions whose inhabitants are utterly different.
But we must return to our main subject. I'm convinced that Forbes's view can be greatly extended. In Europe we see the clearest evidence of the Glacial period, from the western shores of Britain to the Ural range and south to the Pyrenees. From the frozen mammals and the mountain vegetation, we can infer that Siberia was similarly affected. In the Lebanon, according to Dr. Hooker, perpetual snow once covered the central ridge and fed glaciers that rolled 4,000 feet down the valleys. The same observer has recently found great moraines at low elevation on the Atlas range in North Africa. Along the Himalayas, at points 900 miles apart, glaciers have left marks of their former descent to low levels. In Sikkim, Dr. Hooker saw corn growing on ancient and gigantic moraines. South of the Asian continent, on the opposite side of the equator, we know from the excellent research of Dr. J. Haast and Dr. Hector that in New Zealand immense glaciers once descended to low levels. The same plants that Dr. Hooker found on widely separated mountains in New Zealand tell the same story of a former cold period. From facts communicated to me by the Rev. W. B. Clarke, it appears there are also traces of former glacial action on the mountains of southeastern Australia.
Looking to America: in the northern half, ice-carried rock fragments have been observed on the eastern side as far south as latitudes 36-37 degrees, and on the Pacific shores -- where the climate is now so different -- as far south as latitude 46 degrees. Erratic boulders have also been found on the Rocky Mountains. In the Cordillera of South America, nearly under the equator, glaciers once extended far below their present level. In central Chile, I examined a vast mound of debris with great boulders crossing the Portillo valley, which almost certainly was once a huge moraine. Mr. D. Forbes tells me that he found in various parts of the Cordillera, from latitude 13 to 30 degrees south, at about 12,000 feet, deeply grooved rocks resembling those he knew well from Norway, along with great masses of debris including scored pebbles. Along this entire stretch of the Cordillera, true glaciers don't exist today even at much greater heights. Further south, on both sides of the continent from latitude 41 degrees to the southernmost tip, we have the clearest evidence of former glacial action in countless immense boulders transported far from their source.
From all these facts -- that glacial action extended around both the northern and southern hemispheres; that in geological terms the period was recent in both; that it lasted a long time in both, judging by the amount of work it accomplished; and that glaciers recently descended to low levels along the entire Cordillera -- it at first seemed to me that we had to conclude the temperature of the whole world had dropped simultaneously during the Glacial period. But Mr. Croll, in a series of admirable papers, has tried to show that a glacial climate results from various physical causes set in motion by an increase in the eccentricity of Earth's orbit. All these causes work toward the same end, but the most powerful seems to be the indirect effect of orbital eccentricity on ocean currents. According to Mr. Croll, cold periods recur regularly every ten or fifteen thousand years, and at long intervals they become extremely severe, depending on certain conditions -- the most important being, as Sir Charles Lyell has shown, the relative position of land and water. Mr. Croll believes the last great Glacial period began about 240,000 years ago and lasted, with slight climate fluctuations, for about 160,000 years. As for more ancient glacial periods, several geologists are convinced by direct evidence that such episodes occurred during the Miocene and Eocene, not to mention still older formations. But the most important result for us from Mr. Croll's work is this: whenever the northern hemisphere passes through a cold period, the temperature of the southern hemisphere actually rises, with much milder winters, mainly because of changes in ocean current direction. And the reverse happens when the southern hemisphere goes through a glacial period. This conclusion sheds so much light on geographical distribution that I'm strongly inclined to trust it, but let me first present the facts that need explaining.
In South America, Dr. Hooker has shown that besides many closely related species, between forty and fifty of the flowering plants of Tierra del Fuego -- a significant portion of its sparse flora -- are shared with North America and Europe, despite these areas being enormously far apart in opposite hemispheres. On the high mountains of equatorial America, a host of unique species belonging to European genera occur. On the Organ Mountains of Brazil, the botanist Gardner found some temperate European, some Antarctic, and some Andean genera that don't exist in the hot lowlands between them. On the Silla of Caracas, the great Humboldt long ago found species belonging to genera characteristic of the Cordillera.
In Africa, several forms characteristic of Europe, along with a few representatives of the Cape of Good Hope flora, occur on the mountains of Abyssinia. At the Cape of Good Hope itself, a very few European species -- believed not to have been introduced by humans -- and several representative European forms are found on the mountains, though they haven't been discovered in the tropical parts of Africa between. Dr. Hooker has also recently shown that several plants growing on the upper parts of the lofty island of Fernando Po and on the neighboring Cameroon Mountains in the Gulf of Guinea are closely related to those on the mountains of Abyssinia and likewise to those of temperate Europe. It now appears, as I hear from Dr. Hooker, that some of these same temperate plants have been discovered by the Rev. R. T. Lowe on the mountains of the Cape Verde Islands. This extension of the same temperate forms almost under the equator, across the entire continent of Africa and to the mountains of the Cape Verde archipelago, is one of the most astonishing facts ever recorded in plant distribution.
On the Himalayas, on the isolated mountain ranges of the Indian peninsula, on the heights of Ceylon, and on the volcanic cones of Java, many plants occur that are either identical or represent each other, and at the same time represent plants of Europe not found in the intervening hot lowlands. A list of the genera collected on the highest peaks of Java conjures up the image of a collection made on a hillock in Europe. Even more striking is the fact that distinctive Australian forms are represented by certain plants growing on the summits of the mountains of Borneo. Some of these Australian forms, as I hear from Dr. Hooker, extend along the heights of the Malay Peninsula and are thinly scattered on one hand across India, and on the other as far north as Japan.
On the southern mountains of Australia, Dr. F. Muller has discovered several European species. Other species not introduced by humans occur on the lowlands, and a long list can be given, as Dr. Hooker tells me, of European genera found in Australia but not in the tropical regions between. In Dr. Hooker's admirable Introduction to the Flora of New Zealand, similar striking facts are given for the plants of that large island. So we see that certain plants growing on the higher tropical mountains everywhere in the world, and on the temperate plains of the north and south, are either the same species or varieties of the same species. It should be noted, however, that these plants aren't strictly arctic forms. As Mr. H. C. Watson has observed, "in receding from polar toward equatorial latitudes, the alpine or mountain flora really becomes less and less arctic." Besides these identical and closely related forms, many species inhabiting the same widely separated areas belong to genera not now found in the tropical lowlands between them.
These brief remarks apply only to plants, but a few similar facts could be given for land animals. Similar cases occur in marine life too. As an example, I can quote the highest authority, Professor Dana: "It is certainly a wonderful fact that New Zealand should have a closer resemblance in its crustacea to Great Britain, its antipode, than to any other part of the world." Sir J. Richardson also speaks of the reappearance on the shores of New Zealand, Tasmania, and elsewhere of northern forms of fish. Dr. Hooker tells me that twenty-five species of algae are common to New Zealand and Europe but haven't been found in the tropical seas between them.
From all these facts -- the presence of temperate forms on the highlands across equatorial Africa, along the Indian peninsula to Ceylon and the Malay Archipelago, and in a less clear-cut way across tropical South America -- it seems almost certain that at some former period, probably during the most severe part of a Glacial period, the lowlands of these great continents everywhere under the equator were home to a considerable number of temperate forms. At that time, the equatorial climate at sea level was probably similar to what's now experienced at five to six thousand feet at the same latitude, or perhaps even cooler. During this coldest period, the equatorial lowlands must have been clothed with a mixed tropical and temperate vegetation, like that which Hooker describes growing luxuriantly at four to five thousand feet on the lower slopes of the Himalayas, but perhaps with an even greater proportion of temperate forms. Similarly, on the mountainous island of Fernando Po in the Gulf of Guinea, Mr. Mann found temperate European forms beginning to appear at about five thousand feet. On the mountains of Panama, at only two thousand feet, Dr. Seemann found the vegetation resembling that of Mexico, "with forms of the torrid zone harmoniously blended with those of the temperate."
Now let's see whether Mr. Croll's conclusion -- that when the northern hemisphere suffered the extreme cold of the great Glacial period, the southern hemisphere was actually warmer -- sheds any light on the currently puzzling distribution of various organisms in the temperate parts of both hemispheres and on the tropical mountains. The Glacial period, measured in years, must have been very long. And when we remember how far some introduced plants and animals have spread within a few centuries, this period would have been more than enough time for any amount of migration. As the cold grew more and more intense, we know that arctic forms invaded the temperate regions. From the facts just given, there can hardly be a doubt that some of the hardiest, most dominant, and widest-spreading temperate forms invaded the equatorial lowlands. The inhabitants of these hot lowlands would at the same time have migrated to the tropical and subtropical regions of the south, because the southern hemisphere was warmer during this period. As the Glacial period waned and both hemispheres gradually recovered their former temperatures, the northern temperate forms living on the equatorial lowlands would have been driven back to their former homes or destroyed, replaced by equatorial forms returning from the south. Some of the northern temperate forms, however, would almost certainly have climbed any nearby highlands. If these were high enough, those species would have survived there for a long time, just as arctic forms survived on the mountains of Europe. They could have survived even if the climate wasn't perfectly suited to them, because the temperature change must have been very slow, and plants clearly have some capacity to acclimatize -- as shown by their passing on to their offspring different abilities to resist heat and cold.
In the regular course of events, the southern hemisphere would in its turn be subjected to a severe Glacial period, with the northern hemisphere becoming warmer. Then the southern temperate forms would invade the equatorial lowlands. The northern forms that had earlier been stranded on the mountains would now descend and mingle with the southern forms. When the warmth returned, the southern forms would go back to their former homes, leaving some species on the mountains and carrying southward with them some of the northern temperate forms that had descended from their mountain refuges. In this way, we'd end up with some species identical in the northern and southern temperate zones and on the mountains of the tropical regions between them. But species stranded for a long time on these mountains, or in opposite hemispheres, would have to compete with many new forms and would be exposed to somewhat different physical conditions. They would therefore be highly likely to change, and they would generally now exist as varieties or as representative species -- which is exactly what we see. We must also keep in mind that both hemispheres have experienced former Glacial periods. These earlier episodes would account, by the same principles, for the many quite distinct species that inhabit the same widely separated areas and belong to genera not now found in the tropical zones between.
It's a remarkable fact, strongly emphasized by Hooker for America and by Alph. de Candolle for Australia, that many more identical or slightly modified species have migrated from north to south than in the reverse direction. We do see a few southern forms on the mountains of Borneo and Abyssinia. I suspect this predominant north-to-south migration is due to the greater extent of land in the north and to northern forms having existed in larger numbers in their own territories. They would consequently have been pushed to a higher level of perfection -- or dominant power -- through natural selection and competition than the southern forms. So when the two groups became mixed in the equatorial regions during the alternations of Glacial periods, the northern forms were more powerful and were able to hold their places on the mountains and afterward migrate south along with the southern forms. The southern forms couldn't do the same with respect to the north. We see the same pattern today: very many European organisms have established themselves in La Plata, New Zealand, and to a lesser degree in Australia, beating the natives, while extremely few southern forms have become established anywhere in the northern hemisphere. This is true despite the fact that hides, wool, and other materials likely to carry seeds have been imported into Europe in large quantities from La Plata over the last two or three centuries and from Australia over the last forty or fifty years. The Nilgiri Mountains in India, however, offer a partial exception: there, as I hear from Dr. Hooker, Australian forms are rapidly spreading and becoming established. Before the last great Glacial period, the tropical mountains were no doubt stocked with unique alpine species. But these have almost everywhere been displaced by the more dominant forms generated in the larger areas and more effective workshops of the north. On many islands, the native species are nearly equaled or even outnumbered by those that have become established from elsewhere -- and this is the first stage toward extinction. Mountains are islands on the land, and their inhabitants have yielded to species from the larger areas of the north, just as the inhabitants of real islands have everywhere yielded -- and are still yielding -- to continental forms introduced through human activity.
The same principles apply to the distribution of land animals and marine organisms in the northern and southern temperate zones and on the tropical mountains. During the height of the Glacial period, when ocean currents were very different from today, some inhabitants of the temperate seas might have reached the equator. A few of these could perhaps have migrated south immediately by staying in the cooler currents, while others might have remained and survived in the colder depths until the southern hemisphere in its turn experienced a glacial climate that allowed their further progress -- in much the same way that, as Forbes pointed out, isolated pockets inhabited by arctic organisms still exist today in the deeper parts of the northern temperate seas.
I'm far from claiming that all the difficulties regarding the distribution and relationships of the identical and related species now living so widely separated in the north and south -- and sometimes on the mountain ranges between them -- are solved by the views I've given above. We can't trace the exact lines of migration. We can't say why certain species and not others have migrated, why certain species have changed and given rise to new forms while others have remained unchanged. We can't hope to explain such things until we can say why one species and not another becomes established by human introduction in a foreign land, or why one species ranges twice or three times as far and is twice or three times as common as another species within their own homes.
Various specific difficulties also remain to be solved -- for instance, the occurrence, as shown by Dr. Hooker, of the same plants at points as enormously remote as Kerguelen Island, New Zealand, and Tierra del Fuego. Icebergs, as Lyell suggested, may have played a role in their dispersal. The existence at these and other distant points of the southern hemisphere of species that, though distinct, belong to genera exclusively confined to the south is an even more remarkable case. Some of these species are so different that we can't suppose there has been enough time since the last Glacial period began for their migration and subsequent modification to the necessary degree. The facts seem to indicate that distinct species belonging to the same genera have migrated outward in radiating lines from a common center. I'm inclined to look -- in the southern hemisphere as in the northern -- to a former and warmer period before the last Glacial period, when the Antarctic lands, now covered with ice, supported a highly distinctive and isolated flora. It may be that before this flora was wiped out during the last Glacial epoch, a few forms had already been widely dispersed to various points of the southern hemisphere by occasional means of transport, using now-sunken islands as stepping stones. In this way, the southern shores of America, Australia, and New Zealand may have received a slight tinting from the same distinctive forms of life.
Sir Charles Lyell, in a striking passage, speculated in language almost identical to mine on the effects of great climate swings throughout the world on geographical distribution. And we've now seen that Mr. Croll's conclusion -- that successive Glacial periods in one hemisphere coincide with warmer periods in the opposite hemisphere -- together with the acknowledgment that species change slowly, explains a multitude of facts about the distribution of the same and related forms of life across the globe. The living waters have flowed during one period from the north and during another from the south, and in both cases have reached the equator. But the stream of life has flowed with greater force from the north than from the south, and has consequently flooded the southern regions more freely. As the tide leaves its drift in horizontal lines, rising higher on the shores where the tide rises highest, so have the living waters left their living drift on our mountain summits, in a line gently rising from the arctic lowlands to a great height under the equator. The various organisms thus left stranded may be compared with indigenous peoples, driven up and surviving in the mountain strongholds of almost every land -- serving as a record, full of interest to us, of the former inhabitants of the surrounding lowlands.
Since lakes and river systems are separated from each other by barriers of land, you might expect that freshwater organisms wouldn't range widely within a single country. And since the sea is an even more formidable barrier, you'd think they'd never reach distant countries. But the reality is exactly the opposite. Not only do many freshwater species from different groups have enormous ranges, but closely related species show up in a remarkably consistent pattern all around the world. When I first collected freshwater specimens in Brazil, I remember being struck by how similar the freshwater insects and shells were to those of Britain -- while the surrounding land animals were completely different.
But the wide-ranging power of freshwater organisms can, I think, mostly be explained by the fact that they've become well adapted for short, frequent migrations from pond to pond and stream to stream within their own countries. And from this ability, a tendency toward wide dispersal follows almost as a necessary consequence. I can only consider a few cases here, and some of the hardest to explain involve fish. It was once believed that the same freshwater species never existed on two distant continents. But Dr. Gunther has recently shown that Galaxias attenuatus lives in Tasmania, New Zealand, the Falkland Islands, and mainland South America. This is an extraordinary case, and probably points to dispersal from an Antarctic center during a former warm period. The case is made somewhat less surprising by the fact that species in this genus can somehow cross considerable stretches of open ocean -- one species is common to both New Zealand and the Auckland Islands, even though they're separated by about 230 miles. On the same continent, freshwater fish often range widely and almost capriciously: in two neighboring river systems, some species may be identical while others are completely different.
Fish are probably transported from time to time by what you might call accidental means. Living fish are not that rarely dropped at distant points by whirlwinds, and their eggs can survive for a considerable time after being removed from water. But their dispersal is probably best explained by recent changes in land level that caused rivers to flow into one another. There are also cases of this happening during floods, without any change in land level at all. The wide differences between fish on opposite sides of most continuous mountain ranges -- which must have completely prevented the merging of river systems on either side from very early on -- points to the same conclusion. Some freshwater fish belong to very ancient lineages, and in those cases there has been plenty of time for major geographical changes, and therefore time and opportunity for extensive migration. Moreover, Dr. Gunther has recently concluded from several lines of evidence that fish species tend to persist for a very long time. Saltwater fish can, with care, be gradually acclimated to fresh water. And according to Valenciennes, there is hardly a single group in which all members are confined to fresh water, so a marine species belonging to a freshwater group could travel far along the coast and then, quite likely, adapt to the fresh waters of a distant land.
Some species of freshwater shells have very wide ranges, and closely related species -- which, according to my theory, descend from a common ancestor and must have originated from a single source -- are found all over the world. Their distribution puzzled me at first, since their eggs are unlikely to be transported by birds, and both the eggs and the adults are immediately killed by seawater. I couldn't even figure out how some introduced species had spread so quickly across a single country. But two facts that I've personally observed -- and no doubt many more will be discovered -- shed some light on this. When ducks suddenly emerge from a pond covered with duckweed, I've twice seen those little plants clinging to their backs. And it has happened to me that in moving a bit of duckweed from one aquarium to another, I accidentally stocked the second tank with freshwater shells from the first. But another method may be even more effective: I suspended a duck's feet in an aquarium where many freshwater shell eggs were hatching, and I found that large numbers of the extremely tiny, just-hatched shells crawled onto the feet and clung so firmly that they couldn't be shaken off when removed from the water (though at a slightly more advanced age, they would voluntarily drop off). These just-hatched mollusks, though aquatic by nature, survived on the duck's feet in damp air for twelve to twenty hours. In that time, a duck or heron could fly at least six or seven hundred miles, and if blown across the sea to an oceanic island or any other distant point, it would certainly land on a pool or stream. Sir Charles Lyell told me that a Dyticus water beetle has been caught with an Ancylus -- a freshwater shell resembling a limpet -- firmly attached to it. And a water beetle of the same family, a Colymbetes, once flew on board the Beagle when we were forty-five miles from the nearest land. How much farther it might have been carried by a favorable wind, no one can say.
As for plants, it has long been known that many freshwater and even marsh species have enormous ranges, spanning both continents and the most remote oceanic islands. This is strikingly illustrated, according to Alph. de Candolle, in those large groups of land plants that have very few aquatic members -- the aquatic ones seem to immediately acquire a wide range, as if by consequence. I think favorable means of dispersal explain this. I've mentioned before that dirt sometimes sticks in considerable quantity to the feet and beaks of birds. Wading birds, which frequent the muddy edges of ponds, would be the most likely to have muddy feet if suddenly startled into flight. Birds of this group wander more than those of any other order and are occasionally found on the most remote and barren islands in the open ocean. They wouldn't be likely to land on the sea surface, so any dirt on their feet wouldn't get washed off. And when they reached land, they would naturally fly to their freshwater haunts. I don't think botanists realize just how packed the mud of ponds is with seeds. I've tried several little experiments, but I'll give only the most striking one here: in February, I took three tablespoons of mud from three different spots beneath the water at the edge of a little pond. This mud, when dry, weighed only six and three-quarter ounces. I kept it covered in my study for six months, pulling up and counting each plant as it grew. The plants were of many kinds, and altogether numbered 537 -- yet the sticky mud had all fit in a breakfast cup! Considering these facts, I think it would be truly inexplicable if waterbirds did not carry the seeds of freshwater plants to distant, unstocked ponds and streams. The same process may also work with the eggs of some of the smaller freshwater animals.
Other unknown factors have probably played a part as well. I've mentioned that freshwater fish eat some kinds of seeds, though they reject many others after swallowing them. Even small fish swallow seeds of moderate size, like those of the yellow water lily and Potamogeton. Herons and other birds, century after century, have gone on devouring fish day after day. They then take flight and travel to other waters, or get blown across the sea. And as we've seen, seeds retain their ability to germinate when ejected many hours later in pellets or excrement. When I saw the large size of the seeds of the magnificent water lily Nelumbium, and remembered Alph. de Candolle's remarks on its distribution, I thought the means of its dispersal must be inexplicable. But Audubon reported finding seeds of the great southern water lily -- probably, according to Dr. Hooker, the Nelumbium luteum -- in a heron's stomach. Now, this bird must often have flown with its stomach thus well stocked to distant ponds, and then, getting a hearty meal of fish, I believe by analogy that it would have ejected the seeds in a pellet, still able to germinate.
When considering these various means of distribution, we should remember that when a pond or stream is first formed -- on a rising islet, for example -- it will be unoccupied, and a single seed or egg will have a good chance of succeeding. Although there will always be a struggle for life among the inhabitants of any pond, however few in number, the total number of species even in a well-stocked pond is small compared to the number inhabiting an equal area of land. So competition between them will probably be less intense than between land species, and an intruder from foreign waters would have a better chance of establishing itself than a land colonist would. We should also remember that many freshwater organisms rank low in the scale of life, and we have reason to believe such organisms change more slowly than more complex ones -- which gives time for aquatic species to migrate. And we shouldn't forget the likelihood that many freshwater forms once ranged continuously across immense areas and then went extinct at intermediate points. But the wide distribution of freshwater plants and lower animals -- whether retaining the exact same form or modified to some degree -- seems to depend mainly on the wide dispersal of their seeds and eggs by animals, especially by freshwater birds, which are powerful fliers that naturally travel from one body of water to another.
We now come to the last of the three classes of facts that I've selected as presenting the greatest difficulty for the view that not only have all individuals of the same species migrated from a single area, but that closely related species -- though now living at the most distant points on Earth -- have all descended from a single area, the birthplace of their early ancestors. I've already given my reasons for not believing that continental extensions on so enormous a scale occurred within the period of existing species that all the many islands of the world's oceans were stocked with their present land inhabitants by land bridges. This view removes many difficulties, but it doesn't fit all the facts about island life. In the following discussion, I won't limit myself to the question of dispersal alone, but will consider some other cases that bear on the truth of the two competing theories: independent creation versus descent with modification.
The species of all kinds that inhabit oceanic islands are few in number compared with those on equivalent continental areas. Alph. de Candolle acknowledges this for plants, and Wollaston for insects. New Zealand, for instance, with its towering mountains and diverse habitats, stretching over 780 miles of latitude, together with the outlying Auckland, Campbell, and Chatham Islands, contains altogether only 960 kinds of flowering plants. If we compare this modest number with the species that swarm over equal areas in southwestern Australia or at the Cape of Good Hope, we have to conclude that some cause beyond just different physical conditions has produced such a huge difference. Even the uniform county of Cambridge has 847 plants, and the little island of Anglesey 764 -- though a few ferns and some introduced plants are included in these numbers, and the comparison isn't quite fair in some respects. We know that the barren island of Ascension originally had fewer than half a dozen flowering plants, yet many species have now become established there, just as they have on New Zealand and every other oceanic island you could name. On St. Helena, there's good reason to believe that the introduced plants and animals have nearly or completely wiped out many native species. Anyone who accepts the doctrine that each species was separately created must also accept that a sufficient number of well-adapted plants and animals were not created for oceanic islands -- because humans have unintentionally stocked them far more fully and successfully than nature ever did.
Although oceanic islands have few species overall, the proportion of endemic kinds -- those found nowhere else in the world -- is often extremely high. If we compare, for instance, the number of endemic land shells on Madeira, or endemic birds in the Galapagos Archipelago, with the numbers on any continent, and then compare the island's area with the continent's, the point becomes clear. This is what theory would predict: species arriving after long intervals in a new, isolated place, and having to compete with unfamiliar neighbors, would be especially prone to modification and would often produce groups of modified descendants. But it doesn't follow that just because nearly all species of one class on an island are unique, those of another class -- or even another section of the same class -- will also be unique. This difference seems to depend partly on whether non-modified species arrived together as a group, so their relationships to each other weren't much disrupted, and partly on the frequent arrival of unmodified immigrants from the parent country, with which the island forms have interbred. We should keep in mind that the offspring of such crosses would gain in vigor, so even an occasional cross would have more impact than you might expect. Let me give a few examples. In the Galapagos Islands, there are twenty-six land birds, of which twenty-one (or perhaps twenty-three) are unique to the islands. But of the eleven marine birds, only two are unique -- and obviously marine birds could reach these islands much more easily and frequently than land birds. Bermuda, on the other hand, sits at about the same distance from North America as the Galapagos do from South America, and has a very distinctive soil, yet it doesn't possess a single endemic land bird. We know from Mr. J. M. Jones's excellent account that very many North American birds visit Bermuda regularly or occasionally. And almost every year, as Mr. E. V. Harcourt informs me, many European and African birds are blown to Madeira. That island has ninety-nine kinds of birds, of which only one is unique (though very closely related to a European form), and three or four others are confined to Madeira and the Canaries. So Bermuda and Madeira have been stocked from the neighboring continents with birds that have competed and co-adapted with one another for ages. When these birds settled in their new homes, each species was kept by the others in its proper niche, and consequently was unlikely to change much. Any tendency to change would also have been checked by interbreeding with unmodified immigrants arriving regularly from the mainland. Madeira is also home to a remarkable number of unique land shells, while not a single seashell species is unique to its shores. And though we don't fully understand how seashells disperse, we can see that their eggs or larvae -- perhaps attached to seaweed or floating timber, or to the feet of wading birds -- could cross three or four hundred miles of open sea far more easily than land shells. The different orders of insects on Madeira show nearly the same pattern.
Oceanic islands sometimes completely lack animals of certain whole classes, and their roles are filled by other groups. In the Galapagos Islands, reptiles take -- or recently took -- the place of mammals, and in New Zealand, gigantic wingless birds did the same. Although I'm describing New Zealand here as an oceanic island, there's some question about whether it really qualifies. It's large, and it isn't separated from Australia by a very deep sea. Based on its geological character and the direction of its mountain ranges, the Rev. W. B. Clarke has recently argued that New Zealand, along with New Caledonia, should really be considered extensions of Australia. Turning to plants, Dr. Hooker has shown that in the Galapagos the relative proportions of different plant orders are very different from what you'd find elsewhere. All such differences in numbers, and the complete absence of certain groups of animals and plants, are usually chalked up to differences in the islands' physical conditions -- but that explanation is quite doubtful. Ease of immigration seems to have been just as important as the nature of those conditions.
Many remarkable little facts could be pointed out about the inhabitants of oceanic islands. For instance, on certain islands that have no mammals at all, some of the endemic plants have beautifully hooked seeds -- yet few relationships in nature are more obvious than that hooks exist to transport seeds in the wool or fur of four-legged animals. But a hooked seed could have been carried to an island by other means. The plant then evolved into an endemic species while still keeping its hooks, which became a useless feature -- just like the shriveled wings beneath the fused wing covers of many island beetles. Here's another interesting pattern: islands often have trees or bushes belonging to orders that everywhere else consist only of herbaceous species. As Alph. de Candolle has shown, trees generally have restricted ranges, whatever the reason may be. So trees would be unlikely to reach distant oceanic islands. But an herbaceous plant that had no chance of competing with the fully developed trees on a continent could, once established on an island, gain an advantage over other herbaceous plants by growing taller and taller, rising above them. In this situation, natural selection would tend to increase the plant's stature, whatever order it belonged to, first converting it into a bush and then into a tree.
Regarding the absence of whole orders of animals from oceanic islands, Bory St. Vincent pointed out long ago that frogs, toads, and newts are never found on any of the many islands scattered across the great oceans. I've taken pains to verify this claim and found it to be true, with the exceptions of New Zealand, New Caledonia, the Andaman Islands, and perhaps the Solomon Islands and the Seychelles. But I've already noted that it's doubtful whether New Zealand and New Caledonia should really count as oceanic islands, and this is even more doubtful for the Andaman and Solomon groups and the Seychelles. This general absence of frogs, toads, and newts from so many true oceanic islands can't be explained by their physical conditions. In fact, islands seem to be perfectly suited for these animals: frogs introduced to Madeira, the Azores, and Mauritius have multiplied so rapidly that they've become a nuisance. But since these animals and their eggs are immediately killed by seawater (with the exception, as far as is known, of one Indian species), getting them across the ocean would be extremely difficult. That's why they don't exist on truly oceanic islands. But if you believe in separate creation, it would be very hard to explain why they were never created there.
Mammals present a similar case. I've carefully searched the oldest travel accounts and haven't found a single reliable instance of a land mammal (excluding domesticated animals kept by the natives) living on an island more than 300 miles from a continent or large continental island -- and many islands much closer are equally bare. The Falkland Islands, which have a wolf-like fox, come closest to an exception. But this group can't really be considered oceanic, since it sits on an underwater bank connected to the mainland at a distance of about 280 miles. Moreover, icebergs once carried boulders to its western shores, and they may have formerly transported foxes, just as frequently happens in arctic regions today. Yet you can't say that small islands can't support at least small mammals, because they occur on many very small islands close to continents. And hardly an island can be named where our smaller four-legged animals haven't become established and greatly multiplied after being introduced. You also can't argue, on the standard creationist view, that there hasn't been enough time for mammals to be created on these islands. Many volcanic islands are extremely ancient, as shown by the enormous erosion they've undergone and by their Tertiary rock layers. There has also been time for the evolution of endemic species in other groups. And on continents, it's known that new mammal species appear and disappear faster than other, lower animals. Although land mammals don't occur on oceanic islands, flying mammals do appear on nearly every island. New Zealand has two bat species found nowhere else. Norfolk Island, the Fiji Archipelago, the Bonin Islands, the Caroline and Mariana Archipelagoes, and Mauritius all have their own unique bats. Why, one might ask, has the supposed creative force produced bats and no other mammals on remote islands? On my view, the answer is simple: no land mammal can cross a wide stretch of ocean, but bats can fly across. Bats have been seen flying by day far out over the Atlantic Ocean, and two North American species regularly or occasionally visit Bermuda, 600 miles from the mainland. I've learned from Mr. Tomes, who has made a special study of bats, that many species have enormous ranges and are found on both continents and far-flung islands. So we need only suppose that such wide-ranging species have been modified in their new homes to fit their new conditions, and we can understand the presence of endemic bats on oceanic islands alongside the complete absence of all other land mammals.
There's another interesting relationship: the connection between the depth of the sea separating islands from each other or from the nearest continent, and the degree of relatedness among their mammal populations. Mr. Windsor Earl made some striking observations on this point, later greatly expanded by Mr. Wallace's admirable research on the great Malay Archipelago. This archipelago is divided near Celebes by a deep ocean trench that separates two very different mammal faunas. On either side, the islands sit on moderately shallow submarine banks, and these islands are inhabited by the same or closely related four-legged animals. I haven't yet had time to follow up on this across all parts of the world, but as far as I've investigated, the relationship holds. For instance, Britain is separated from Europe by a shallow channel, and the mammals are the same on both sides. The same is true of the islands near Australia's shores. The West Indian Islands, on the other hand, sit on a deeply submerged bank nearly a thousand fathoms deep, and while we find American forms there, the species and even the genera are quite distinct. Since the amount of change that animals undergo depends partly on the passage of time, and since islands separated by shallow channels are more likely to have been connected within the recent past than those separated by deeper channels, we can understand why there's a relationship between sea depth and the degree of relatedness between two mammal faunas -- a relationship that is completely inexplicable under the theory of independent creation.
The facts I've laid out about the inhabitants of oceanic islands -- the small number of species with a high proportion of endemic forms; the modification of some groups but not others in the same class; the absence of entire orders like frogs and land mammals despite the presence of bats; the unusual proportions of certain plant orders; the evolution of herbaceous plants into trees -- all of this seems to me to fit much better with the idea that occasional means of transport, operating over long periods of time, have stocked these islands, rather than with the belief that all oceanic islands were once connected to the nearest continent. Under that latter view, we'd expect the different groups to have immigrated more evenly, and because the species would have arrived together as a body, their relationships to each other wouldn't have been much disrupted. They either wouldn't have changed at all, or would all have changed in a more uniform way.
I don't deny that there are many serious difficulties in understanding how many of the inhabitants of the more remote islands -- whether still in their original form or subsequently modified -- reached their present homes. But we shouldn't overlook the probability that other islands once existed as stepping stones, of which no trace now remains. Let me describe one difficult case. Almost all oceanic islands, even the most isolated and smallest, are home to land shells -- usually endemic species, but sometimes species found elsewhere, as Dr. A. A. Gould has strikingly demonstrated for the Pacific. Now, it's well known that land shells are easily killed by seawater. Their eggs, at least the ones I've tested, sink in it and die. Yet there must be some unknown but occasionally effective means of getting them across the ocean. Could the just-hatched young sometimes cling to the feet of birds roosting on the ground and get carried that way? It occurred to me that land shells, when hibernating and having sealed the opening of their shell with a membrane, might float in crevices of driftwood across moderately wide stretches of sea. And I find that several species in this state can survive unharmed in seawater for seven days. One shell, Helix pomatia, after being treated this way and then allowed to hibernate again, was placed in seawater for twenty days and recovered perfectly. In that time, the shell could have been carried by an average ocean current about 660 geographical miles. Since this species has a thick, chalky operculum, I removed it, and when it had formed a new membranous one, I immersed it again for fourteen days in seawater. Once again, it recovered and crawled away. Baron Aucapitaine later tried similar experiments. He placed 100 land shells of ten species in a box pierced with holes and submerged it in the sea for two weeks. Of the hundred shells, twenty-seven survived. The presence of an operculum seemed important: of twelve specimens of Cyclostoma elegans, which has one, eleven revived. It's remarkable that while Helix pomatia survived so well in my experiments, not one of fifty-four specimens from four other Helix species survived in Aucapitaine's tests. Still, it's not very likely that land shells have often been transported this way. The feet of birds remain a more probable method.
The most striking and important fact for us is that the species inhabiting islands are related to those of the nearest mainland, without being exactly the same. Countless examples could be given. The Galapagos Archipelago, sitting right on the equator, lies between 500 and 600 miles from the coast of South America. Here, almost every product of the land and water bears the unmistakable stamp of the American continent. There are twenty-six land birds. Of these, twenty-one, or perhaps twenty-three, are classified as distinct species and would normally be assumed to have originated there. Yet the close relationship of most of these birds to American species is obvious in every detail -- their habits, gestures, and tones of voice. The same is true of the other animals and of a large proportion of the plants, as Dr. Hooker showed in his excellent study of the archipelago's flora. The naturalist looking at the inhabitants of these volcanic islands in the Pacific, several hundred miles from the continent, feels as if he's standing on American soil. Why should this be? Why should species supposedly created in the Galapagos, and nowhere else, so clearly bear the stamp of kinship with those created in America? There is nothing in the conditions of life, the geological nature of the islands, their height or climate, or the proportions of the different groups that closely resembles conditions on the South American coast. In fact, there's a considerable difference in all these respects. On the other hand, there's a good deal of resemblance in the volcanic soil, climate, height, and size of the islands between the Galapagos and the Cape Verde Archipelagos -- but what a total, absolute difference in their inhabitants! The inhabitants of the Cape Verde Islands are related to those of Africa, just as those of the Galapagos are related to America. Facts like these simply cannot be explained by the theory of independent creation. But on the view I'm presenting here, it's obvious that the Galapagos would be likely to receive colonists from America, whether by occasional means of transport or (though I don't believe it) by a former land connection, and the Cape Verde Islands from Africa. Such colonists would be prone to modification -- with the principle of inheritance still revealing their original birthplace.
Many similar facts could be given. Indeed, it's almost a universal rule that the endemic species of islands are related to those of the nearest continent or nearest large island. The exceptions are few, and most can be explained. For example, although Kerguelen Land lies closer to Africa than to America, its plants are related -- very closely, as we know from Dr. Hooker's account -- to those of America. But on the view that this island was mainly stocked by seeds carried with dirt and stones on icebergs, driven by the prevailing currents, the puzzle disappears. New Zealand's endemic plants are much more closely related to those of Australia, the nearest mainland, than to any other region -- exactly what you'd expect. But New Zealand is also clearly related to South America, which, though the next nearest continent, is so enormously remote that the fact becomes puzzling. This difficulty partly disappears if we accept that New Zealand, South America, and the other southern lands were partly stocked from a nearly intermediate, though distant, point -- namely, the Antarctic islands, when they were covered with vegetation during a warmer Tertiary period, before the last Glacial period began. The affinity between the flora of southwestern Australia and the Cape of Good Hope -- which, though faint, Dr. Hooker assures me is real -- is an even more remarkable case. But this affinity is confined to plants and will no doubt someday be explained.
The same principle that determines the relationship between the inhabitants of islands and the nearest mainland sometimes plays out on a smaller scale, in a most interesting way, within a single archipelago. Each separate island of the Galapagos is home to many distinct species -- a truly marvelous fact -- yet these species are far more closely related to each other than to the inhabitants of the American continent or any other part of the world. This is what you'd expect, since islands so close together would almost inevitably receive immigrants from the same original source, and from each other. But how is it that many of these immigrants have been modified differently, even if only slightly, on islands within sight of one another, with the same geology, the same elevation, the same climate? This long seemed a great difficulty to me. But it arises mainly from the deeply rooted error of treating physical conditions as the most important factor, when in reality the other species each organism has to compete with are at least as important -- and usually far more so. Now, if we look at the Galapagos species that also occur elsewhere in the world, we find that they differ considerably from island to island. This is exactly what we'd expect if the islands were stocked by occasional transport -- one plant's seed brought to one island, another plant's seed to another, though all from the same general source. So when an immigrant first settled on one island, or later spread from one to another, it would encounter different conditions on each island because it would compete with a different set of organisms. A plant, for instance, would find its preferred ground occupied by somewhat different species on different islands, and would face attacks from somewhat different enemies. If it then varied, natural selection would likely favor different varieties on different islands. Some species, however, might spread and keep the same form throughout the group, just as some species spread across an entire continent without changing.
The truly surprising fact about the Galapagos Archipelago -- and to a lesser degree about some similar cases -- is that each new species, after forming on one island, didn't quickly spread to the others. But although the islands are within sight of each other, they're separated by deep ocean channels, in most cases wider than the English Channel, and there's no reason to think they were ever continuously joined. The ocean currents between them are swift and deep, and strong winds are extraordinarily rare, so the islands are far more effectively separated than they appear on a map. Nevertheless, some species -- both those found elsewhere in the world and those confined to the archipelago -- are common to several islands, and from their present distribution we can infer that they spread from one island to another. But I think we often take a mistaken view of how readily closely related species will invade each other's territory when given free access. If one species has any advantage over another, it will quickly replace it, wholly or in part. But if both are equally well adapted to their own niches, both will probably hold their ground for almost any length of time. Because we're familiar with the fact that species introduced by humans have spread with astonishing speed across wide areas, we tend to assume most species would do the same. But we should remember that species that become established in new countries are generally not closely related to the native inhabitants -- they're very different forms, belonging in a large proportion of cases, as Alph. de Candolle has shown, to entirely different genera. In the Galapagos, even many of the birds, despite being well adapted for flying from island to island, differ on the different islands. There are three closely related species of mockingbird, each confined to its own island. Now suppose the Chatham Island mockingbird were blown to Charles Island, which has its own mockingbird. Why should it succeed in establishing itself there? We can safely assume that Charles Island is well stocked with its own species, since every year more eggs are laid and more young birds hatch than can possibly survive. And we can assume that the Charles Island mockingbird is at least as well fitted for its home as the Chatham Island species is for its. Sir Charles Lyell and Mr. Wollaston shared with me a remarkable fact on this point: Madeira and the nearby islet of Porto Santo possess many distinct but closely related species of land shells, some living in rock crevices. Although large quantities of stone are shipped annually from Porto Santo to Madeira, the Porto Santo shell species have not colonized Madeira -- yet both islands have been colonized by some European land shells, which presumably had some advantage over the native species. From these considerations, I think we needn't be too surprised that the endemic species on the various Galapagos islands haven't all spread from island to island. On continents too, prior occupancy has probably played an important role in preventing the mixing of species that live in different districts with nearly identical physical conditions. The southeast and southwest corners of Australia, for example, have almost the same physical conditions and are connected by continuous land, yet they're inhabited by a vast number of distinct mammals, birds, and plants. The same is true, according to Mr. Bates, of the butterflies and other animals in the great, open, continuous valley of the Amazon.
The same principle that governs the general character of oceanic island inhabitants -- the relationship to the source from which colonists could most easily have come, combined with their subsequent modification -- applies throughout all of nature. We see it on every mountaintop, in every lake and marsh. Alpine species, except where they've become widely spread during the Glacial period, are related to those of the surrounding lowlands. In South America, we find alpine hummingbirds, alpine rodents, alpine plants -- all strictly American forms. And obviously, as a mountain was slowly uplifted, it would be colonized from the surrounding lowlands. The same is true for the inhabitants of lakes and marshes, except where easy transport has allowed the same forms to prevail across large parts of the world. We see the same principle in the character of most blind animals inhabiting the caves of America and Europe. Other similar examples could be given. I believe it will prove universally true that wherever two regions, no matter how distant, share many closely related or representative species, some identical species will also be found there. And wherever many closely related species occur, there will be many forms that some naturalists rank as distinct species and others as mere varieties -- these doubtful forms showing us the steps in the process of modification.
The connection between the power and extent of migration in certain species -- whether now or in some former period -- and the existence of closely related species at remote points around the world, shows up in another, more general way. Mr. Gould pointed out to me long ago that in bird genera with worldwide ranges, many of the species individually have very wide ranges. I'm fairly confident this rule is generally true, though it's hard to prove. Among mammals, we see it strikingly in bats, and to a lesser degree in the cat and dog families. The same pattern appears in butterflies and beetles, and in most freshwater organisms, since many genera in very different groups range across the world, and many individual species have enormous ranges. I don't mean that all species in such genera range widely, but that some do. Nor do I mean that species in widely ranging genera have, on average, very wide ranges -- that will largely depend on how far modification has progressed. For instance, two varieties of the same species might inhabit America and Europe, giving the species an immense range. But if variation went a little further, the two varieties would be ranked as distinct species, and their ranges would shrink dramatically. Still less do I mean that species with the ability to cross barriers and range widely -- certain powerful-winged birds, for example -- will necessarily range widely. We should never forget that ranging widely requires not just the power to cross barriers, but the more important power of winning the struggle for life with foreign competitors in distant lands. But according to the view that all species of a genus, though scattered to the most remote points of the world, descend from a single ancestor, we ought to find -- and I believe as a general rule we do find -- that at least some species in such genera range very widely.
We should keep in mind that many genera in all groups are of ancient origin, and in these cases the species have had ample time for dispersal and subsequent modification. There's also geological evidence to suggest that within each major group, lower organisms change more slowly than higher ones. This means they've had a better chance of ranging widely while still keeping the same specific form. This fact, combined with the observation that the seeds and eggs of most simply organized forms are very small and better suited for long-distance transport, probably explains a long-observed law that Alph. de Candolle has recently discussed for plants: the lower any group of organisms stands, the more widely it ranges.
In these two chapters, I've tried to show that if we make proper allowance for our ignorance of the full effects of climate changes and shifts in land level -- which have certainly occurred in the recent period -- along with other changes that have probably occurred; if we remember how little we know about the many curious means of occasional transport; and if we keep in mind (and this is very important) how often a species may have once ranged continuously over a wide area and then gone extinct in the intervening regions -- then the difficulty is not insurmountable in believing that all individuals of the same species, wherever they're found, descend from common parents. We're led to this conclusion -- which many naturalists have reached under the label of "single centers of creation" -- by various general considerations, especially the importance of barriers of all kinds and the patterns in the distribution of subgenera, genera, and families.
As for distinct species within the same genus, which on my theory have spread from a single parent source -- if we make the same allowances for our ignorance and remember that some forms of life change very slowly, with enormous periods of time thus available for their migration -- the difficulties are far from insurmountable, though they are often great, just as with individuals of the same species.
To illustrate the effects of climate change on distribution, I've tried to show how important the last Glacial period was. It affected even the equatorial regions. And during the alternation of cold in the north and south, it allowed organisms from opposite hemispheres to mingle and left some of them stranded on mountaintops all over the world. To show how varied the means of occasional transport are, I've discussed at some length how freshwater organisms disperse.
If the difficulties are not insurmountable in accepting that, over the long course of time, all individuals of the same species -- and likewise all the species within the same genus -- have descended from a single source, then all the grand leading facts of geographical distribution become explicable through the theory of migration, combined with subsequent modification and the multiplication of new forms. We can then understand the great importance of barriers, whether of land or water, in not only separating but apparently creating the various zoological and botanical regions. We can understand why related species are concentrated in the same areas, and how it is that under different latitudes -- in South America, for instance -- the inhabitants of the plains and mountains, the forests, marshes, and deserts, are linked together in such a mysterious way, and are likewise linked to the extinct creatures that once lived on the same continent. Keeping in mind that the relationships between organisms are of the highest importance, we can see why two areas with nearly the same physical conditions should often be inhabited by very different forms of life. Depending on how long ago colonists entered one or both regions; depending on whether the routes of communication allowed certain forms in and kept others out, in greater or lesser numbers; depending on whether those that entered happened to compete more or less directly with each other and with the natives; and depending on how rapidly the immigrants were able to vary -- there would arise, in two or more regions and quite independently of their physical conditions, an almost infinitely diverse array of living conditions. There would be an almost endless amount of interaction and counteraction among organisms, and we would find some groups greatly modified, others only slightly; some flourishing in great numbers, others barely hanging on -- and this is exactly what we do find across the great geographical regions of the world.
On these same principles, we can understand -- as I've tried to show -- why oceanic islands should have few inhabitants, but why a large proportion of these should be endemic. We can understand why, depending on the means of migration, one group of organisms should have all its species unique to the island while another group in the same class should have all its species identical to those on the nearest continent. We can see why whole groups of organisms, like frogs and land mammals, should be absent from oceanic islands, while even the most isolated islands possess their own unique species of bats. We can see why there should be some relationship on islands between the presence of mammals, in more or less modified form, and the depth of the sea between such islands and the mainland. We can clearly see why all the inhabitants of an archipelago, though distinct on the individual islands, should be closely related to each other, and also related -- though less closely -- to those of the nearest continent or other likely source of immigrants. We can see why, if two areas however distant share very closely related or representative species, some identical species will almost always be found there too.
As the late Edward Forbes often emphasized, there is a striking parallel between the laws of life through time and through space. The laws governing the succession of forms in the past are nearly the same as those governing the differences between forms in different areas today. We see this in many facts. The persistence of each species and group of species is continuous in time; the apparent exceptions are so few that they can fairly be attributed to our not yet having found certain forms in intermediate deposits where they're absent but occur both above and below. Similarly in space, the general rule is that the area inhabited by a single species, or by a group of species, is continuous, and the exceptions -- which aren't rare -- can, as I've tried to show, be explained by past migrations under different conditions, by occasional means of transport, or by extinction in the intervening regions. Both in time and space, species and groups of species have their points of maximum development. Groups of species living during the same period of time, or living within the same area, are often characterized by minor shared features, like patterns of surface texture or color. Looking at the long succession of past ages, as at distant regions throughout the world, we find that species in certain classes differ little from each other, while those in another class, or even in a different section of the same order, differ enormously. In both time and space, the simpler organisms in each class generally change less than the more complex ones, though there are striking exceptions to this rule in both cases. According to my theory, all these relationships through time and space make sense. Whether we look at related forms that changed during successive ages or at those that changed after migrating to distant regions, they are connected by the same bond of ordinary generation. In both cases, the laws of variation have been the same, and modifications have been accumulated by the same means -- natural selection.
From the earliest periods of life on Earth, living things have been found to resemble each other in descending degrees, so that they can be organized into groups within groups. This classification isn't arbitrary, like grouping stars into constellations. It would be simple enough if one group were exclusively suited to live on land and another in water, one to eat meat and another to eat plants, and so on. But the reality is very different -- it's well known that members of even the same subgroup often have completely different habits of life. In the second and fourth chapters, on Variation and on Natural Selection, I tried to show that within each region, it's the widely ranging, widespread, and common species -- the dominant species belonging to the larger genera in each class -- that vary the most. The varieties, or emerging species, produced this way ultimately become new and distinct species. These, following the principle of inheritance, tend to produce still more new and dominant species. As a result, the groups that are already large, and that generally include many dominant species, tend to keep growing. I also tried to show that because the varying descendants of each species try to occupy as many different niches as possible in the natural world, they constantly tend to diverge in character. This conclusion is supported by observing the great diversity of forms that come into the closest competition in any small area, and by certain facts about the establishment of introduced species.
I also tried to show that there's a steady tendency for forms that are increasing in number and diverging in character to replace and drive to extinction the earlier, less divergent, and less improved forms. I'd ask the reader to look back at the diagram illustrating the action of these several principles, as I explained earlier. You'll see that the inevitable result is that the modified descendants from one ancestor become divided into groups within groups. In the diagram, each letter on the uppermost line may represent a genus containing several species, and all the genera along this upper line form together one class, since all are descended from one ancient parent and have therefore inherited something in common. But the three genera on the left have, by this same principle, much in common and form a subfamily, distinct from the one containing the next two genera on the right, which diverged from a common parent at the fifth stage of descent. These five genera also have much in common, though less than when grouped into subfamilies, and they form a family distinct from the one containing the three genera still further to the right, which diverged at an earlier period. And all these genera, descended from A, form an order distinct from the genera descended from I. So here we have many species descended from a single ancestor, grouped into genera, and the genera into subfamilies, families, and orders, all under one great class. The grand fact that living things are naturally organized in groups within groups -- something so familiar that it doesn't always strike us as remarkable -- is, in my judgment, explained this way. Of course, organisms, like any other objects, can be classified in many ways, either artificially by single traits or more naturally by a combination of traits. We know, for instance, that minerals and chemical elements can be arranged this way. In that case, there's obviously no connection to genealogical descent, and no cause can currently be given for why they fall into groups. But with living things the case is different, and the view I've described fits with their natural arrangement in groups within groups. No other explanation has ever even been attempted.
As we've seen, naturalists try to arrange the species, genera, and families in each class according to what's called the Natural System. But what does this system actually mean? Some authors see it merely as a scheme for grouping similar organisms together and separating dissimilar ones, or as an efficient way of stating general propositions -- that is, using one sentence to describe the traits common to all mammals, another for all carnivores, another for the dog genus, and then adding a single sentence to give a full description of each kind of dog. The cleverness and usefulness of this system are undeniable. But many naturalists think the Natural System means something more than this. They believe it reveals the plan of the Creator. But unless someone specifies whether they mean order in time, in space, or both, or something else entirely, it seems to me that this adds nothing to our knowledge. Expressions like the famous one by Linnaeus, which we often encounter in more or less disguised form -- that the characters don't make the genus, but the genus gives the characters -- seem to imply that our classifications involve some deeper connection than mere resemblance. I believe this is the case, and that shared descent -- the one known cause of close similarity in living things -- is the bond that, though obscured by various degrees of modification, our classifications partially reveal.
Now let's consider the rules followed in classification and the difficulties encountered, whether we think classification reveals some unknown plan of creation or is simply a scheme for making general statements and grouping similar forms together. You might think (and people did think, in ancient times) that the parts of an organism's body that determine its way of life and general role in nature would be extremely important for classification. Nothing could be more wrong. No one considers the outward resemblance between a mouse and a shrew, a dugong and a whale, or a whale and a fish to be of any importance. These resemblances, though intimately connected with the animal's whole way of life, are dismissed as merely "adaptive or analogical characters" -- but we'll come back to them later. In fact, a general rule can be stated: the less any part of the body is connected with special habits of life, the more important it becomes for classification. As an example, Owen, speaking of the dugong, says: "The reproductive organs, being those most remotely related to the habits and food of an animal, I have always regarded as providing very clear indications of its true relationships. We are least likely in these organs to mistake a merely adaptive trait for an essential one." In plants, it's remarkable that the organs of nutrition, on which the plant's life depends, have little significance for classification, while the reproductive organs, along with their product the seed and embryo, are of the highest importance. So too, as I discussed earlier, certain structural features that aren't functionally important are often extremely useful for classification. This is because they remain constant throughout many related groups -- and their constancy mainly depends on the fact that slight variations in these features haven't been preserved and accumulated by natural selection, which acts only on useful traits.
That the mere physiological importance of an organ doesn't determine its value for classification is almost proved by the fact that in related groups, where the same organ presumably has nearly the same physiological importance, its classificatory value is wildly different. No naturalist can have worked on any group without being struck by this, and it has been acknowledged by almost every author. To quote the highest authority, the botanist Robert Brown, speaking of certain organs in the Proteaceae, says their importance at the genus level, "like that of all their parts, not only in this but, as I understand it, in every natural family, is very unequal, and in some cases seems to be entirely lost." In another work he says the genera of the Connaraceae "differ in having one or more ovaries, in the presence or absence of albumen, in the overlapping or valve-like arrangement of their bud coverings. Any one of these characters alone is frequently of more than generic importance, though here, even when all taken together, they appear insufficient to separate Cnestis from Connarus." To give an example from insects: in one great division of the Hymenoptera, the antennae, as Westwood has pointed out, are highly constant in structure. In another division they vary greatly, and those differences are of quite minor value for classification. Yet no one would say the antennae in these two divisions of the same order differ in physiological importance. Any number of similar examples could be given, showing how the same organ varies in classificatory importance within the same group.
No one would say that rudimentary or shrunken organs are of high physiological importance. Yet organs in this condition are undeniably often very valuable in classification. No one would argue that the rudimentary teeth in the upper jaws of young ruminants, or certain rudimentary leg bones, aren't highly useful for showing the close relationship between ruminants and pachyderms. Robert Brown has strongly emphasized that the position of rudimentary florets is of the highest importance in classifying grasses.
Many examples could be given of traits derived from parts that must be considered physiologically trivial, yet are universally recognized as highly useful for defining whole groups. For instance: whether or not there's an open passage from the nostrils to the mouth -- the only character, according to Owen, that absolutely distinguishes fish from reptiles. The angle of the lower jaw in marsupials. The way insects fold their wings. Color alone in certain algae. Hairiness on parts of the flower in grasses. The nature of the outer covering -- hair or feathers -- in vertebrates. If the platypus had been covered with feathers instead of fur, naturalists would have considered this superficial trait an important clue to its relationship with birds.
The importance of seemingly trivial characters for classification mainly depends on their being linked to many other characters of greater or lesser importance. The value of a combination of characters is indeed well recognized in natural history. This is why a species may differ from its relatives in several traits of high physiological importance and nearly universal occurrence, yet leave us in no doubt about where to rank it. This is also why classification based on any single character, however important, has always failed -- because no part of the body is invariably constant. The value of a combination of characters, even when none individually are important, is what explains Linnaeus's saying that the characters don't make the genus, but the genus gives the characters. This seems to be based on an appreciation of many slight points of resemblance, too subtle to define. Certain plants belonging to the Malpighiaceae bear both perfect and degraded flowers. In the degraded ones, as A. de Jussieu has remarked, "The greater number of the characters proper to the species, to the genus, to the family, to the class, disappear, and thus laugh at our classification." When Aspicarpa produced only these degraded flowers in France for several years, departing remarkably in many important structural features from the typical form of the order, the botanist Richard wisely recognized, as Jussieu notes, that this genus should still be kept among the Malpighiaceae. This case perfectly illustrates the spirit of our classification system.
In practice, when naturalists are working, they don't trouble themselves about the physiological importance of the traits they use to define a group or place a particular species. If they find a trait that's nearly uniform and common to a great number of forms but not to others, they treat it as highly valuable. If it's common to a smaller number, they treat it as less important. This principle has been openly acknowledged by some naturalists as the correct one, and by none more clearly than by the excellent botanist Aug. St. Hilaire. If several trivial traits always appear together, though no obvious connection between them can be discovered, they're given special weight. Since in most animal groups the important organs -- those for pumping blood, for breathing, or for reproduction -- are found to be nearly uniform, they're considered highly useful for classification. But in some groups, all of these vital organs offer traits of only minor value. For example, as Fritz Muller has recently pointed out, in the same group of crustaceans, Cypridina has a heart, while in two closely related genera, Cypris and Cytherea, there is no such organ. One species of Cypridina has well-developed gills, while another species has none at all.
We can see why traits from the embryo should be as important as those from the adult -- a natural classification obviously includes all ages. But it's not at all obvious, under the ordinary view, why the embryo's structure should be more important than the adult's, since it's the adult that plays its full role in nature. Yet the great naturalists Milne Edwards and Agassiz have argued forcefully that embryological traits are the most important of all, and this view has been widely accepted. Their importance has sometimes been exaggerated, however, because adaptive traits of larvae haven't been excluded. To demonstrate this, Fritz Muller arranged the great class of crustaceans using larval traits alone, and the result was not a natural classification. But there can be no doubt that embryonic traits (as opposed to larval adaptations) are of the highest value for classification, in both animals and plants. The main divisions of flowering plants, for instance, are based on differences in the embryo -- the number and position of the cotyledons, and the way the shoot and root develop. We'll see shortly why these traits are so valuable for classification: it's because the natural system is genealogical in its arrangement.
Our classifications are often clearly influenced by chains of relationships. Nothing is easier than defining a set of traits common to all birds. But with crustaceans, no such definition has ever been possible. There are crustaceans at opposite ends of the series that have hardly any traits in common. Yet the species at both ends, by being clearly related to others, and those to still others, can be unmistakably recognized as belonging to this class and no other among the Articulata.
Geographic distribution has often been used in classification, though perhaps not quite logically -- especially in very large groups of closely related forms. Temminck insists on the usefulness, even necessity, of this practice in certain groups of birds, and several entomologists and botanists have followed it.
Finally, regarding the relative value of the various levels of classification -- orders, suborders, families, subfamilies, genera -- these seem to be, at least at present, almost arbitrary. Several of the best botanists, such as Mr. Bentham and others, have strongly insisted on their arbitrary nature. Examples could be given among plants and insects of groups first ranked by experienced naturalists as merely genera, then elevated to subfamilies or families. This was done not because further research uncovered important structural differences that had initially been overlooked, but because numerous related species with slightly different grades of difference were subsequently discovered.
All the rules, aids, and difficulties in classification that I've just described can be explained, if I'm not greatly mistaken, by the view that the natural system is founded on descent with modification. The traits that naturalists consider as showing true relationship between species are those inherited from a common parent. All true classification is genealogical. Shared descent is the hidden bond that naturalists have been unconsciously seeking -- not some unknown plan of creation, not general propositions, not the mere grouping of similar objects together.
But I need to explain what I mean more fully. I believe that the arrangement of groups within each class, in proper subordination and relationship to each other, must be strictly genealogical to be natural. But the amount of difference between the various branches or groups -- though they may be equally related by blood to their common ancestor -- can differ greatly, depending on how much modification they've undergone. This is expressed by ranking the forms under different genera, families, sections, or orders. The reader will best understand this by looking at the diagram in the fourth chapter. Suppose the letters A through L represent related genera that existed during the Silurian period, descended from some still earlier form. In three of these genera (A, F, and I), a species has transmitted modified descendants to the present day, represented by the fifteen genera (a14 to z14) on the uppermost horizontal line. Now, all these modified descendants from a single species are related by blood in the same degree. They could metaphorically be called cousins to the same millionth degree -- yet they differ widely and in different amounts from each other. The forms descended from A, now split into two or three families, form a distinct order from those descended from I, also split into two families. Nor can the existing species descended from A be placed in the same genus as their ancestor A, or those from I in the same genus as ancestor I. But the existing genus F14 may be supposed to have been only slightly modified, and so it would rank with the parent genus F -- just as some still-living organisms belong to Silurian genera. So the comparative value of the differences between these organisms, all related in the same degree by blood, has come to be very different. Nevertheless, their genealogical arrangement remains strictly accurate, not only now but at each successive stage of descent. All the modified descendants from A will have inherited something in common from their shared ancestor, as will all the descendants from I. The same holds for each subordinate branch of descendants at each successive stage. If, however, any descendant of A or I were to become so profoundly modified that all traces of its ancestry were lost, its place in the natural system would be lost too -- as seems to have happened with a few existing organisms. All the descendants of genus F, along its entire line of descent, are supposed to have been only slightly modified, and they form a single genus. But this genus, though quite isolated, will still occupy its proper intermediate position. The representation of groups as shown in the diagram on a flat surface is much too simple. The branches ought to have diverged in all directions. If the group names had simply been written down in a linear series, the representation would have been even less natural. And it's well known that we can't represent on a flat surface the relationships we discover in nature among members of the same group. The natural system is genealogical in its arrangement, like a family tree. But the amount of modification that different groups have undergone has to be expressed by ranking them under different genera, subfamilies, families, sections, orders, and classes.
It may be worthwhile to illustrate this view of classification using the case of languages. If we had a perfect family tree of humanity, a genealogical arrangement of human populations would provide the best classification of all the languages now spoken throughout the world. And if all extinct languages, along with every intermediate and slowly changing dialect, were included, such an arrangement would be the only possible one. Yet some ancient languages might have changed very little and given rise to few new languages, while others had changed greatly -- due to the spread, isolation, and cultural development of their descendant populations -- and thus given rise to many new dialects and languages. The various degrees of difference between languages of the same stock would have to be expressed by groups within groups. But the proper, or even the only possible, arrangement would still be genealogical. This would be strictly natural, as it would connect together all languages, extinct and recent, by the closest relationships, and would give the lineage and origin of each tongue.
To confirm this view, let's glance at the classification of varieties known or believed to descend from a single species. These are grouped under the species, with subvarieties under the varieties, and in some cases, as with the domestic pigeon, with several other grades of difference. Nearly the same rules are followed as in classifying species. Authors have insisted on arranging varieties on a natural rather than artificial system. We're warned, for instance, not to group two varieties of pineapple together merely because their fruit -- the most important part -- happens to be nearly identical. No one puts the Swedish and common turnip together, even though their edible, thickened stems are so similar. Whatever part is found to be most constant is used for classifying varieties. The great agriculturist Marshall says horns are very useful for this purpose with cattle, because they're less variable than body shape or color. With sheep, by contrast, horns are much less useful because they're less constant. In classifying varieties, I think that if we had a real pedigree, a genealogical classification would be universally preferred -- and it has been attempted in some cases. We could be confident that, whether there had been more or less modification, the principle of inheritance would keep the most closely related forms together. In tumbler pigeons, for example, though some subvarieties differ in the important character of beak length, all are kept together because they share the common habit of tumbling in flight. The short-faced breed has nearly or completely lost this habit. Nevertheless, without any conscious thought about it, these tumblers are kept in the same group because they're related by blood and alike in other respects.
With species in nature, every naturalist has in fact used descent in their classification. At the most basic level -- species -- they include both sexes, and how enormously the sexes sometimes differ in the most important traits is well known to every naturalist. Scarcely a single trait can be found in common between the adult males and hermaphrodites of certain barnacles, yet no one dreams of separating them. As soon as the three orchid forms Monachanthus, Myanthus, and Catasetum -- previously ranked as three distinct genera -- were found to sometimes grow on the same plant, they were immediately reclassified as varieties. I've since been able to show that they are the male, female, and hermaphrodite forms of the same species. The naturalist includes the various larval stages of the same individual as one species, however different they may be from each other and from the adult -- as well as the so-called alternating generations described by Steenstrup, which can only technically be considered the same individual. The naturalist includes monstrous forms and varieties not because of their partial resemblance to the parent form, but because they descend from it.
Since descent has universally been used to group the individuals of a species -- though the males, females, and larvae are sometimes extremely different -- and since it's been used to group varieties that have undergone considerable modification, might not this same element of descent have been unconsciously used to group species under genera, and genera under higher groups, all under the so-called natural system? I believe it has been unconsciously used, and only this can explain the various rules and guides that our best taxonomists have followed. Since we have no written pedigrees, we're forced to trace shared descent through resemblances of any kind. We therefore choose traits that are least likely to have been modified by the conditions of life each species has recently experienced. Rudimentary structures are, from this perspective, as useful as -- or even sometimes more useful than -- other parts of the body. We don't care how trivial a trait may be. Let it be the mere angle of the jaw, the way an insect's wing is folded, whether the skin is covered by hair or feathers -- if it's found throughout many different species, especially those with very different ways of life, it takes on high value. We can explain its presence in so many forms with such different habits only by inheritance from a common ancestor. We may make mistakes about individual structural features, but when several traits, however trivial, are consistently found together across a large group of organisms with different habits, we can be almost certain, on the theory of descent, that these traits have been inherited from a common ancestor. And we know that such combined traits have special value in classification.
We can understand why a species or group of species may differ from its relatives in several of its most important traits, yet still be safely classified with them. This can be done, and often is done, as long as a sufficient number of other traits -- however unimportant -- reveal the hidden bond of shared descent. Let two forms have not a single trait in common. Yet if these extreme forms are connected by a chain of intermediate groups, we can immediately infer their shared descent and put them all in the same class. Since we find that organs of high physiological importance -- those that preserve life under the most diverse conditions -- are generally the most constant, we attach special value to them. But if these same organs, in another group or section of a group, are found to vary greatly, we immediately value them less for classification. We'll soon see why embryological traits are so important for classification. Geographic distribution can sometimes be usefully employed in classifying large genera, because all species of the same genus inhabiting any distinct and isolated region are almost certainly descended from the same parents.
Based on the views I've described, we can understand the very important distinction between real relationships and analogical or adaptive resemblances. Lamarck first drew attention to this subject, and he has been ably followed by Macleay and others. The resemblance in body shape and fin-like front limbs between dugongs and whales, and between these two orders of mammals and fish, is analogical. So is the resemblance between a mouse and a shrew (Sorex), which belong to different orders. And so is the even closer resemblance, pointed out by Mivart, between the mouse and a small marsupial (Antechinus) of Australia. These latter resemblances can be explained, it seems to me, by adaptation for similarly active movement through dense vegetation, combined with the need to hide from enemies.
Among insects there are countless examples. Linnaeus, misled by external appearance, actually classified a homopterous insect as a moth. We see the same thing even among our domestic varieties -- for example, the strikingly similar body shape of the improved Chinese and common pig, which descend from different species, or the similarly thickened stems of the common and specifically distinct Swedish turnip. The resemblance between the greyhound and the racehorse is hardly more fanciful than the analogies some authors have drawn between wildly different animals.
If traits are important for classification only insofar as they reveal descent, we can clearly understand why analogical or adaptive traits -- though of the utmost importance to the organism's survival -- are almost worthless to the taxonomist. Animals belonging to two completely different lineages may have adapted to similar conditions and thus assumed a close outward resemblance. But such resemblances won't reveal their blood relationship -- they'll actually tend to conceal it. This also explains an apparent paradox: the very same traits are analogical when one group is compared with another, but reveal true relationships when members of the same group are compared among themselves. The shape of the body and fin-like limbs are only analogical when whales are compared with fish -- both are adaptations for swimming. But among the various members of the whale family, body shape and fin-like limbs reveal true kinship, because these parts are so similar throughout the whole family that they must have been inherited from a common ancestor. The same is true with fish.
Many examples could be given of striking resemblances between quite unrelated organisms in single parts or organs adapted for the same function. A good case is the close resemblance between the jaws of the dog and the Tasmanian wolf, or Thylacinus -- animals widely separated in the natural system. But this resemblance is only superficial, limited to the general appearance: the prominent canines and the cutting shape of the molars. The teeth actually differ a great deal. The dog has four premolars and only two molars on each side of the upper jaw, while the Thylacinus has three premolars and four molars. The molars also differ considerably between the two animals in relative size and structure. The adult teeth are preceded by very different sets of baby teeth. Anyone may, of course, deny that the teeth in either case were adapted for tearing flesh through natural selection of successive variations. But if this is accepted in one case, it's inexplicable to me why it should be denied in the other. I'm glad to find that so high an authority as Professor Flower has reached this same conclusion.
The extraordinary cases I gave in a previous chapter -- of very different fish possessing electric organs, of very different insects possessing light-producing organs, and of orchids and milkweeds having pollen masses with sticky discs -- fall under this same heading of analogical resemblance. These cases are so remarkable that I introduced them as difficulties for our theory. But in all such cases, some fundamental difference in the growth or development of the parts, and generally in their mature structure, can be detected. The end result is the same, but the means, though appearing superficially identical, are essentially different. The principle I discussed earlier under the term analogical variation has probably often been at work in these cases. That is, members of the same class, though only distantly related, have inherited enough in common that they tend to vary in similar ways under similar conditions. This would obviously help in the acquisition through natural selection of parts or organs strikingly like each other, independently of direct inheritance from a shared ancestor.
Since species from different classes have often been adapted by gradual steps to live under nearly similar conditions -- inhabiting, for instance, the three elements of land, air, and water -- we can perhaps understand why a numerical parallelism has sometimes been observed between subgroups of different classes. A naturalist, struck by such a parallelism, could easily extend it over a wide range by arbitrarily raising or lowering the rank of groups in different classes (and all our experience shows that their ranking is arbitrary). This is probably how the septenary, quinary, quaternary, and ternary systems of classification arose.
There's another curious class of cases where close external resemblance doesn't depend on adaptation to similar habits but has been acquired for protection. I'm referring to the wonderful way certain butterflies mimic other, quite distinct species, as first described by Mr. Bates. This excellent observer showed that in some districts of South America, where, for instance, an Ithomia butterfly abounds in gaudy swarms, another butterfly, a Leptalis, is often found mingled in the same flock. The Leptalis so closely resembles the Ithomia in every shade and stripe of color, and even in wing shape, that Bates, with his eyes sharpened by eleven years of collecting, was continually deceived despite always being on his guard. When the mimics and the models are caught and compared, they turn out to be very different in essential structure, belonging not only to different genera but often to different families. If this mimicry occurred in only one or two instances, it might be dismissed as a strange coincidence. But if we move from a district where one Leptalis imitates an Ithomia, we find another pair of mimic and model species, belonging to the same two genera, equally close in their resemblance. Altogether, no fewer than ten genera contain species that imitate other butterflies. The mimics and models always live in the same region -- we never find an imitator living far from the form it imitates. The mimics are almost always rare insects; the models almost always swarm in great numbers. In the same district where a Leptalis closely imitates an Ithomia, there are sometimes other butterflies mimicking the same Ithomia. So in the same place, species from three genera of butterflies and even a moth are all found closely resembling a butterfly from a fourth genus. It's worth noting that many of the mimicking forms of Leptalis, as well as the mimicked forms, can be shown through a graded series to be merely varieties of the same species, while others are undoubtedly distinct species. But why, you might ask, are certain forms treated as the models and others as the mimics? Bates answers this satisfactorily by showing that the imitated form keeps the usual appearance of its group, while the imitators have changed their appearance and don't resemble their nearest relatives.
This leads us to ask what reason there could be for certain butterflies and moths so often adopting the appearance of another, quite distinct form. Why, to the confusion of naturalists, has nature stooped to the tricks of the theater? Bates has undoubtedly hit on the true explanation. The model species, which always exist in great numbers, must regularly escape destruction to a large extent -- otherwise they couldn't persist in such swarms. And a large body of evidence has now been collected showing that they're distasteful to birds and other insect-eating animals. The mimics, on the other hand, living in the same district, are comparatively rare and belong to rare groups. They must therefore regularly face some danger, for otherwise, given the number of eggs all butterflies lay, they would within three or four generations swarm over the entire countryside. Now, if a member of one of these rare, persecuted groups were to assume an appearance so like that of a well-protected species that it continually fooled even the practiced eyes of an entomologist, it would often fool predatory birds and insects too, and thus often escape being eaten. Bates can almost be said to have actually witnessed the process by which the mimics came to resemble the models so closely. He found that some of the Leptalis forms that mimic many other butterflies varied enormously. In one district, several varieties occurred, and one alone resembled, to some extent, the common Ithomia of that district. In another district there were two or three varieties, one of which was much commoner than the others and closely mimicked another form of Ithomia. From facts like these, Bates concludes that the Leptalis first varies. When a variety happens to resemble some degree a common butterfly living in the same district, this variety -- because of its resemblance to a thriving, unpersecuted species -- has a better chance of escaping destruction from predators and is therefore more often preserved. "The less perfect degrees of resemblance being generation after generation eliminated, and only the others left to propagate their kind." Here we have an excellent illustration of natural selection in action.
Wallace and Trimen have described several equally striking cases of mimicry among the butterflies and moths of the Malay Archipelago and Africa, and in some other insects. Wallace has also found one case in birds, but we know of none in larger four-legged animals. The much greater frequency of mimicry in insects than in other animals is probably because of their small size. Insects can't defend themselves -- except for those with stings, and I've never heard of a stinging insect mimicking others, though they themselves are mimicked. Insects can't easily escape by flight from the larger animals that prey on them. So, speaking metaphorically, they're reduced, like most weak creatures, to trickery and disguise.
It's worth noting that the process of mimicry probably never started between forms that were wildly different in color. But starting with species that already somewhat resembled each other, the closest possible resemblance, if beneficial, could easily be achieved by the means I've described. And if the model species gradually changed in appearance over time through any cause, the mimic would be led along the same path and thus altered almost without limit, until it might eventually assume an appearance totally unlike that of other members of its own family. There is, however, some difficulty here. We have to suppose that in some cases, ancient members of several distinct groups, before they had diverged to their present extent, accidentally resembled a member of another, protected group closely enough to gain some slight protection. This would have provided the starting point for the later acquisition of the most perfect resemblance.
As the modified descendants of dominant species, belonging to the larger genera, tend to inherit the advantages that made their groups large and their parents dominant, they are almost certain to spread widely and seize more and more niches in the natural world. The larger and more dominant groups within each class thus tend to keep growing, and they consequently replace many smaller and weaker groups. This explains the fact that all organisms, recent and extinct, fall within a few great orders and still fewer classes. As a striking illustration of how few the higher groups are and how widely they're spread throughout the world: the discovery of Australia hasn't added a single insect belonging to a new class, and in the plant kingdom, as I learn from Dr. Hooker, it has added only two or three families of small size.
In the chapter on geological succession, I tried to show, based on the principle that each group generally diverges considerably during the long process of modification, how it is that the more ancient forms of life often show traits somewhat intermediate between existing groups. Because some of these old, intermediate forms have transmitted descendants down to the present day with only slight modification, these constitute our so-called aberrant groups. The more aberrant any form is, the greater must be the number of connecting forms that have been driven to extinction and completely lost. And we have evidence that aberrant groups have suffered severely from extinction, since they're almost always represented by very few species. The species that do survive are generally very distinct from each other, which again implies that many intermediate forms have gone extinct. The genera Ornithorhynchus (platypus) and Lepidosiren (lungfish), for example, would be no less aberrant if each were represented by a dozen species instead of one, or two or three. I think we can explain this only by viewing aberrant groups as forms that have been beaten by more successful competitors, with just a few members surviving under unusually favorable conditions.
Mr. Waterhouse has remarked that when a member of one animal group shows a resemblance to a quite different group, the resemblance is usually general rather than specific. For example, according to Waterhouse, of all rodents, the viscacha is most closely related to marsupials. But in the features where it approaches that group, its resemblance is general -- it's not more closely related to any one marsupial species than to another. Since these resemblances are believed to be real and not merely adaptive, they must, according to our theory, be due to inheritance from a common ancestor. We must therefore suppose either that all rodents, including the viscacha, branched off from some ancient marsupial that would naturally have been more or less intermediate in character among all existing marsupials, or that both rodents and marsupials branched off from a common ancestor and both groups have since been greatly modified in different directions. On either view, the viscacha has retained, by inheritance, more of the character of its ancient ancestor than other rodents have. It therefore won't be specially related to any one existing marsupial but indirectly to all or nearly all of them, by having partially retained traits from their shared ancestor. On the other hand, of all marsupials, as Waterhouse has noted, the wombat (Phascolomys) most closely resembles not any one species but the general order of rodents. In this case, however, we may strongly suspect that the resemblance is only analogical, because the wombat has become adapted to habits similar to those of a rodent. The elder De Candolle has made nearly similar observations about the general nature of relationships between distinct plant families.
Based on the principle that species descended from a common ancestor multiply and gradually diverge in character while retaining some shared traits through inheritance, we can understand the enormously complex and radiating relationships that connect all members of the same family or higher group. The common ancestor of a whole family, now fragmented by extinction into distinct groups and subgroups, will have passed on some of its traits, modified in various ways and degrees, to all the species. They'll consequently be related to each other by indirect lines of relationship of various lengths (as shown in the diagram I've often referred to), tracing back through many ancestors. Just as it's difficult to show the blood relationships among the many relatives of any ancient and noble family even with a genealogical chart -- and almost impossible without one -- we can understand why naturalists have had such extraordinary difficulty describing the various relationships they perceive among the many living and extinct members of the same great natural class.
Extinction, as we saw in the fourth chapter, has played an important role in defining and widening the gaps between the various groups in each class. This is how we can account for the distinctness of entire classes from each other. Birds, for instance, are distinct from all other vertebrates because many ancient forms of life have been utterly lost -- forms through which the early ancestors of birds were once connected with the early ancestors of the other, then less differentiated, vertebrate classes. Much less extinction has occurred among the forms that once connected fish with amphibians. Still less has occurred within some whole classes -- the Crustacea, for instance, where wonderfully diverse forms are still linked together by a long, only partially broken chain of relationships. Extinction has only sharpened the boundaries between groups; it hasn't created them. If every form that has ever lived on this earth were suddenly to reappear, it would be impossible to write definitions distinguishing each group. Yet a natural classification -- or at least a natural arrangement -- would still be possible. We can see this by looking at the diagram: the letters A through L represent eleven Silurian genera, some of which have produced large groups of modified descendants. Imagine every link in each branch and subbranch still alive, with the differences between links no greater than those between existing varieties. In this case, it would be impossible to write definitions distinguishing the members of each group from their immediate parents and descendants. Yet the arrangement in the diagram would still hold and would still be natural. Based on the principle of inheritance, all forms descended from A, for instance, would have something in common. In a tree, we can point to this or that branch, even though at the actual fork the two merge and blend. We couldn't define the groups, as I've said, but we could pick out types or forms representing most of the traits of each group, large or small, and thus give a general idea of the differences between them. This is what we'd have to do if we ever succeeded in collecting every form in any class that has lived throughout all time and space. We'll certainly never achieve so perfect a collection. But in some classes, we're moving in that direction. Milne Edwards has recently emphasized, in an excellent paper, the great importance of looking at types, whether or not we can precisely separate and define the groups to which they belong.
To sum up: we've seen that natural selection, which follows from the struggle for existence and almost inevitably leads to extinction and divergence of character among descendants from any one parent species, explains that great and universal feature of all living things -- their organization in groups within groups. We use the element of descent to classify individuals of both sexes and all ages under one species, even though they may have few traits in common. We use descent to classify acknowledged varieties, however different they may be from their parents. And I believe that this element of descent is the hidden bond that naturalists have sought under the term "Natural System." On this view of the natural system being genealogical in its arrangement, with the grades of difference expressed by the terms varieties, species, genera, families, orders, and classes, we can understand the rules we're compelled to follow in classification. We can understand why we value certain resemblances far more than others, why we use rudimentary and useless organs or others of trivial physiological importance, why we reject analogical or adaptive traits when relating one group to another yet use these same traits within the limits of the same group. We can see clearly how all living and extinct forms can be grouped within a few great classes, and how the members of each class are connected by the most complex and radiating lines of relationship. We'll probably never untangle the web of relationships among the members of any one class. But when we have a clear objective and don't look for some unknown plan of creation, we may hope to make slow but steady progress.
Professor Haeckel, in his Generelle Morphologie and other works, has recently brought his great knowledge and abilities to bear on what he calls phylogeny -- the lines of descent of all living things. In constructing these lineages, he relies mainly on embryological traits but also draws on homologous and rudimentary organs, as well as the geological periods when various forms of life are believed to have first appeared. He has boldly made a great beginning and shows us how classification will be treated in the future.
We've seen that members of the same class resemble each other in the general plan of their body structure, regardless of their habits of life. This resemblance is often described by the term "unity of type," or by saying that the various parts and organs in the different species of a class are homologous. This whole subject falls under the general heading of morphology -- one of the most fascinating branches of natural history, which may almost be called its very soul. What could be more remarkable than that the hand of a human, formed for grasping, the hand of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat should all be built on the same pattern and include similar bones in the same relative positions? How curious it is, to give a secondary but striking example, that the hind feet of the kangaroo, so well suited for bounding over open plains -- the feet of the climbing, leaf-eating koala, equally well suited for grasping tree branches -- those of the ground-dwelling, insect- and root-eating bandicoots -- and those of some other Australian marsupials -- should all be constructed on the same extraordinary pattern, with the bones of the second and third toes extremely slender and wrapped in the same skin, so that they appear like a single toe with two claws. Despite this similarity of pattern, it's obvious that the hind feet of these animals are used for purposes as wildly different as you could imagine. The case is made even more striking by the American opossums, which follow nearly the same habits of life as some of their Australian relatives but have feet built on the ordinary plan. Professor Flower, from whom I've taken these facts, remarks in conclusion: "We may call this conformity to type, without getting much nearer to an explanation of the phenomenon." He then adds: "But is it not powerfully suggestive of true relationship, of inheritance from a common ancestor?"
Geoffroy St. Hilaire strongly insisted on the importance of relative position or connection in homologous parts. They may differ to almost any extent in form and size, yet remain connected in the same invariable order. We never find, for instance, the bones of the upper arm and forearm, or the thigh and shin, transposed. This is why the same names can be given to the homologous bones in wildly different animals. We see the same great law in the construction of insect mouths: what could be more different than the immensely long spiral tongue of a hawk moth, the curiously folded mouthparts of a bee or a bug, and the great jaws of a beetle? Yet all these organs, serving such wildly different purposes, are formed by countless modifications of an upper lip, mandibles, and two pairs of maxillae. The same law governs the construction of the mouths and limbs of crustaceans. And the same is true for the flowers of plants.
It's hopeless to try to explain this similarity of pattern within members of the same class by function or by the doctrine of design. The hopelessness of the attempt has been expressly admitted by Owen in his fascinating work On the Nature of Limbs. Under the view of the independent creation of each organism, we can only say that this is how it is -- that it pleased the Creator to build all the animals and plants in each great class on a uniform plan. But this isn't a scientific explanation.
The explanation is, to a large extent, straightforward under the theory of selection of successive slight modifications, each useful in some way to the modified form, but often affecting other parts of the body through correlated development. In changes of this kind, there will be little or no tendency to alter the original pattern or to rearrange the parts. The bones of a limb might be shortened and flattened to any extent, becoming wrapped in thick membrane to serve as a fin. Or a webbed hand might have all its bones, or some of them, lengthened to any extent, with the connecting membrane enlarged to serve as a wing. Yet none of these changes would tend to alter the framework of the bones or the way the parts connect to each other. If we suppose that the ancient ancestor -- the archetype, as it might be called -- of all mammals, birds, and reptiles had its limbs built on the existing general pattern, whatever purpose they served, we can immediately see the plain meaning of homologous limb structure throughout the class. Similarly with insect mouths: we only need to suppose that their common ancestor had an upper lip, mandibles, and two pairs of maxillae, perhaps very simple in form. Natural selection then accounts for the infinite diversity in the structure and function of insect mouths. It's conceivable, however, that the general pattern of an organ might become so obscured as to be completely lost -- through the reduction and ultimately complete disappearance of some parts, the fusion of other parts, and the doubling or multiplication of still others. All of these variations are known to be possible. In the paddles of the gigantic extinct marine reptiles and in the mouths of certain parasitic crustaceans, the general pattern does seem to have become partially obscured.
There's another equally curious branch of this subject: serial homology, or the comparison of different parts or organs within the same individual, rather than of the same parts in different members of the same class. Most anatomists believe that the bones of the skull are homologous -- that is, they correspond in number and relative position -- with the components of a certain number of vertebrae. The front and hind limbs in all the higher vertebrate classes are plainly homologous. So are the wonderfully complex jaws and legs of crustaceans. It's familiar to almost everyone that in a flower, the arrangement of sepals, petals, stamens, and pistils, as well as their detailed structure, makes sense on the view that they consist of modified leaves arranged in a spiral. In abnormal plants, we often get direct evidence that one organ can be transformed into another. And we can actually see, during the early or embryonic stages of development in flowers as well as in crustaceans and many other animals, that organs which become extremely different when mature are at first exactly alike.
How inexplicable serial homologies are under the ordinary view of creation! Why should the brain be enclosed in a box made of so many extraordinarily shaped pieces of bone that apparently represent vertebrae? As Owen has remarked, the benefit of having separate pieces that yield during birth in mammals doesn't explain the same construction in the skulls of birds and reptiles. Why should similar bones have been created to form the wing and the leg of a bat, used as they are for totally different purposes -- flying and walking? Why should a crustacean with an extremely complex mouth made of many parts always have fewer legs? Or conversely, why do those with many legs have simpler mouths? Why should the sepals, petals, stamens, and pistils of each flower, though adapted for such different purposes, all be built on the same pattern?
Under the theory of natural selection, we can answer these questions to a certain extent. We needn't consider here how the bodies of some animals first became divided into a series of segments, or into right and left halves with matching organs -- such questions are almost beyond investigation. It's likely, however, that some serial structures result from cells multiplying by division, producing a multiplication of the parts that develop from those cells. For our purposes, it's enough to remember that indefinite repetition of the same part or organ is the hallmark of all simple or unspecialized forms, as Owen has pointed out. The unknown ancestor of the vertebrates probably had many vertebrae. The unknown ancestor of the arthropods probably had many segments. The unknown ancestor of flowering plants probably had many leaves arranged in one or more spirals. As I showed earlier, parts that are repeated many times are especially prone to vary, not only in number but in form. Such parts, being already present in large numbers and being highly variable, would naturally provide the raw material for adaptation to the most different purposes. Yet they would generally retain, through the force of inheritance, clear traces of their original resemblance. They would retain this resemblance all the more because the variations that formed the basis for their later modification through natural selection would tend to be similar from the start -- the parts being alike at an early stage of growth and subject to nearly the same conditions. Such parts, whether more or less modified, would be serially homologous unless their common origin had become completely obscured.
In the great class of mollusks, although the parts of different species can be shown to be homologous, only a few serial homologies -- such as the shell plates of chitons -- can be identified. That is, we can rarely say that one part of a mollusk is homologous with another part of the same individual. This makes sense, because even in the simplest mollusks, we don't find nearly as much indefinite repetition of any one part as we do in the other great classes of the animal and plant kingdoms.
But morphology is a more complex subject than it first appears, as has recently been well shown in a remarkable paper by E. Ray Lankester, who has drawn an important distinction between cases that naturalists have all equally called homologous. He proposes to call structures that resemble each other in different animals because of descent from a common ancestor with subsequent modification "homogenous." Resemblances that can't be explained this way he proposes to call "homoplastic." For instance, he believes that the hearts of birds and mammals are, as a whole, homogenous -- derived from a common ancestor. But the four chambers of the heart in the two classes are homoplastic -- they developed independently. Lankester also discusses the close resemblance of the right and left sides of the body, and of successive segments of the same animal. Here we have parts commonly called homologous that have no connection to the descent of different species from a common ancestor. Homoplastic structures are the same as what I've called, though rather imperfectly, analogical modifications or resemblances. Their formation can be attributed partly to distinct organisms, or distinct parts of the same organism, having varied in similar ways, and partly to similar modifications having been preserved for the same general function -- of which I've given many examples.
Naturalists frequently speak of the skull as being made of modified vertebrae, the jaws of crabs as modified legs, the stamens and pistils of flowers as modified leaves. But it would be more accurate in most cases, as Professor Huxley has pointed out, to say that both skull and vertebrae, both jaws and legs, and so on, have been modified from some shared, simpler element -- not directly from each other as they currently exist. Most naturalists, however, use such language only metaphorically. They're far from meaning that over a long course of descent, primordial organs of any kind -- vertebrae in one case, legs in the other -- have actually been converted into skulls or jaws. Yet the appearance of this having occurred is so strong that naturalists can hardly help using language that implies it. According to the views I'm defending here, such language can be used literally. And the wonderful fact that the jaws of a crab, for instance, retain numerous traits that they would probably have retained through inheritance if they really had been transformed from true, though extremely simple, legs -- this fact is partly explained.
This is one of the most important subjects in all of natural history. The metamorphoses of insects, with which everyone is familiar, are generally carried out abruptly through a few stages. But the transformations are in reality numerous and gradual, though hidden. A certain mayfly (Chloeon), as shown by Sir John Lubbock, molts more than twenty times during its development, undergoing some degree of change each time. Here we can see metamorphosis performed in a gradual, step-by-step manner. Many insects, and especially certain crustaceans, show us what astonishing changes of structure can occur during development. Such changes reach their peak in the so-called alternating generations of some lower animals. It is, for instance, an astonishing fact that a delicate, branching coral colony, studded with polyps and attached to an underwater rock, should produce -- first by budding and then by splitting crosswise -- a host of huge floating jellyfish. These jellyfish then produce eggs, from which hatch tiny swimming larvae that attach themselves to rocks and develop into branching coral colonies, and so on in an endless cycle. The belief that alternating generations and ordinary metamorphosis are essentially the same process has been greatly strengthened by Wagner's discovery that the larva, or maggot, of a fly called Cecidomyia can produce other larvae asexually, and these produce still others, which finally develop into adult males and females that reproduce in the ordinary way through eggs.
It may be worth mentioning that when Wagner's remarkable discovery was first announced, I was asked how it was possible to explain how the larvae of this fly had acquired the power of asexual reproduction. As long as the case was unique, no answer could be given. But Grimm has since shown that another fly, a Chironomus, reproduces in nearly the same way, and he believes this occurs frequently in the order. In Chironomus, it's the pupa, not the larva, that has this power. Grimm further shows that this case, to some extent, "bridges the gap between the Cecidomyia and the parthenogenesis of the Coccidae" -- parthenogenesis meaning that mature female Coccidae (scale insects) can produce fertile eggs without mating. Certain animals in several classes are now known to be able to reproduce at an unusually early age. We only need to imagine parthenogenetic reproduction being gradually pushed to an earlier and earlier age -- with Chironomus showing us an almost exactly intermediate stage, namely the pupal stage -- and we can perhaps explain the remarkable case of the Cecidomyia.
As I've already mentioned, various parts of the same individual that are exactly alike during early embryonic development become widely different and serve very different purposes in the adult. Similarly, the embryos of the most distinct species within the same class are closely similar but become widely dissimilar when fully developed. No better proof of this can be given than the statement by the embryologist Von Baer: "The embryos of mammals, birds, lizards, and snakes, probably also of turtles, are in their earliest stages exceedingly like one another, both as a whole and in how their parts develop -- so much so that we can often distinguish the embryos only by size. I have two little embryos preserved in alcohol whose labels I've forgotten to attach, and at present I'm quite unable to say what class they belong to. They may be lizards or small birds or very young mammals, so complete is the similarity in the formation of the head and trunk. The limbs, however, are still absent in these embryos. But even if they had been present at the earliest stage of development, we would learn nothing from them, for the feet of lizards and mammals, the wings and feet of birds, and the hands and feet of humans all arise from the same basic form." The larvae of most crustaceans, at corresponding stages of development, closely resemble each other, however different the adults may become. The same is true for many other animals. A trace of this law of embryonic resemblance sometimes lasts until quite late in development. Birds of the same genus, and of related genera, often resemble each other in their immature plumage -- as we see in the spotted feathers of young thrushes. In the cat family, most species as adults are striped or spotted, and stripes or spots can be clearly seen in the cubs of both the lion and the puma. We occasionally, though rarely, see something similar in plants. The first leaves of the gorse (Ulex) and the first leaves of the phyllode-bearing acacias are divided like the ordinary leaves of legumes.
The structural features that make embryos of widely different animals within the same class resemble each other often have no direct connection to their conditions of life. We can't suppose, for instance, that in vertebrate embryos, the peculiar looping arteries near the gill slits are related to similar conditions -- in the young mammal nourished in its mother's womb, in the bird egg hatched in a nest, and in frog spawn under water. We have no more reason to believe in such a connection than to believe that the similar bones in the human hand, the bat's wing, and the porpoise's flipper are related to similar conditions of life. No one supposes that the stripes on a lion cub, or the spots on a young blackbird, are of any use to these animals.
The case is different, however, when an animal is active at some point during its embryonic development and has to fend for itself. This period of activity may come earlier or later in life, but whenever it arrives, the larva's adaptation to its conditions is just as perfect and beautiful as in the adult. Sir John Lubbock has recently shown how important this is, by pointing out the close similarity of larvae among some insects from very different orders and the dissimilarity of larvae within the same order, depending on their habits. Because of such adaptations, the similarity of larvae in related animals is sometimes greatly obscured -- especially when there's a division of labor during different stages of development, as when the same larva has to search for food during one stage and for a place to attach during another. There are even cases where the larvae of related species or groups of species differ from each other more than the adults do. In most cases, however, larvae, though active, still more or less follow the law of embryonic resemblance. Barnacles provide a good example: even the great Cuvier didn't recognize that a barnacle was a crustacean. But a glance at the larva shows it unmistakably. Similarly, the two main divisions of barnacles -- stalked and sessile -- look very different externally, but their larvae are barely distinguishable at all stages.
The embryo generally becomes more complex in organization as it develops. I use this expression while acknowledging that it's hard to define precisely what "higher" or "lower" organization means. But probably no one would dispute that a butterfly is more complex than a caterpillar. In some cases, however, the adult must be considered simpler than the larva, as with certain parasitic crustaceans. To return once more to barnacles: the larvae in the first stage have three pairs of swimming legs, a simple single eye, and a functioning mouth with which they feed actively, growing considerably in size. In the second stage, corresponding to the chrysalis stage of butterflies, they have six pairs of beautifully constructed swimming legs, a pair of magnificent compound eyes, and extremely complex antennae. But they have a sealed, nonfunctional mouth and can't feed. Their purpose at this stage is to use their well-developed sense organs and active swimming ability to find a suitable place to attach and undergo their final transformation. Once this is complete, they're fixed in place for life. Their legs are now converted into grasping organs. They again have a well-constructed mouth, but they have no antennae, and their two eyes have been reduced back to a tiny, simple eyespot. In this final and complete state, barnacles could be considered either more or less complex than they were as larvae. But in some genera, the larvae develop either into hermaphrodites with the usual structure or into what I've called complemental males. In the latter case, the development has certainly been retrograde -- the male is a mere sac that lives for only a short time and lacks a mouth, stomach, and every other important organ except those for reproduction.
We're so used to seeing a difference in structure between the embryo and the adult that we're tempted to view this difference as somehow inevitably tied to growth. But there's no reason why, for instance, the wing of a bat or the fin of a porpoise shouldn't have been sketched out with all their parts in proper proportion as soon as any part became visible. In some whole groups of animals, and in certain members of other groups, this is exactly the case -- the embryo doesn't differ significantly from the adult at any stage. Owen has noted this about cuttlefish: "There is no metamorphosis; the cephalopod characteristics appear long before the embryo's parts are fully developed." Land snails and freshwater crustaceans are born in their adult form, while the marine members of these same two great classes pass through considerable, often dramatic changes during development. Spiders barely undergo any metamorphosis. The larvae of most insects pass through a wormlike stage, whether they're active and adapted to various habits, or inactive because they're surrounded by food or fed by their parents. But in a few cases, like the aphid -- as shown in Professor Huxley's excellent drawings of its development -- there's hardly any trace of the wormlike stage.
Sometimes it's only the earlier developmental stages that are skipped. Fritz Muller made the remarkable discovery that certain shrimp-like crustaceans (related to Penaeus) first appear in the simple nauplius form and, after passing through two or more zoea stages and then a mysis stage, finally achieve their adult structure. In the whole great order Malacostraca, to which these crustaceans belong, no other member is yet known to start as a nauplius, though many appear as zoeas. Nevertheless, Muller gives reasons for believing that if no developmental stages had been skipped, all these crustaceans would have started as nauplii.
How, then, can we explain these facts about embryology? The very general, though not universal, difference between the embryo and the adult. The various parts of the same embryo that ultimately become very different and serve diverse purposes, yet are alike at an early stage. The common, but not invariable, resemblance between embryos or larvae of the most different species within the same class. The embryo often retaining, while still in the egg or womb, structures that are of no use to it at that or any later stage. On the other hand, larvae that must fend for themselves being perfectly adapted to their surroundings. And finally, certain larvae being more complex in organization than the adults they eventually become. I believe all these facts can be explained as follows.
It's commonly assumed -- perhaps from abnormalities that affect the embryo very early -- that slight variations necessarily appear at an equally early stage. We have little evidence on this, but what we do have certainly points the other way. Breeders of cattle, horses, and various fancy animals are well known to be unable to tell, until some time after birth, what the strengths and weaknesses of their young animals will be. We see this plainly in our own children -- we can't tell whether a child will be tall or short, or what their precise features will be. The question isn't at what stage of life a variation may have been caused, but at what stage the effects show up. The cause may have acted -- and I believe often has acted -- on one or both parents before conception. It's worth noting that it doesn't matter to a very young animal, as long as it's nourished and protected by its parent, whether most of its adult traits are acquired a little earlier or later. It wouldn't matter, for instance, to a bird that feeds with a strongly curved beak whether it had that beak shape when young, as long as it was being fed by its parents.
I stated in the first chapter that variations tend to reappear in the offspring at a corresponding age to when they first appeared in the parent. Some variations can only appear at specific ages -- peculiarities of the caterpillar, cocoon, or adult stages of the silkworm moth, for example, or the full-grown horns of cattle. But variations that, for all we can tell, might have appeared either earlier or later also tend to reappear at the corresponding age in the offspring. I'm far from claiming this is always the case, and I could give several examples of variations that appeared at an earlier age in the child than in the parent.
These two principles -- that slight variations generally appear at a not very early period of life and are inherited at a corresponding period -- explain, as I believe, all the major facts of embryology that I've listed above. But first, let's look at some comparable cases in our domestic varieties. Some authors who have written about dogs maintain that the greyhound and bulldog, though so different, are really close relatives descended from the same wild stock. So I was curious to see how much their puppies differed. Breeders told me the puppies differed just as much as the adults, and judging by eye, this seemed almost true. But when I actually measured the adult dogs and their six-day-old puppies, I found the puppies hadn't nearly acquired their full degree of proportional difference. Similarly, I was told that the foals of cart horses and racehorses -- breeds formed almost entirely by selection under domestication -- differed as much as the adults. But careful measurements of the mares and their three-day-old foals showed this was by no means the case.
Since we have conclusive evidence that pigeon breeds descend from a single wild species, I compared young pigeons within twelve hours of hatching. I carefully measured the proportions (though I won't give the details here) of the beak, width of mouth, length of nostril and eyelid, size of feet, and length of leg in the wild parent species and in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Some of these birds, when mature, differ so extraordinarily in the length and form of beak and in other traits that they would certainly be ranked as distinct genera if found in nature. But when the nestling birds of these several breeds were placed in a row, though most could just be told apart, the proportional differences were incomparably less than in the full-grown birds. Some characteristic differences -- the width of the mouth, for instance -- could hardly be detected in the young. But there was one remarkable exception: the young of the short-faced tumbler differed from the young of the wild rock pigeon and of the other breeds in almost exactly the same proportions as in the adult stage.
These facts are explained by the two principles I've described. Breeders select their dogs, horses, pigeons, and so on for breeding when they're nearly grown. They don't care whether the desired qualities appear earlier or later in life, as long as the adult animal has them. The cases I've just given, especially the pigeons, show that the distinctive traits accumulated by human selection, which give value to these breeds, don't generally appear very early in life and are inherited at a correspondingly late stage. The case of the short-faced tumbler, which had its characteristic features at twelve hours old, proves this isn't universal. Here, the distinctive differences must either have appeared at an earlier age than usual, or, if not, they must have been inherited at an earlier age than the one at which they first appeared.
Now let's apply these two principles to species in nature. Take a group of birds descended from some ancient form and modified through natural selection for different habits. Since the many slight successive variations appeared at a not early age and were inherited at a corresponding age, the young will have been only slightly modified, and they'll still resemble each other much more closely than the adults do -- just as we saw with the pigeon breeds. We can extend this view to widely different structures and to whole classes. The forelimbs, for instance, which once served as legs for a remote ancestor, may have become adapted in one descendant to function as hands, in another as paddles, in another as wings. But based on our two principles, the forelimbs won't have been much modified in the embryos of these various forms, even though in each one the forelimb will differ greatly in the adult state. Whatever influence long-continued use or disuse may have had in modifying the limbs or other parts of any species, this will mainly have affected it at maturity, when it had to use its full powers to make its own living. The effects would then be transmitted to offspring at a correspondingly mature age. So the young won't be modified, or will be modified only slightly, by the effects of increased use or disuse of parts.
In some animals, successive variations may have appeared very early in life, or may have been inherited at an earlier age than when they first occurred. In either case, the young or embryo will closely resemble the mature parent form, as we saw with the short-faced tumbler. This is the rule of development in certain whole groups, or in certain subgroups alone, as with cuttlefish, land snails, freshwater crustaceans, spiders, and some members of the great class of insects. Regarding why the young in such groups don't undergo metamorphosis, we can see that this would follow from the young having to fend for themselves at a very early age and following the same way of life as their parents. In that case, it would be essential for their survival that they be modified in the same way as their parents. Again, regarding the curious fact that many land and freshwater animals don't undergo metamorphosis while marine members of the same groups pass through various transformations, Fritz Muller has suggested that the process of slowly adapting an animal to live on land or in fresh water instead of in the sea would be greatly simplified if it didn't pass through any larval stage. It's unlikely that places well suited for both the larval and adult stages, under such drastically changed conditions, would commonly be found unoccupied by other organisms. In these circumstances, the gradual acquisition of the adult structure at earlier and earlier ages would be favored by natural selection, and all traces of former metamorphoses would eventually be lost.
If, on the other hand, it benefited the young of an animal to follow habits slightly different from those of the parent form, and consequently to be built on a slightly different plan -- or if it benefited a larva already different from its parent to change still further -- then, by the principle of inheritance at corresponding ages, the young or larvae could be made through natural selection more and more different from their parents to any extent imaginable. Differences in the larva might also become linked to successive stages of its development, so that the larva in the first stage might come to differ greatly from the larva in the second stage, as is the case with many animals. The adult might also become suited to environments or habits where organs of movement, of the senses, and so on would be useless. In that case, the metamorphosis would be retrograde.
From these observations, we can see how, through changes in the structure of the young that match changed habits of life, together with inheritance at corresponding ages, animals might come to pass through developmental stages completely different from the original condition of their adult ancestors. Most of our best authorities are now convinced that the various larval and pupal stages of insects have been acquired through adaptation, not inherited from some ancient form. The curious case of Sitaris -- a beetle that passes through unusual developmental stages -- illustrates how this might occur. The first larval form is described by Fabre as an active, tiny insect with six legs, two long antennae, and four eyes. These larvae are hatched in the nests of bees. When the male bees emerge from their burrows in the spring (they do so before the females), the larvae spring onto them and later crawl onto the females during mating. As soon as the female bee deposits her eggs on the surface of the honey stored in the cells, the Sitaris larvae leap onto the eggs and devour them. Afterward, they undergo a complete transformation: their eyes disappear, their legs and antennae become rudimentary, and they feed on honey -- so that they now more closely resemble the ordinary larvae of insects. They ultimately undergo a further transformation and finally emerge as the adult beetle. If an insect undergoing transformations like those of Sitaris were to become the ancestor of a whole new class of insects, the course of development in the new class would be wildly different from that of our existing insects. The first larval stage certainly would not represent the former condition of any adult ancestral form.
On the other hand, it's very likely that in many animals the embryonic or larval stages show us, more or less completely, the condition of the group's ancestor in its adult state. In the great class of crustaceans, forms as different from each other as parasitic sucking forms, barnacles, tiny entomostraca, and even the large malacostracans all first appear as nauplius larvae. Since these larvae live and feed in the open sea and aren't adapted for any special way of life -- and for other reasons given by Fritz Muller -- it's likely that at some very remote period, an independent adult animal resembling the nauplius existed and subsequently produced, along several divergent lines of descent, the great crustacean groups named above. Similarly, from what we know of the embryos of mammals, birds, fish, and reptiles, it's likely that these animals are the modified descendants of some ancient ancestor that in its adult state had gills, a swim bladder, four fin-like limbs, and a long tail -- all suited for aquatic life.
Since all living things, extinct and recent, can be arranged within a few great classes, and since all within each class have, according to our theory, been connected by fine gradations, the best arrangement -- and if our collections were nearly perfect, the only possible arrangement -- would be genealogical. Descent is the hidden bond that naturalists have been seeking under the term "Natural System." On this view, we can understand why most naturalists consider the embryo's structure even more important for classification than the adult's. If two or more groups of animals differ greatly from each other in structure and habits as adults but pass through closely similar embryonic stages, we can be confident that they all descend from one parent form and are therefore closely related. Shared embryonic structure reveals shared descent. But differences in embryonic development don't prove the absence of shared descent, because in one of two groups the developmental stages may have been suppressed or so greatly modified through adaptation to new habits of life as to be unrecognizable. Even in groups where the adults have been modified to an extreme degree, shared ancestry is often revealed by the larvae. We've seen, for instance, that barnacles, though externally resembling shellfish, are immediately recognized by their larvae as members of the great class of crustaceans. Since the embryo often shows us, more or less clearly, the structure of the less modified and ancient ancestor of the group, we can see why ancient and extinct forms so often resemble the embryos of existing species in the same class. Agassiz believes this to be a universal law of nature, and we may hope to see it proved true in the future. It can, however, be proved only in cases where the ancient condition of the group's ancestor hasn't been completely erased -- either by variations appearing very early in development, or by variations being inherited at an earlier age than that at which they first appeared. We should also keep in mind that the law may be true, yet because the geological record doesn't extend far enough back in time, it may remain impossible to demonstrate for a long period, or even forever. The law won't strictly hold in cases where an ancient form became adapted in its larval state to some special way of life and transmitted that same larval state to an entire group of descendants, because such a larval state wouldn't resemble any still more ancient form in its adult state.
So, as it seems to me, the leading facts of embryology -- second to none in importance -- are explained by the principle that variations in the many descendants from some ancient ancestor have appeared at a not very early period of life and have been inherited at a corresponding period. Embryology becomes enormously more interesting when we look at the embryo as a picture, more or less blurred, of the ancestor -- either in its adult or its larval state -- of all the members of the same great class.
Organs or parts in this strange condition, bearing the plain stamp of uselessness, are extremely common -- virtually universal -- throughout nature. It would be impossible to name a single higher animal in which some part isn't in a rudimentary condition. In mammals, for instance, males possess rudimentary nipples. In snakes, one lobe of the lungs is rudimentary. In birds, the "bastard wing" can safely be considered a rudimentary digit, and in some species the entire wing is so reduced that it can't be used for flight. What could be more curious than the presence of teeth in fetal whales, which when grown up have no teeth at all? Or the teeth that never break through the gums in the upper jaws of unborn calves?
Rudimentary organs plainly declare their origin and meaning in various ways. There are beetles of closely related species, or even the same species, that have either full-sized and perfect wings or mere remnants of membrane, which often lie under wing covers firmly fused together. In these cases, it's impossible to doubt that the remnants represent wings. Rudimentary organs sometimes retain their potential: this occasionally happens with the nipples of male mammals, which have been known to become fully developed and produce milk. Similarly, in the udders of the genus Bos, there are normally four developed and two rudimentary teats. But in our domestic cows, the rudimentary ones sometimes become fully developed and yield milk. In some plants, petals are sometimes rudimentary and sometimes well developed in individuals of the same species. In certain plants with separate sexes, Kolreuter found that by crossing a species whose male flowers included a rudiment of a pistil with a hermaphrodite species having a well-developed pistil, the rudiment in the hybrid offspring was much enlarged. This clearly shows that the rudimentary and perfect pistils are essentially the same in nature. An animal may have various parts in a perfect state that are nevertheless, in a sense, rudimentary because they're useless. As G. H. Lewes remarks, the tadpole of the common salamander or water newt "has gills and lives in the water. But the alpine salamander (Salamandra atra), which lives high in the mountains, gives birth to its young fully formed. This animal never lives in the water. Yet if we open a pregnant female, we find tadpoles inside her with exquisitely feathered gills. When placed in water, they swim about like the tadpoles of the water newt. Obviously this aquatic organization has no relevance to the future life of the animal, nor any adaptation to its embryonic condition. It has reference solely to ancestral adaptations -- it repeats a phase in the development of its ancestors."
An organ serving two purposes may become rudimentary or completely disappear for one purpose, even the more important one, while remaining perfectly functional for the other. In plants, for instance, the pistil's job is to let pollen tubes reach the ovules inside the ovary. The pistil consists of a stigma supported on a style. But in some plants of the daisy family, the male florets -- which of course can't be fertilized -- have a rudimentary pistil with no stigma. The style, however, remains well developed and is clothed in the usual way with hairs that serve to brush pollen out of the surrounding fused anthers. An organ may also become rudimentary for its original function and be repurposed for a different one. In certain fish, the swim bladder seems to be rudimentary for its proper function of providing buoyancy but has been converted into an emerging breathing organ, or lung. Many similar examples could be given.
Useful organs, however little developed, shouldn't be considered rudimentary unless we have reason to think they were once more highly developed. They may be in a nascent state, in the process of developing further. Rudimentary organs, on the other hand, are either completely useless, like teeth that never break through the gums, or nearly useless, like the wings of an ostrich, which serve only as sails. Since organs in this condition would have been even less useful when they were still less developed, they can't have been produced through variation and natural selection, which works only by preserving useful modifications. They've been partially retained by the power of inheritance and relate to an earlier state of things. It's often difficult, however, to distinguish between rudimentary and nascent organs, since we can only judge by analogy whether a part is capable of further development -- and only then does it deserve to be called nascent. Organs in a nascent state will always be somewhat rare, because organisms possessing them will usually have been replaced by competitors with the same organ in a more perfected form and will have gone extinct long ago. The penguin's wing is highly functional, serving as a flipper. It might therefore represent the nascent state of a wing -- not that I believe this; it's more likely a reduced organ modified for a new function. The wing of the kiwi (Apteryx), on the other hand, is completely useless and truly rudimentary. Owen considers the simple, threadlike limbs of the lungfish (Lepidosiren) to be "the beginnings of organs which reach full functional development in higher vertebrates." But according to the view recently defended by Dr. Gunther, they're probably remnants -- the persistent central axis of a fin, with the side branches lost. The mammary glands of the platypus (Ornithorhynchus) may be considered nascent compared with the udders of a cow. The egg-holding appendages of certain barnacles, which have ceased to hold eggs and are feebly developed, are nascent gills.
Rudimentary organs in individuals of the same species are very prone to vary in the degree of their development and in other respects. In closely related species, too, how much the same organ has been reduced can differ considerably. This is well illustrated by the wings of female moths within the same family. Rudimentary organs may be completely lost. This means that in certain animals or plants, parts are entirely absent that we would expect to find based on comparison with related forms, and that occasionally appear in abnormal individuals. In most species of the Scrophulariaceae, for instance, the fifth stamen is entirely absent. Yet we can conclude that a fifth stamen once existed, because a rudiment of it is found in many species of the family, and this rudiment occasionally becomes fully developed -- as can sometimes be seen in the common snapdragon. In tracing the homologies of any part across different members of the same class, nothing is more common or more useful than the discovery of rudiments. This is well shown in the drawings Owen has given of the leg bones of the horse, ox, and rhinoceros.
It's an important fact that rudimentary organs, such as teeth in the upper jaws of whales and ruminants, can often be detected in the embryo but later disappear completely. It's also, I believe, a universal rule that a rudimentary part is relatively larger in the embryo compared with the adjoining parts than in the adult. So at the early stage, the organ is less rudimentary, or may not be rudimentary at all. This is why rudimentary organs in the adult are often said to have retained their embryonic condition.
I've now laid out the main facts about rudimentary organs. In reflecting on them, everyone must be struck with astonishment. The same reasoning that tells us most parts and organs are exquisitely adapted for certain purposes tells us, with equal clarity, that rudimentary or shrunken organs are imperfect and useless. In natural history books, rudimentary organs are generally said to have been created "for the sake of symmetry" or to "complete the scheme of nature." But this isn't an explanation -- it's merely a restatement of the fact. And it's not even consistent with itself: the boa constrictor has rudiments of hind limbs and a pelvis, but if these bones were retained "to complete the scheme of nature," why, as Professor Weismann asks, haven't they been retained by other snakes that lack even a trace of them? What would we think of an astronomer who maintained that satellites revolve in elliptical orbits around their planets "for the sake of symmetry" because the planets revolve that way around the sun? One eminent physiologist explains rudimentary organs by supposing they serve to excrete excess or harmful substances. But can we suppose that the minute bump of tissue that often represents the pistil in male flowers, made of mere cellular tissue, could serve this function? Can we suppose that rudimentary teeth, which are later absorbed, benefit the rapidly growing embryonic calf by removing something as precious as calcium phosphate? When a person's fingers have been amputated, imperfect nails have been known to appear on the stumps. I could as easily believe these nail vestiges are developed to excrete horny matter as believe that the rudimentary nails on the flipper of the manatee serve the same purpose.
Under the view of descent with modification, the origin of rudimentary organs is relatively straightforward, and we can largely understand the laws governing their imperfect development. We have plenty of examples of rudimentary organs in our domestic breeds: the stump of a tail in tailless breeds, the vestige of an ear in earless breeds of sheep, the reappearance of tiny dangling horns in hornless breeds of cattle (especially, according to Youatt, in young animals), and the state of the entire flower in cauliflower. We often see rudiments of various parts in abnormal individuals. But I doubt any of these cases shed much light on the origin of rudimentary organs in nature, beyond showing that rudiments can be produced. The balance of evidence clearly indicates that species in nature don't undergo great and sudden changes. But we learn from studying our domestic breeds that disuse of parts leads to their reduced size, and that this reduction is inherited.
It seems likely that disuse has been the main force behind the reduction of organs to a rudimentary state. It would at first lead, step by step, to increasingly complete reduction of a part, until it finally became rudimentary -- as in the case of the eyes of animals living in dark caves, or the wings of birds on oceanic islands, which have rarely been forced by predators to fly and have eventually lost the power of flight. Again, an organ useful under certain conditions might become harmful under others, as with the wings of beetles living on small, exposed islands. In such cases, natural selection would have helped reduce the organ until it was rendered harmless and rudimentary.
Any change in structure and function that can be achieved by small steps is within the power of natural selection. So an organ rendered useless or harmful for one purpose by changed habits might be modified and used for another. Or it might be retained for just one of its former functions. Organs originally shaped by natural selection, once rendered useless, may well be variable, since their variations can no longer be checked by natural selection. All this fits with what we see in nature. Moreover, at whatever period of life either disuse or selection reduces an organ -- and this will generally be at maturity, when the organism must use its full powers to make its own living -- the principle of inheritance at corresponding ages will tend to reproduce the organ in its reduced state at the same mature age, but will rarely affect it in the embryo. This is why rudimentary organs are relatively larger in the embryo than in the adult. If, for instance, the digit of an adult animal was used less and less over many generations, due to some change of habits, or if an organ or gland was exercised less and less, we can predict that it would become reduced in size in the adult descendants but retain nearly its original level of development in the embryo.
There remains, however, this difficulty. After an organ has stopped being used and has become much reduced, how can it be reduced still further until only the tiniest vestige remains? And how can it eventually be completely eliminated? It's hard to see how disuse can produce any further effect once an organ has become nonfunctional. Some additional explanation is needed, which I can't provide. If, for instance, it could be proved that every part of the body tends to vary more toward smaller size than larger, we could understand how a useless organ would become rudimentary and eventually disappear entirely, independent of disuse -- because variations toward smaller size would no longer be checked by natural selection. The principle of economy of growth, which I explained in an earlier chapter -- whereby materials forming any part not useful to the organism are saved as far as possible -- may also play a role in reducing useless organs. But this principle would almost certainly apply only to the earlier stages of reduction. We can't suppose that a minute bump of tissue, for instance, representing in a male flower the pistil of the female flower and composed of nothing more than cells, could be further reduced or absorbed for the sake of saving nutrients.
Finally, since rudimentary organs -- by whatever steps they may have been degraded to their present useless condition -- are the record of a former state of things and have been retained solely through the power of inheritance, we can understand, on a genealogical view of classification, why taxonomists have often found rudimentary parts as useful as, or even more useful than, parts of high physiological importance for placing organisms in their proper positions in the natural system. Rudimentary organs may be compared with the letters in a word that are still retained in the spelling but have become silent in pronunciation, yet serve as a clue to the word's origin. Under the view of descent with modification, we may conclude that the existence of organs in a rudimentary, imperfect, and useless condition, or completely absent, far from presenting a strange difficulty -- as it certainly does under the old doctrine of creation -- might even have been predicted by the views I've explained.
In this chapter, I've tried to show that the arrangement of all living things throughout all time in groups within groups -- the nature of the relationships by which all living and extinct organisms are connected through complex, radiating, and indirect lines of kinship into a few grand classes -- the rules followed and the difficulties encountered by naturalists in their classifications -- the value placed on traits that are constant and widespread, whether of high importance or the most trivial, or, as with rudimentary organs, of no importance at all -- the sharp contrast between analogical or adaptive traits and traits of true kinship -- and other such rules -- all follow naturally if we accept the common parentage of related forms, together with their modification through variation and natural selection, with the contingencies of extinction and divergence of character. In considering this view of classification, we should keep in mind that the element of descent has universally been used to group together the sexes, ages, dimorphic forms, and acknowledged varieties of the same species, however much they may differ in structure. If we extend the use of this element of descent -- the one known cause of similarity in living things -- we can understand what is meant by the Natural System: it is genealogical in its attempted arrangement, with the grades of acquired difference marked by the terms varieties, species, genera, families, orders, and classes.
Under this same view of descent with modification, most of the great facts of morphology become intelligible -- whether we look at the same pattern displayed by the different species of the same class in their homologous organs, applied to whatever purpose, or at the serial and lateral homologies within each individual animal and plant.
Based on the principle that slight variations generally don't appear at a very early period of life and are inherited at a corresponding period, we can understand the leading facts of embryology. The close resemblance between parts in the individual embryo that are homologous and that become widely different in structure and function when mature. The resemblance of homologous parts or organs in related but distinct species, though the adults are adapted for the most different habits imaginable. Larvae are active embryos that have become specially modified to varying degrees for their way of life, with their modifications inherited at a corresponding early age. Based on these same principles -- and keeping in mind that when organs are reduced in size, whether through disuse or natural selection, this generally happens at maturity when the organism must fend for itself, and keeping in mind how powerful inheritance is -- the occurrence of rudimentary organs might even have been predicted. The importance of embryological traits and rudimentary organs for classification makes sense on the view that a natural arrangement must be genealogical.
Finally, the several classes of facts I've considered in this chapter seem to me to proclaim so clearly that the countless species, genera, and families that populate this world are all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that I would unhesitatingly adopt this view even if it were unsupported by other facts or arguments.
Since this entire book is one long argument, it may help the reader to have the main facts and conclusions briefly summarized.
I don't deny that many serious objections can be raised against the theory of descent with modification through variation and natural selection. I've tried to give these objections their full force. Nothing at first seems harder to believe than that complex organs and instincts were perfected not by some intelligence superior to human reason (though working in an analogous way), but by the accumulation of countless slight variations, each beneficial to the individual that possessed it. Nevertheless, this difficulty, though it seems overwhelmingly great to our imagination, can't be considered a real obstacle if we accept the following propositions: that all parts of an organism's structure and instincts show at least individual differences; that there is a struggle for existence leading to the preservation of beneficial variations in structure or instinct; and lastly, that gradations in the perfection of each organ may have existed, each useful in its own way. The truth of these propositions cannot, I think, be disputed.
It is, no doubt, extremely difficult even to guess by what gradations many structures were perfected, especially among fragmentary and declining groups of organisms that have suffered heavy extinction. But we see so many strange gradations in nature that we should be extremely cautious about saying that any organ, instinct, or whole structure could not have reached its present state through many gradual steps. There are, admittedly, cases of special difficulty for the theory of natural selection. One of the most puzzling is the existence of two or three distinct castes of workers or sterile female ants within the same community. But I've tried to show how these difficulties can be overcome.
Regarding the nearly universal sterility of species when first crossed -- which forms such a remarkable contrast with the nearly universal fertility of varieties when crossed -- I must refer the reader to the summary of facts given at the end of the ninth chapter. These facts seem to me to show conclusively that this sterility is no more a special gift than the inability of two different kinds of trees to be grafted together. Rather, it's an incidental result of differences confined to the reproductive systems of the crossed species. We see the truth of this conclusion in the vast difference in results when the same two species are crossed in opposite directions -- that is, when one species is first used as the father and then as the mother. The analogy from dimorphic and trimorphic plants clearly leads to the same conclusion, because when the forms are illegitimately united, they produce few or no seeds, and their offspring are more or less sterile -- and yet these forms belong to the same undoubted species, differing from each other in nothing except their reproductive organs and functions.
Although many authors have asserted that the fertility of varieties when crossed, and of their hybrid offspring, is universal, this can't be considered entirely correct given the facts reported on the high authority of Gartner and Kolreuter. Most of the varieties that have been tested were produced under domestication, and since domestication (I don't mean mere confinement) almost certainly tends to eliminate the sterility that, judging from analogy, would have affected the parent species if crossed, we shouldn't expect that domestication would also cause sterility in their modified descendants when crossed. This elimination of sterility apparently follows from the same cause that allows our domestic animals to breed freely under varied circumstances -- and this in turn apparently follows from their having been gradually accustomed to frequent changes in their living conditions.
A double and parallel set of facts seems to shed much light on the sterility of species when first crossed and of their hybrid offspring. On one side, there's good reason to believe that slight changes in living conditions give vigor and fertility to all organisms. We also know that crossing between distinct individuals of the same variety, and between distinct varieties, increases the number of offspring and certainly gives them greater size and vigor. This is mainly because the forms being crossed have been exposed to somewhat different conditions of life. I've confirmed through a painstaking series of experiments that if all individuals of the same variety are kept under the same conditions for several generations, the benefit from crossing is often much reduced or disappears entirely. That's one side of the picture. On the other side, we know that species that have long been exposed to nearly uniform conditions, when subjected in confinement to new and greatly changed conditions, either die or, if they survive, become sterile while retaining perfect health. This doesn't happen, or happens only very slightly, with our domesticated animals, which have long been exposed to fluctuating conditions. So when we find that hybrids produced by crossing two distinct species are few in number -- because they die soon after conception or at a very early age, or if they survive are rendered more or less sterile -- it seems highly probable that this result is due to their having been subjected, in effect, to a great change in their conditions of life, from being composed of two distinct biological organizations. Whoever can explain in a definite way why, for instance, an elephant or a fox won't breed in confinement in its native country, while the domestic pig or dog breeds freely under the most varied conditions, will at the same time be able to give a definite answer to why two distinct species when crossed, as well as their hybrid offspring, are generally rendered more or less sterile, while two domesticated varieties when crossed and their hybrid offspring are perfectly fertile.
Turning to geographical distribution, the difficulties for the theory of descent with modification are serious enough. All the individuals of the same species, and all the species of the same genus or even higher group, are descended from common parents. Therefore, in however distant and isolated parts of the world they may now be found, they must over the course of successive generations have traveled from some one point to all the others. We are often completely unable even to guess how this could have happened. Yet since we have reason to believe that some species have retained the same form for very long periods -- immensely long as measured by years -- we shouldn't place too much weight on the occasional wide distribution of the same species. During very long periods there will always have been a good chance for extensive migration by many means. A broken or interrupted range can often be explained by the extinction of the species in the regions between. We can't deny that we are still very ignorant of the full extent of the climatic and geographical changes that have affected the earth during modern periods, and such changes will often have made migration easier. As an example, I've tried to show how powerful the influence of the Glacial period has been on the distribution of the same and related species throughout the world. We are still deeply ignorant of the many occasional means of transport. With respect to distinct species of the same genus inhabiting distant and isolated regions, since the process of modification has necessarily been slow, all means of migration will have been possible over a very long period. Consequently, the difficulty of explaining the wide distribution of species within the same genus is somewhat reduced.
Since, according to the theory of natural selection, an enormous number of intermediate forms must have existed, linking together all the species in each group by gradations as fine as our existing varieties, the question arises: why don't we see these linking forms all around us? Why aren't all organisms blended together in an inextricable chaos? Regarding existing forms, we should remember that we have no right to expect (except in rare cases) to discover directly connecting links between them -- only between each and some extinct, supplanted form. Even across a wide area that has remained continuous for a long period, where climatic and other conditions change gradually as you move from a district occupied by one species into another occupied by a closely related species, we have no right to expect often to find intermediate varieties in the intermediate zones. We have reason to believe that only a few species in a genus ever undergo change, while the other species go completely extinct without leaving modified descendants. Of the species that do change, only a few within the same region change at the same time, and all modifications happen slowly. I've also shown that the intermediate varieties that probably first existed in the intermediate zones would be liable to replacement by the related forms on either side. The latter, existing in greater numbers, would generally be modified and improved at a faster rate than the intermediate varieties, which existed in smaller numbers. So in the long run, the intermediate varieties would be replaced and driven to extinction.
Given this process of extinction of countless connecting links between the living and extinct inhabitants of the world, and at each successive period between the extinct and still older species, why isn't every geological formation packed with such links? Why doesn't every collection of fossil remains offer clear evidence of the gradation and transformation of life forms? Although geological research has undoubtedly revealed many former links, bringing numerous forms of life much closer together, it doesn't yield the infinitely many fine gradations between past and present species that the theory requires -- and this is the most obvious of the many objections that can be raised against it. Why, again, do whole groups of related species appear (though this appearance is often misleading) to have arrived suddenly in successive geological stages? Although we now know that organisms appeared on this globe at an incalculably remote period, long before the lowest bed of the Cambrian system was deposited, why don't we find beneath this system great piles of rock layers stored with the remains of the ancestors of Cambrian fossils? For according to the theory, such layers must somewhere have been deposited at these ancient and utterly unknown epochs of the world's history.
I can answer these questions and objections only on the assumption that the geological record is far more imperfect than most geologists believe. The number of specimens in all our museums is absolutely nothing compared with the countless generations of countless species that have certainly existed. The parent form of any two or more species would not be intermediate in all its features between its modified descendants, any more than the rock pigeon is intermediate in crop and tail between its descendants, the pouter and fantail pigeons. We wouldn't be able to recognize a species as the parent of another modified species, even if we examined the two extremely closely, unless we had most of the intermediate links. And owing to the imperfection of the geological record, we have no right to expect to find so many links. If two or three, or even more linking forms were discovered, they would simply be classified by many naturalists as so many new species, especially if found in different geological layers, however slight their differences. Numerous existing doubtful forms could be named that are probably varieties, but who will claim that in future ages enough fossil links will be discovered for naturalists to decide whether these doubtful forms should be called varieties? Only a small portion of the world has been geologically explored. Only organisms of certain kinds can be preserved as fossils, at least in any great number. Many species, once formed, never undergo any further change but go extinct without leaving modified descendants. And the periods during which species have undergone modification, though long as measured by years, have probably been short compared to the periods during which they retained the same form. It's the dominant and widely distributed species that vary most frequently and vary most, and varieties are often at first local -- both factors making the discovery of intermediate links in any one formation less likely. Local varieties won't spread into other and distant regions until they are considerably modified and improved, and when they have spread and are discovered in a geological formation, they appear as if suddenly created there and will simply be classified as new species. Most formations have been intermittent in their accumulation, and their duration has probably been shorter than the average duration of species. Successive formations are in most cases separated from each other by blank intervals of great length, because fossiliferous formations thick enough to resist future erosion can, as a general rule, accumulate only where much sediment is deposited on a subsiding sea floor. During the alternate periods of elevation and of stationary level, the record will generally be blank. During these latter periods there will probably be more variability in life forms; during periods of subsidence, more extinction.
Regarding the absence of fossil-rich strata beneath the Cambrian formation, I can only return to the hypothesis given in the tenth chapter: that although our continents and oceans have endured for an enormous period in nearly their present relative positions, we have no reason to assume this has always been the case. Consequently, formations much older than any now known may lie buried beneath the great oceans. Regarding the objection that not enough time has passed since our planet solidified for the assumed amount of organic change -- and this objection, as urged by Sir William Thompson, is probably one of the most serious yet advanced -- I can only say, first, that we don't know at what rate species change as measured by years, and second, that many scientists are not yet willing to admit that we know enough about the structure of the universe and the interior of our globe to speculate safely on its past duration.
Everyone will admit that the geological record is imperfect. But few will be inclined to admit that it's imperfect to the degree our theory requires. If we look at long enough intervals of time, geology plainly declares that species have all changed, and they have changed in the manner the theory requires -- slowly and gradually. We see this clearly in the fossil remains from consecutive formations, which are invariably much more closely related to each other than are fossils from widely separated formations.
Such is the sum of the main objections and difficulties that can justly be raised against the theory, and I've now briefly summarized the answers and explanations that, as far as I can see, may be given. I've felt these difficulties far too heavily during many years to underestimate their weight. But it's worth noting that the most important objections relate to questions about which we are admittedly ignorant -- nor do we know how ignorant we are. We don't know all the possible transitional gradations between the simplest and the most perfect organs. No one can claim that we know all the varied means of dispersal over the long course of time, or how imperfect the geological record really is. Serious as these objections are, in my judgment they are by no means enough to overthrow the theory of descent with subsequent modification.
Now let's turn to the other side of the argument. Under domestication we see much variability, caused -- or at least triggered -- by changed conditions of life, but often in such an obscure way that we're tempted to consider the variations spontaneous. Variability is governed by many complex laws: by correlated growth, compensation, the increased use and disuse of parts, and the direct action of surrounding conditions. It's very difficult to determine how extensively our domestic animals and plants have been modified, but we can safely conclude that the changes have been large and that modifications can be inherited for long periods. As long as living conditions remain the same, we have reason to believe that a modification already inherited for many generations may continue to be inherited for an almost infinite number of generations. On the other hand, we have evidence that variability, once it comes into play, doesn't stop under domestication for a very long period -- nor do we know that it ever stops, since new varieties are still occasionally produced by our oldest domesticated animals and plants.
Variability is not actually caused by humans; we only unintentionally expose organisms to new conditions of life, and then nature acts on their organization and causes them to vary. But humans can and do select the variations that nature provides, and accumulate them in any desired direction. We thus adapt animals and plants for our own benefit or pleasure. We may do this methodically, or we may do it unconsciously by preserving the individuals most useful or pleasing to us without any intention of altering the breed. It's certain that we can greatly influence the character of a breed by selecting, in each successive generation, individual differences so slight as to be undetectable except by a trained eye. This unconscious process of selection has been the great force behind the formation of our most distinct and useful domestic breeds. That many breeds produced by humans have largely the character of natural species is shown by the persistent doubts over whether many of them are varieties or originally distinct species.
There is no reason why the principles that have worked so effectively under domestication should not have worked under nature. In the survival of favored individuals and populations, during the constantly recurring struggle for existence, we see a powerful and ever-acting form of selection. The struggle for existence inevitably follows from the high geometric rate of increase common to all organisms. This high rate of increase is proved by calculation, by the rapid increase of many animals and plants during a run of favorable seasons, and when they become established in new countries. More individuals are born than can possibly survive. A tiny advantage may determine which individuals live and which die -- which variety or species increases in number, and which decreases or finally goes extinct. Since individuals of the same species compete with each other in every respect, the struggle will generally be most severe among them. It will be almost equally severe between varieties of the same species, and next in severity between species of the same genus. On the other hand, the struggle will often be severe between organisms far apart on the scale of nature. The slightest advantage in certain individuals, at any age or during any season, over those they compete with -- or better adaptation in however slight a degree to the surrounding physical conditions -- will, in the long run, tip the balance.
In animals with separate sexes, there will in most cases be a struggle between males for possession of the females. The most vigorous males, or those that have most successfully dealt with their conditions of life, will generally leave the most offspring. But success will often depend on the males having special weapons, means of defense, or attractive features -- and a slight advantage will lead to victory.
Since geology plainly shows that every land has undergone great physical changes, we might have expected organisms to vary under nature just as they have under domestication. And if there has been any variability under nature, it would be an inexplicable fact if natural selection had not come into play. It has often been asserted, but the assertion can't be proved, that the amount of variation under nature is strictly limited. Humans, though acting on external characters alone and often capriciously, can produce great results within a short period by adding up mere individual differences in domestic animals and plants -- and everyone admits that species show individual differences. But besides such differences, all naturalists admit that natural varieties exist, considered distinct enough to be recorded in systematic works. No one has drawn any clear line between individual differences and slight varieties, or between more clearly marked varieties and subspecies and species. On separate continents, and on different parts of the same continent when divided by barriers of any kind, and on outlying islands, what a multitude of forms exist that some experienced naturalists rank as varieties, others as geographical races or subspecies, and others as distinct though closely related species!
If, then, animals and plants do vary, however slightly or slowly, why should not variations or individual differences that are in any way beneficial be preserved and accumulated through natural selection, or the survival of the fittest? If humans can patiently select variations useful to them, why, under changing and complex conditions of life, should not variations useful to nature's living creatures often arise and be preserved or selected? What limit can be put to this power, acting over long ages and rigorously scrutinizing the whole constitution, structure, and habits of each creature -- favoring the good and rejecting the bad? I can see no limit to this power, in slowly and beautifully adapting each form to the most complex web of life. The theory of natural selection, even if we look no further than this, seems in the highest degree probable. I've already summarized, as fairly as I could, the opposing difficulties and objections. Now let's turn to the special facts and arguments in favor of the theory.
On the view that species are only strongly marked and permanent varieties, and that each species first existed as a variety, we can see why no line of demarcation can be drawn between species -- commonly supposed to have been produced by special acts of creation -- and varieties, which are acknowledged to have been produced by natural laws. On this same view we can understand why, in a region where many species of a genus have been produced and where they now flourish, these same species should show many varieties. Where the factory of species has been active, we'd expect, as a general rule, to find it still in action -- and this is the case if varieties are emerging species. Moreover, the species of larger genera, which provide the greater number of varieties or emerging species, retain to some degree the character of varieties, for they differ from each other by a smaller amount than do the species of smaller genera. The closely related species of larger genera apparently have restricted ranges, and in their relationships they cluster in little groups around other species -- in both respects resembling varieties. These are strange patterns if each species was independently created, but they make sense if each first existed as a variety.
Since each species tends by its geometric rate of reproduction to increase excessively in number, and since the modified descendants of each species will be able to increase as they become more diversified in habits and structure -- so as to seize on many and widely different places in the natural world -- there will be a constant tendency in natural selection to preserve the most divergent offspring of any one species. So during a long-continued course of modification, the slight differences characteristic of varieties of the same species tend to be amplified into the greater differences characteristic of species of the same genus. New and improved varieties will inevitably replace and drive to extinction the older, less improved, and intermediate varieties. Thus species are rendered largely defined and distinct. Dominant species belonging to the larger groups within each class tend to give rise to new and dominant forms, so that each large group tends to become still larger and at the same time more divergent in character. But since all groups can't keep increasing in size -- the world wouldn't hold them -- the more dominant groups beat the less dominant. This tendency of large groups to keep growing in size and diverging in character, together with the inevitable outcome of much extinction, explains the arrangement of all forms of life in groups subordinate to groups, all within a few great classes, which has prevailed throughout all time. This grand fact -- the grouping of all organisms under what is called the Natural System -- is utterly inexplicable on the theory of creation.
Since natural selection acts solely by accumulating slight, successive, favorable variations, it can produce no great or sudden changes. It can act only by short and slow steps. Hence the principle of "Natura non facit saltum" -- nature does not make leaps -- which every new addition to our knowledge tends to confirm, makes sense under this theory. We can see why throughout nature the same general end is achieved by an almost infinite diversity of means, because every trait, once acquired, is long inherited, and structures already modified in many different ways have to be adapted for the same general purpose. We can, in short, see why nature is lavish in variety yet stingy with innovation. But why this should be a law of nature if each species was independently created, no one can explain.
Many other facts are, it seems to me, explained by this theory. How strange it is that a bird shaped like a woodpecker should prey on insects on the ground; that upland geese, which rarely or never swim, should have webbed feet; that a thrush-like bird should dive and feed on underwater insects; and that a petrel should have the habits and structure fitting it for the life of an auk -- and so on in endless other cases. But on the view that each species is constantly trying to increase in number, with natural selection always ready to adapt the slowly varying descendants of each to any unoccupied or poorly occupied place in nature, these facts stop being strange and might even have been expected.
We can to some extent understand why there is so much beauty throughout nature, for this may be largely attributed to the action of selection. That beauty, as we perceive it, is not universal must be admitted by anyone who will look at some venomous snakes, some fish, and certain hideous bats with a distorted resemblance to the human face. Sexual selection has given the most brilliant colors, elegant patterns, and other ornaments to the males, and sometimes to both sexes of many birds, butterflies, and other animals. In birds it has often made the male's voice musical to the female, as well as to our ears. Flowers and fruit have been made conspicuous by brilliant colors contrasting with green foliage, so that flowers may be easily seen, visited, and fertilized by insects, and seeds spread by birds. How it comes that certain colors, sounds, and forms give pleasure to humans and other animals -- that is, how the sense of beauty in its simplest form was first acquired -- we don't know, any more than we know how certain smells and flavors first became agreeable.
Since natural selection acts through competition, it adapts and improves the inhabitants of each country only in relation to their fellow inhabitants. So we need feel no surprise that the species of any one country, although supposedly created and specially adapted for that country, can be beaten and replaced by introduced species from another land. Nor should we be astonished if all the designs in nature are not, as far as we can judge, absolutely perfect -- as in the case even of the human eye. Or if some of them are disturbing to our ideas of fitness. We need not be astonished at the sting of the bee, when used against an enemy, causing the bee's own death; at drones being produced in such great numbers for one single act and then slaughtered by their sterile sisters; at the astonishing waste of pollen by fir trees; at the instinctive hatred of the queen bee for her own fertile daughters; at parasitic wasps feeding within the living bodies of caterpillars; and at other such cases. The wonder, indeed, on the theory of natural selection, is that more cases of the lack of absolute perfection have not been found.
The complex and little-known laws governing the production of varieties are the same, as far as we can judge, as the laws that have governed the production of distinct species. In both cases, physical conditions seem to have produced some direct and definite effect, but how much we can't say. Thus, when varieties enter any new habitat, they occasionally take on some of the characters typical of the species in that habitat. With both varieties and species, use and disuse seem to have produced considerable effect; it's impossible to resist this conclusion when we look, for instance, at the loggerhead duck, whose wings are incapable of flight in nearly the same condition as in the domestic duck; or at the burrowing tuco-tuco, which is occasionally blind, and then at certain moles, which are habitually blind and have their eyes covered with skin; or at the blind animals inhabiting the dark caves of America and Europe. With both varieties and species, correlated variation seems to have played an important part, so that when one part has been modified, other parts have necessarily been modified too. With both varieties and species, reversions to long-lost characters occasionally occur. How inexplicable on the theory of creation is the occasional appearance of stripes on the shoulders and legs of the various species of the horse family and their hybrids! How simply this fact is explained if we believe that these species are all descended from a striped ancestor, in the same way that the various domestic breeds of pigeon are descended from the blue and barred rock pigeon!
On the conventional view that each species was independently created, why should specific characters -- those by which species of the same genus differ from each other -- be more variable than the generic characters on which they all agree? Why, for instance, should the color of a flower be more likely to vary in any one species of a genus, if the other species possess differently colored flowers, than if all possess the same colored flowers? If species are only well-marked varieties whose characters have become highly stable, we can understand this. They have already varied since branching off from a common ancestor in certain characters, by which they've come to be specifically distinct from each other, and therefore these same characters would be more likely to vary again than the generic characters that have been inherited unchanged for an immense period. It's inexplicable on the theory of creation why a part developed in a very unusual way in one species alone of a genus -- and therefore, as we may naturally infer, of great importance to that species -- should be especially liable to variation. But on our view, this part has undergone, since the several species branched off from a common ancestor, an unusual amount of variability and modification, and therefore we'd expect the part to still be variable. But a part may be developed in the most unusual manner, like the wing of a bat, and yet not be more variable than any other structure, if the part is common to many subordinate forms -- that is, if it has been inherited for a very long period -- because in that case it will have been made stable by long-continued natural selection.
Looking at instincts, marvelous as some are, they offer no greater difficulty than do bodily structures under the theory of natural selection of successive, slight, but beneficial modifications. We can thus understand why nature moves by graduated steps in giving different animals of the same class their various instincts. I've tried to show how much light the principle of gradation throws on the admirable architectural powers of the honeybee. Habit no doubt often plays a role in modifying instincts, but it certainly isn't essential, as we see in the case of sterile worker insects, which leave no offspring to inherit the effects of long-continued habit. On the view that all species of the same genus have descended from a common parent and inherited much in common, we can understand why related species, when placed under widely different conditions of life, still follow nearly the same instincts -- why the thrushes of tropical and temperate South America, for instance, line their nests with mud like our British species. On the view that instincts have been slowly acquired through natural selection, we need not be surprised that some instincts are imperfect and liable to mistakes, and that many instincts cause other animals to suffer.
If species are only well-marked and permanent varieties, we can immediately see why their crossed offspring should follow the same complex laws in their degrees and kinds of resemblance to their parents -- in being absorbed into each other by successive crosses, and in other such points -- as do the crossed offspring of acknowledged varieties. This similarity would be a strange fact if species had been independently created and varieties had been produced through natural laws.
If we accept that the geological record is imperfect to an extreme degree, then the facts the record does provide strongly support the theory of descent with modification. New species have appeared slowly and at successive intervals, and the amount of change after equal intervals of time is widely different in different groups. The extinction of species and of whole groups of species, which has played so conspicuous a part in the history of life, almost inevitably follows from the principle of natural selection, because old forms are replaced by new and improved forms. Neither single species nor groups of species reappear once the chain of ordinary reproduction is broken. The gradual spread of dominant forms, with the slow modification of their descendants, causes life forms, after long intervals of time, to appear as if they had changed simultaneously throughout the world. The fact that fossil remains from each formation are in some degree intermediate in character between fossils from the formations above and below is simply explained by their intermediate position in the chain of descent. The grand fact that all extinct organisms can be classified with all living organisms naturally follows from both the living and the extinct being the offspring of common parents. Since species have generally diverged in character during their long course of descent and modification, we can understand why the more ancient forms, or early ancestors of each group, so often occupy a position somewhat intermediate between existing groups. Recent forms are generally considered to be, on the whole, higher in the scale of organization than ancient forms -- and they must be higher insofar as the later and more improved forms have conquered the older and less improved forms in the struggle for life. They have also generally had their organs more specialized for different functions. This fact is perfectly compatible with many organisms still retaining simple and barely improved structures, suited for simple conditions of life. It's likewise compatible with some forms having declined in organization, by having become at each stage of descent better fitted for new and less demanding ways of life. Lastly, the wonderful pattern of the long persistence of related forms on the same continent -- of marsupials in Australia, of edentates in America, and other such cases -- makes sense, because within the same country the existing and the extinct will be closely related by descent.
Looking at geographical distribution, if we accept that during the long course of ages much migration has occurred from one part of the world to another, owing to former climatic and geographical changes and to the many occasional and unknown means of dispersal, then we can understand, on the theory of descent with modification, most of the great leading facts about distribution. We can see why there should be so striking a parallel between the distribution of organisms throughout space and their geological succession through time -- because in both cases the organisms have been connected by the bond of ordinary reproduction, and the means of modification have been the same. We see the full meaning of the wonderful fact, which has struck every traveler, that on the same continent, under the most diverse conditions -- heat and cold, mountain and lowland, desert and marsh -- most of the inhabitants within each great class are plainly related, because they are the descendants of the same ancestors and early colonists. On this same principle of former migration, combined in most cases with modification, we can understand, with the help of the Glacial period, why some identical plants and many closely related ones occur on the most distant mountains and in the northern and southern temperate zones. Likewise, the close relationship of some sea creatures in the northern and southern temperate latitudes, though separated by the whole tropical ocean. Although two countries may present physical conditions as closely similar as the same species ever require, we need feel no surprise at their inhabitants being widely different, if they have been completely separated from each other for a long period. Since the relationship of organism to organism is the most important of all relationships, and since the two countries will have received colonists at various times and in different proportions -- from some other country or from each other -- the course of modification in the two areas will inevitably have been different.
On this view of migration with subsequent modification, we see why oceanic islands are inhabited by only a few species, but why many of these are peculiar or endemic forms. We clearly see why species belonging to those groups of animals that can't cross wide stretches of ocean, such as frogs and terrestrial mammals, don't inhabit oceanic islands -- and why, on the other hand, new and peculiar species of bats, animals that can cross the ocean, are often found on islands far distant from any continent. Such cases as the presence of peculiar bat species on oceanic islands and the absence of all other terrestrial mammals are utterly inexplicable on the theory of independent acts of creation.
The existence of closely related representative species in any two areas implies, on the theory of descent with modification, that the same parent forms formerly inhabited both areas -- and we almost invariably find that wherever many closely related species inhabit two areas, some identical species are still common to both. Wherever many closely related yet distinct species occur, doubtful forms and varieties belonging to the same groups likewise occur. It's a rule of high generality that the inhabitants of each area are related to the inhabitants of the nearest source from which immigrants might have been derived. We see this in the striking relationship of nearly all the plants and animals of the Galapagos Archipelago, of Juan Fernandez, and of the other American islands, to those of the neighboring American mainland -- and of those of the Cape Verde Archipelago and the other African islands to the African mainland. These facts receive no explanation on the theory of creation.
The fact, as we've seen, that all past and present organisms can be arranged within a few great classes, in groups subordinate to groups, with extinct groups often falling between recent groups, is understandable on the theory of natural selection with its consequences of extinction and divergence of character. On these same principles we see why the mutual relationships of forms within each class are so complex and circuitous. We see why certain characters are far more useful than others for classification; why adaptive characters, though of the highest importance to the organisms, are of hardly any importance in classification; why characters derived from rudimentary parts, though of no use to the organisms, are often of high classificatory value; and why embryological characters are often the most valuable of all. The true relationships of all organisms, as distinct from their adaptive resemblances, are due to inheritance -- to community of descent. The Natural System is a genealogical arrangement, with the acquired degrees of difference marked by the terms varieties, species, genera, families, and so on. And we have to discover the lines of descent by the most stable characters, whatever they may be and however slight their vital importance.
The similar framework of bones in the hand of a human, wing of a bat, fin of a porpoise, and leg of a horse -- the same number of vertebrae forming the neck of the giraffe and of the elephant -- and countless other such facts, explain themselves at once on the theory of descent with slow and slight successive modifications. The similarity of pattern in the wing and leg of a bat, though used for such different purposes -- in the jaws and legs of a crab -- in the petals, stamens, and pistils of a flower -- is likewise, to a large extent, understandable on the view of the gradual modification of parts or organs that were originally alike in an early ancestor in each of these classes. On the principle that successive variations don't always appear at an early age and are inherited at a corresponding period of life, we clearly see why the embryos of mammals, birds, reptiles, and fish should be so closely similar, and so unlike the adult forms. We may stop marveling at the embryo of an air-breathing mammal or bird having gill slits and arteries running in loops, like those of a fish that has to breathe the air dissolved in water with the help of well-developed gills.
Disuse, sometimes aided by natural selection, will often have reduced organs when rendered useless under changed habits or conditions of life, and we can understand on this view the meaning of rudimentary organs. But disuse and selection will generally act on each creature when it has come to maturity and has to play its full part in the struggle for existence, and will thus have little effect on an organ during early life. Hence the organ will not be reduced or rendered rudimentary at this early age. The calf, for instance, has inherited teeth that never cut through the gums of the upper jaw, from an early ancestor that had well-developed teeth. We may believe that the teeth in the adult animal were formerly reduced by disuse, because the tongue and palate, or lips, had become excellently fitted through natural selection to browse without their help -- whereas in the calf, the teeth have been left unaffected and, on the principle of inheritance at corresponding ages, have been inherited from a remote period to the present day. On the view that each organism with all its separate parts was specially created, how utterly inexplicable it is that organs bearing the plain stamp of uselessness -- such as the teeth in the embryonic calf or the shriveled wings under the fused wing covers of many beetles -- should so frequently occur. Nature may be said to have taken pains to reveal her scheme of modification by means of rudimentary organs, embryological and homologous structures -- but we are too blind to understand her meaning.
I have now summarized the facts and considerations that have thoroughly convinced me that species have been modified during a long course of descent. This has been achieved chiefly through the natural selection of numerous successive, slight, favorable variations, aided in an important way by the inherited effects of the use and disuse of parts, and in a less important way -- that is, in relation to adaptive structures, whether past or present -- by the direct action of external conditions, and by variations that in our ignorance seem to arise spontaneously. It appears that I formerly underestimated the frequency and value of these latter forms of variation, as leading to permanent modifications of structure independent of natural selection. But since my conclusions have recently been much misrepresented, and it has been stated that I attribute the modification of species exclusively to natural selection, I may be permitted to point out that in the first edition of this work, and in later editions, I placed in a most prominent position -- namely, at the close of the Introduction -- the following words: "I am convinced that natural selection has been the main but not the exclusive means of modification." This has been to no avail. Great is the power of persistent misrepresentation; but the history of science shows that fortunately this power does not last long.
It can hardly be supposed that a false theory would explain, in so satisfactory a manner as the theory of natural selection does, the several large classes of facts specified above. It has recently been objected that this is an unsafe method of arguing, but it's a method used in judging the ordinary events of life and has often been used by the greatest natural philosophers. The wave theory of light was arrived at in this way, and the belief in the revolution of the earth on its own axis was until recently supported by hardly any direct evidence. It's no valid objection that science so far sheds no light on the far higher problem of the essence or origin of life. Who can explain what the essence of the attraction of gravity is? No one now objects to following out the results of this unknown force of attraction, notwithstanding that Leibniz formerly accused Newton of introducing "occult qualities and miracles into philosophy."
I see no good reason why the views presented in this book should shock anyone's religious feelings. It's reassuring, as a reminder of how fleeting such reactions are, to recall that the greatest discovery ever made by humanity -- the law of the attraction of gravity -- was also attacked by Leibniz "as subversive of natural, and inferentially of revealed, religion." A celebrated author and clergyman has written to me that "he has gradually learned to see that it is just as noble a conception of the Deity to believe that He created a few original forms capable of self-development into other and needful forms, as to believe that He required a fresh act of creation to supply the voids caused by the action of His laws."
Why, it may be asked, did nearly all the most eminent living naturalists and geologists until recently disbelieve in the mutability of species? It can't be asserted that organisms in a state of nature are subject to no variation. It can't be proved that the amount of variation over long ages is limited. No clear distinction has been, or can be, drawn between species and well-marked varieties. It can't be maintained that species when crossed are invariably sterile and varieties invariably fertile, or that sterility is a special gift and sign of creation. The belief that species were immutable was almost unavoidable as long as the history of the world was thought to be short. And now that we've gained some idea of the immensity of time, we're too inclined to assume, without proof, that the geological record is so complete that it would have given us clear evidence of the transformation of species, if they had undergone transformation.
But the chief cause of our natural reluctance to admit that one species has given rise to other and distinct species is that we are always slow to accept any great change whose steps we can't see. The difficulty is the same as that felt by so many geologists when Lyell first argued that long lines of inland cliffs had been formed, and great valleys excavated, by the very forces we still see at work. The mind cannot possibly grasp the full meaning of even a million years; it cannot add up and perceive the full effects of many slight variations accumulated during an almost infinite number of generations.
Although I am fully convinced of the truth of the views given in this book in the form of an abstract, I by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine. It's so easy to hide our ignorance under expressions like "the plan of creation," "unity of design," and so on, and to think that we're giving an explanation when we're only restating a fact. Anyone whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject the theory. A few naturalists, blessed with much flexibility of mind and who have already begun to doubt the immutability of species, may be influenced by this book. But I look with confidence to the future -- to young and rising naturalists, who will be able to view both sides of the question with impartiality. Whoever is led to believe that species are mutable will do good service by conscientiously expressing his conviction, for only in this way can the load of prejudice by which this subject is overwhelmed be removed.
Several eminent naturalists have recently published their belief that a multitude of supposed species in each genus are not real species, but that other species are real -- that is, were independently created. This seems to me a strange conclusion. They admit that a multitude of forms, which until recently they themselves considered special creations, and which are still viewed that way by the majority of naturalists, and which consequently have all the external characteristics of true species -- they admit that these have been produced by variation, but they refuse to extend the same view to other and slightly different forms. Nevertheless, they don't claim that they can define, or even guess, which are the created forms of life and which are those produced by natural laws. They accept variation as a genuine cause in one case; they arbitrarily reject it in another, without pointing to any distinction between the two cases. The day will come when this will be cited as a curious illustration of the blindness of preconceived opinion. These authors seem no more startled by a miraculous act of creation than by an ordinary birth. But do they really believe that at innumerable periods in the earth's history certain atoms were commanded to suddenly flash into living tissue? Do they believe that at each supposed act of creation one individual or many were produced? Were all the infinitely numerous kinds of animals and plants created as eggs or seeds, or fully grown? And in the case of mammals, were they created bearing the false marks of nourishment from the mother's womb? Undoubtedly some of these same questions can't be answered by those who believe in the appearance or creation of only a few forms of life, or of some one form alone. It has been argued by several authors that it's as easy to believe in the creation of a million beings as of one. But Maupertuis's philosophical axiom of "least action" leads the mind more readily to accept the smaller number -- and certainly we ought not to believe that innumerable organisms within each great class have been created with plain but deceptive marks of descent from a single parent.
As a record of a former state of things, I've retained in the preceding paragraphs, and elsewhere, several sentences implying that naturalists believe in the separate creation of each species -- and I've been much criticized for expressing myself this way. But undoubtedly this was the general belief when the first edition of this work appeared. I used to speak to very many naturalists on the subject of evolution and never once met with any sympathetic agreement. It's probable that some did then believe in evolution, but they were either silent or expressed themselves so ambiguously that it was hard to understand their meaning. Now things are wholly changed, and almost every naturalist accepts the great principle of evolution. There are, however, some who still think that species have suddenly given birth, through quite unexplained means, to new and totally different forms. But, as I've tried to show, strong evidence can be brought against the idea of great and abrupt changes. From a scientific standpoint, and as a guide to further investigation, little advantage is gained by believing that new forms suddenly developed in an inexplicable manner from old and widely different forms, compared with the old belief in the creation of species from the dust of the earth.
It may be asked how far I extend the doctrine of the modification of species. The question is difficult to answer, because the more distinct the forms we consider, the fewer and weaker become the arguments for community of descent. But some of the strongest arguments extend very far. All the members of whole classes are connected together by a chain of relationships, and all can be classified on the same principle, in groups subordinate to groups. Fossil remains sometimes tend to fill very wide gaps between existing orders.
Organs in a rudimentary condition plainly show that an early ancestor had the organ in a fully developed state, and this in some cases implies an enormous amount of modification in the descendants. Throughout whole classes, various structures are formed on the same pattern, and at a very early age the embryos closely resemble each other. Therefore I cannot doubt that the theory of descent with modification embraces all the members of the same great class or kingdom. I believe that animals are descended from at most only four or five ancestors, and plants from an equal or lesser number.
Analogy would lead me one step further -- to the belief that all animals and plants are descended from some one prototype. But analogy may be a deceptive guide. Nevertheless, all living things have much in common: their chemical composition, their cellular structure, their laws of growth, and their susceptibility to harmful influences. We see this even in so small a fact as that the same poison often similarly affects plants and animals, or that the poison secreted by the gall fly produces monstrous growths on the wild rose or oak tree. In all organisms, except perhaps some of the very lowest, sexual reproduction seems to be essentially similar. In all, as far as is presently known, the germinal vesicle is the same -- so that all organisms start from a common origin. If we look even to the two main divisions, the animal and vegetable kingdoms, certain low forms are so intermediate in character that naturalists have disputed which kingdom they should be assigned to. As Professor Asa Gray has remarked, "the spores and other reproductive bodies of many of the lower algae may claim to have first a characteristically animal, and then an unequivocally vegetable existence." Therefore, on the principle of natural selection with divergence of character, it does not seem incredible that, from some such low and intermediate form, both animals and plants may have developed. And if we admit this, we must likewise admit that all the organisms that have ever lived on this earth may be descended from some one primordial form. But this inference is chiefly grounded on analogy, and it is immaterial whether or not it be accepted. No doubt it is possible, as Mr. G. H. Lewes has argued, that at the first beginning of life many different forms evolved. But if so, we may conclude that only very few have left modified descendants. For, as I have recently remarked regarding the members of each great kingdom, such as the Vertebrata, Articulata, and so on, we have distinct evidence in their embryological, homologous, and rudimentary structures that within each kingdom all the members are descended from a single ancestor.
When the views advanced by me in this book, and by Mr. Wallace, or when analogous views on the origin of species are generally accepted, we can dimly foresee that there will be a considerable revolution in natural history. Systematists will be able to pursue their work as at present, but they will not be constantly haunted by the shadowy doubt whether this or that form is a true species. This, I feel sure -- and I speak from experience -- will be no small relief. The endless disputes about whether some fifty species of British brambles are good species will cease. Systematists will have only to decide (not that this will be easy) whether any form is sufficiently stable and distinct from other forms to be capable of definition, and if definable, whether the differences are important enough to deserve a species name. This latter point will become a far more essential consideration than it is at present, because differences, however slight, between any two forms, if not bridged by intermediate gradations, are currently considered by most naturalists sufficient to raise both forms to the rank of species.
In the future we shall be compelled to acknowledge that the only distinction between species and well-marked varieties is that the latter are known, or believed, to be connected at the present day by intermediate gradations, whereas species were formerly thus connected. So, without dismissing the importance of intermediate gradations between any two forms, we shall be led to weigh more carefully and value more highly the actual amount of difference between them. It's quite possible that forms now generally acknowledged to be merely varieties may later be thought worthy of species names, and in this case scientific and common language will come into agreement. In short, we shall have to treat species in the same way that those naturalists treat genera who admit that genera are merely artificial groupings made for convenience. This may not be a cheerful prospect, but we shall at least be freed from the futile search for the undiscovered and undiscoverable essence of the term species.
The other and more general branches of natural history will rise greatly in interest. The terms used by naturalists -- affinity, relationship, community of type, paternity, morphology, adaptive characters, rudimentary and aborted organs, and so on -- will cease to be metaphorical and will have a plain meaning. When we no longer look at an organism as a savage looks at a ship, as something wholly beyond his comprehension; when we regard every product of nature as one that has had a long history; when we contemplate every complex structure and instinct as the summing up of many devices, each useful to the possessor, in the same way that any great mechanical invention is the summing up of the labor, the experience, the reason, and even the blunders of numerous workers -- when we view each organism this way, how far more interesting, I speak from experience, does the study of natural history become!
A grand and almost unexplored field of inquiry will be opened, on the causes and laws of variation, on correlation, on the effects of use and disuse, on the direct action of external conditions, and so forth. The study of domestic animals and plants will rise immensely in value. A new variety raised by humans will be a far more important and interesting subject for study than one more species added to the already vast catalog of recorded species. Our classifications will come to be, as far as they can be made, genealogies -- and will then truly give what may be called the plan of creation. The rules for classifying will no doubt become simpler when we have a definite goal in view. We possess no pedigrees or coats of arms; and we have to discover and trace the many diverging lines of descent in our natural genealogies by characters of any kind that have long been inherited. Rudimentary organs will speak unmistakably about the nature of long-lost structures. Species and groups of species that are called aberrant, and which may fancifully be called living fossils, will help us form a picture of the ancient forms of life. Embryology will often reveal to us the structure, somewhat obscured, of the prototypes of each great class.
When we can feel assured that all individuals of the same species, and all the closely related species of most genera, have within a not very remote period descended from one parent and have migrated from some one birthplace; and when we better understand the many means of migration; then, by the light that geology now throws, and will continue to throw, on former changes of climate and of the level of the land, we shall surely be able to trace in a wonderful way the former migrations of the inhabitants of the whole world. Even at present, by comparing the differences between the inhabitants of the sea on the opposite sides of a continent, and the nature of the various inhabitants of that continent in relation to their apparent means of immigration, some light can be shed on ancient geography.
The noble science of geology loses glory from the extreme imperfection of the record. The crust of the earth, with its embedded remains, must not be looked at as a well-filled museum, but as a poor collection made at random and at rare intervals. The accumulation of each great fossiliferous formation will be recognized as having depended on an unusual combination of favorable circumstances, and the blank intervals between the successive stages as having been of vast duration. But we shall be able to gauge with some confidence the duration of these intervals by comparing the preceding and succeeding organic forms. We must be cautious in trying to match up two formations as strictly contemporaneous if they don't share many identical species, relying instead on the general succession of life forms. Since species are produced and driven to extinction by slowly acting and still existing causes, and not by miraculous acts of creation -- and since the most important of all causes of organic change is one that is almost independent of altered, and perhaps suddenly altered, physical conditions, namely the mutual relationship of organism to organism, the improvement of one organism requiring the improvement or the extinction of others -- it follows that the amount of organic change in the fossils of consecutive formations probably serves as a fair measure of the relative, though not actual, passage of time. A number of species, however, staying together as a group might remain unchanged for a long period, while within the same period several of these species, by migrating into new countries and coming into competition with foreign inhabitants, might become modified. So we must not overestimate the accuracy of organic change as a measure of time.
In the future I see open fields for far more important researches. Psychology will be securely based on the foundation already well laid by Mr. Herbert Spencer, that of the necessary acquisition of each mental power and capacity by gradation. Much light will be thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with the view that each species has been independently created. To my mind it accords better with what we know of the laws impressed on matter by the Creator, that the production and extinction of the past and present inhabitants of the world should have been due to secondary causes, like those determining the birth and death of the individual. When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Cambrian system was deposited, they seem to me to become ennobled. Judging from the past, we may safely infer that not one living species will transmit its unaltered likeness to a distant future. And of the species now living, very few will transmit offspring of any kind to a far distant future; for the manner in which all organisms are grouped shows that the greater number of species in each genus, and all the species in many genera, have left no descendants but have become utterly extinct. We can so far take a prophetic glance into the future as to foretell that it will be the common and widely spread species, belonging to the larger and dominant groups within each class, which will ultimately prevail and give rise to new and dominant species. As all the living forms of life are the lineal descendants of those which lived long before the Cambrian epoch, we may feel certain that the ordinary succession by generation has never once been broken, and that no cataclysm has desolated the whole world. Hence we may look with some confidence to a secure future of great length. And as natural selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection.
It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.