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.
O. LATA O. LAMARCKIANA O. NANELLA.
Fig. 5.
Mutation in Oenothera lamarckiana. The parent species (in the middle) with two of the 'sports' from it.
From De Vries, The Evolution Theory. By permission of The Open Court Publishing Co.
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.
Fig. 6.—Horse's Foot, with well-developed Side Digit.