Evolution

CHAPTER I. The Evidence for Evolution

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.

+--------------+------------------+---------------+------------------+ | | First Appearance |Dominant Types.| | | | of Types. | | | +--------------+------------------+---------------+------------------+ | Modern | ... | MAN | } Post-tertiary, | | Diluvium | Man | ... | } 1/2 per cent. | +--------------+------------------+---------------+------------------+ | Pliocene | ... | } | } Tertiary or | | Miocene | Monkeys | } MAMMALS | } Cænozoic, | | Oligocene | ... | } | } 2-1/2 per cent.| | Eocene | Lemurs | } | } | +--------------+------------------+---------------+------------------+ | Cretaceous | Higher mammals | } | } Secondary or | | Jurassic | { Birds | } REPTILES | } Mesozoic, | | | { Marsupials | } | } 11 per cent. | | Triassic | Monotremes | } | } | +--------------+------------------+---------------+------------------+ | Permian | Reptiles | AMPHIBIANS | } Primary or | | Carboniferous| Amphibians | } FISHES | } Palæozoic, | | Devonian | Lung fishes | } | } 32 per cent. | | Silurian | Lower fishes | ... | } | +--------------+------------------+---------------+------------------+ | Cambrian | ... | ... | } Archäen, 54 | | Laurentian | ... | ... | } per cent. | +--------------+------------------+---------------+------------------+

FIG. 2.--Table showing the chronological succession of the stratified rocks, the subdivision of geological time, the approximate position of the earliest fossils of each of the main types of vertebrates, and the period of domination of each group.

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 for the whole truth from them, for their memory is imperfect; and yet they can tell us a great number of important facts.

FIG. 3. From The Guide to the American Museum of Natural History.]

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 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 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.

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