THE EVIDENCE FOR EVOLUTION

The idea of Evolution is an old one. It is older than the Darwinian hypothesis; it is older than Lamarck, who published his particular theory in 1809, the year that Darwin was born; it is older than Buffon or Kant. In a fairly definite form it is as old as Aristotle. The Evolution idea has thus itself evolved, and is the product of many centuries of thought. Yet it was only the last generation that began to give the idea serious consideration, and it is perhaps only the present that has granted it any general measure of acceptance; and it was Darwin who wrought this change, who raised the conception of Evolution from the status of a vague speculative idea to that of a well-grounded theory, which appeals to the majority of educated minds as satisfactory and reasonable.

We do not here propose to sketch the development of the idea, either before or after Darwin; but only, in the first place, to state the grounds on which the belief in Evolution is based, and, in the second, to trace roughly the lines along which animal Evolution has proceeded. In the first few pages of this book, then, we shall endeavour to bring forward some of the evidence on which the modern Evolution theory rests.

As our first witness, we may call the rocks which constitute the outer portion of the earth, and ask them to tell us what they remember of the history of life upon the planet. We cannot hope [2]
[3]
[4]
[5]for the whole truth from them, for their memory is imperfect; and yet they can tell us a great number of important facts.

From the time when the world was sufficiently cooled for water to condense on its surface, a continual process of unbuilding and rebuilding of rocks has gone on. Wind and water, heat and cold have laid their hands to the work, making sand and dust and gravel out of solid stone, and these products of their labours have been carried off to other places, laid down, and cemented together into new rocks. We do not know the exact age of any particular rock that has been made in this way, nor how long the process has been going on. At a rough guess it may be three or four hundreds of millions of years. The chronological succession of the different rock formations is, however, known, and their relative ages may be judged with considerable accuracy. Here and there, as time went on, the body of a plant or an animal was deposited in the sand or mud or chalk, and has remained in the resulting rocks, in the form of a fossil, through all the ages. If, then, we study the occurrence of fossils in this succession of deposits, we ought to get some indications as to the inhabitants of the globe at various stages of its history. And if we do so, we meet unmistakable evidence that the lower and simpler types, both of animals and of plants, were in existence before the higher. Fig. 2 shows the facts with regard to the vertebrates, the great upper class of the animal kingdom. The first appearance of vertebrate fossils is in the Upper Silurian rocks, that is to say, somewhere after the middle of geological time. The fossils represent the lowest group of fishes. In the next great formation, the Devonian, fossils of two higher groups of fishes are to be found. The first land vertebrates, the amphibians, are doubtfully represented in the upper or newer layers of the same formation, and definitely so in the next, the Carboniferous. Towards the end of the Carboniferous or early in the Permian epoch, the first reptiles appear, and in the following period, or after about three-fourths of geological time had passed, the earliest fossils of mammals occur. The significance of this sequence will become plainer when [6]
[7]
[8]the differences and likenesses of these various groups are explained. Each of these great groups in turn formed the dominant animal population of the globe, and each in turn was superseded, although not entirely, by the next. The mammal group itself appears to be on the wane, overcome in the struggle for dominance by its own latest and most remarkable member, man himself.

THE VARIATION OF PIGEONS UNDER DOMESTICATION.

Centre—Rock Doves.

1. Carrier.
2. Pouter.
3. Almond Tumbler.
4. Trumpeter.
5. Barb.
6. Fantail.
7. Jacobin.
8. Capuchin.
9. Dragoon.
10. Modena.
11. Scandaroon.
12. Turbit.
13. English Owl.
14. Nun.
15. Mottle Tumbler.
16. Saddle Tumbler.
17. English Beard.
18. Baldhead.
19. Runt.
20. Magpie.
21. Show Homer.
22. Archangel.
23. Oriental Roller.
24. Norwich Cropper.
25. Cumulet.
26. Tippler.
27. African Owl.
28. Working Homer.
29. Mane.
30. Domino.
31. Oriental Turbit.
32. Blondinette.
33. Satinette.
34. Shortfaced Antwerp.
35. Priest.
36. Fairy.
37. Frillback.
38. Swallow.
39. Suabian.
40. Fire Spot.

The broad facts in the history of living things upon the earth are, then, in accordance with the theory of Evolution. The chain of types is indeed a broken one, the gaps being many, and some of them wide. But this is readily to be understood from the comparative scarcity of fossils, and the imperfection of the geological record.

In certain particular instances, however, very complete series of fossil forms have been discovered, connecting, by small gradations, modern animals with greatly different extinct types. One of the most complete of such series has been discovered for the horse. The changes that have occurred in the evolution of this animal have been mainly in three directions—increase in size, reduction in the number of toes from the original five to the final one, and deepening of the crowns of the teeth, so as to render them capable of longer wear. From the Eohippus of early tertiary times, an animal of about the size of a fox terrier, with five toes behind, and four with the vestige of the fifth in front, there is a complete connecting series reaching up to the modern horse, with its single remaining toe and the vestiges of two others. A few of the main links in this chain are illustrated in Fig. 3. It is impossible to regard such a series without having the idea of Evolution strongly suggested to the mind.

In the second place, there is evidence for Evolution in the fact that marked changes can and do occur in the characters of living races of organisms. There is ample evidence, for example, that all our modern breeds of pigeons are descended from the wild rock-dove. How markedly some of these differ from their wild ancestor, and among themselves, may be seen from Fig. 4. The size of some is twice as great as that of others. The bill in some is greatly increased in length, is almost ludicrously reduced [9]
[10]
[11]in others. Colour, feathering, build, even the instincts and the voice, vary enormously as between different varieties. In short, there is hardly any obvious character that has not, in one or other of the breeds, undergone great modification. As Darwin remarked, any naturalist coming upon such a group of forms in nature would have no hesitation in placing them in different species or genera, or even perhaps in different families. Even granting that the conditions of domestication are peculiar, we must admit that if such large changes can occur in a few centuries, it is possible that man has evolved from the lowest of living organisms during a period some hundreds of thousands of times as long.

But marked changes of type occur not only under conditions of domestication; nor is it necessary to infer the occurrence of any such changes without actual direct evidence. The formation of new types occurs in nature, and has taken place under the very eyes of scientific observers. Perhaps the most striking case that can be quoted is that of Lamarck's Evening Primrose, which, under the observation of Prof. De Vries in Amsterdam, produced some half-dozen of 'sports' which seem well entitled to rank as new species. Fig. 5 shows the parent plant and two of the new types that were produced by it. One is a dwarf in habit, the other is characterised by the greatly increased breadth of its foliage. Others showed different peculiarities. One might quote many other instances of violent changes of type—of the appearance of six-fingered children, whose peculiarity was afterwards inherited; of web-footed pigeons, and of new varieties of fruits, flowers, and vegetables. The causes of such 'sports' or mutations are unknown, but their moderately frequent occurrence is abundantly demonstrated. Such facts show, at all events, that the old conception of species as permanently fixed, unchanging types, can no longer reasonably be held.

Not all of the abnormalities which thus suddenly appear, we know not how or wherefore, are new. Many recall characters in lower or older groups, and may reasonably be interpreted as 'reversions.' Thus the horse's leg shown in Fig. 6 bears a well-developed side toe, in place of the small vestige that is normally present. Horses with this peculiarity have occurred with some frequency, probably before, and certainly since, the most famous of their kind, which Julius CÆsar rode. It seems reasonable to regard this peculiarity as a return to the old ancestral condition illustrated before, in which the side toes were well developed. The same applies to the instance of a persistent tail and persistent gill slits in man (Figs. 7 and 8), and to many other instances that might be quoted. One must indeed deal carefully with such cases, for it is always difficult to say what changes are new departures, and what are returns to ancestral types. There is danger of arguing in a circle—of supposing the ancestry from the abnormality, and of terming the latter a reversion because it suggests the supposed ancestry. Nevertheless, when variations occur, suggesting characters which are believed, on other grounds, to be ancestral, they must tend to strengthen the other evidence as to the evolution of the type in question.

Another and a very strong evidence of Evolution is to be found in what are termed vestigeal structures, two of which are illustrated in Figs. 9 and 10. They are, for the most part, obviously useless, and their occurrence has never been satisfactorily explained except by supposing them to be remnants of organs that were functional in the past history of their possessors' race. The appendix of man, for instance, is not only useless, but is frequently a source of danger. But its presence is readily explained by supposing that it represents the blind-gut, which is large and functional in many of the lower animals. Again, how should we account for the presence of small functionless wing-bones in the cassowary, unless by supposing that its ancestors were accustomed to fly like ordinary birds? How should we explain the bones which represent the hind limbs of the whale, unless by regarding the whale as descended from an animal which had functional hind limbs, or the representatives of eyes in animals that live in the dark, unless by supposing that these are descended from ancestors which saw? It has been well said that the bodies of many animals are veritable antiquarian museums, filled with relics of their own ancestors.

The next argument for Evolution to which we would refer is based on the similar structure and origin of organs or members that have entirely different uses. In Fig. 11 are figured the bones of the fore limbs of four different mammals, a whale, a bat, a dog, and man. The first is used for swimming, the second for flight, the third for locomotion on land, and the fourth as a grasping and holding organ. If these organs had been specially designed, each for its specific purpose, we should expect to find fundamental differences in structure. Actually the general arrangement of bones is the same in each case. A fact like this points strongly to a common origin of the four types mentioned, and to a general primitive arrangement of the bones of the limb. This primary type, it seems natural to suppose, has been modified for various special purposes in many different directions, the general features remaining recognisable. Many other cases of homology, or similarity of structure and origin, in organs whose function is dissimilar, might be quoted. Thus the poison gland of the poison snakes is not an organ which has been specially developed, but is a modified portion of one of the salivary glands. The hoof of the horse and the finger nail of man can evidently be satisfactorily explained as modifications of a[15]
[16]
[17] general type of terminal claw, and the scales of the scaly ant-eater and the quills of the porcupine are only modified hairs. The significance of facts like these, when carefully considered, is very great.

The study of the geographical distribution of animals has brought forth a great mass of facts which, considered by themselves, seem chaotic and meaningless, but which, in the light of Evolution, are full of significance. Observe, for example, the distribution of the Marsupials or pouch-bearing animals, shown on the accompanying map (Fig. 12). Australia is full of them, while they are relatively meagrely represented in a few other parts of the world. At the same time the greater and higher group of mammals was represented in Australia, at the time of its discovery, only by the bushman and his dog and a few species of mice. It is not as if the Australian environment were specially well adapted for marsupials, or specially ill-adapted for higher mammals; for the sheep has proved itself splendidly adapted for the conditions, and the rabbit most inconveniently so. Why, then, this curious state of affairs? It is an undoubted fact that the marsupials are both lower in their position in the animal kingdom, and older, than the main group to which all our European mammals belong. Now it is believed that Australia was once connected by land with the Asiatic Continent, and that it was finally separated from it before the higher mammals were in existence. The great step of further progress occurred elsewhere than in Australia, and the mammals of the latter continent were left in their obsolete condition, preserved through lack of competition of that higher type which elsewhere became dominant.

Madagascar offers a similar case. It abounds with forest vegetation and seems to offer a highly suitable environment for the monkey tribe. Yet there are no apes on the island. Their place is occupied by the Lemur tribe, which, there is every reason to believe, is the older group of the two, and that from which the apes have sprung. It is supposed, then, that Madagascar was separated from Africa before the ape had evolved. The lemurs thenceforward were free from the competition of their more highly developed relatives, and have branched out into a great variety of types, while still remaining on a relatively low plane of intelligence and specialisation. The distribution of the Lemurs is shown in Fig. 13.

In Dr. Alfred Russel Wallace's book on Island Life there are set forth a great number of interesting facts on the subject of the animal population of islands, and many striking interpretations of these facts in the light of the Evolution theory. Coral islands, and those caused by volcanic eruptions, are peopled with inhabitants which have accidentally come thither by flight, or have been brought, for example, on floating timber by ocean currents. On the other hand, islands which represent separated fragments of continents have usually a fauna of the same general type as that of the continent of which they have formed a part. But the actual species are frequently different, and if the separation is of more ancient date, the differences are still more marked. The fact of this divergence of an isolated animal population from that from which it has originated is sufficiently striking, and would remain an inexplicable problem, were we without an Evolution theory. According to the Evolution hypothesis, however, the restricted and somewhat special environment favours a modification of the original types with which the island was provided, and a satisfactory explanation is offered.

Finally, we may mention the evidence that has been gathered from the study of embryology and development. It has been stated, in a metaphor which is perhaps more clever than it is exact, that every animal climbs up its own ancestral tree; and while it would be absurd to say, for instance, that a mammalian embryo resembles successively a fish, an amphibian, and a reptile, still many of the broad facts in the evolution of a race seem to be repeated, in a more or less blurred and indistinct fashion, in the development of the individual. Thus, for example, gill-slits [20]
[21]
[22]and a tail are possessed in common by the embryos of all higher animals, only afterwards to disappear in those types in which the adult animal is without these structures. The heart of the mammal or bird is at first simple, then two chambered like that of a fish, then three chambered like an amphibian's, and finally four chambered. Some of the main phases in the development of the rabbit and of man are shown in Figs. 14 and 15 respectively.

The young flat-fish is like an ordinary member of the fish tribe, with an eye on either side of its head, and its body built on the ordinary symmetrical lines. It is only later, when it begins habitually to be upon one side on the sea bottom, that the eye from the under side wanders round to the opposite aspect beside its fellow, and the upper side becomes pigmented, while the lower remains white.

In similar fashion a primitive form of kidney is, as it were, sketched in, in the development of the higher animals, only to be erased at a later stage and replaced by a better form. The human child has a complete body covering of hair, which disappears soon after birth. In these and many more instances, one cannot avoid the impression that the organism has not been specially designed for what it finally comes to be. It cannot forget, and must needs repeat, or so it seems, some considerable part of the history of its race.

Manifestly, then, all this evidence, gleaned from many different sources, points to a common origin of living things, and to the gradual evolution of the higher from the lower types. It may also be said that there is no scientific evidence against such a view.

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