"Further results obtained from the study of fossils have a bearing even on the deepest problems of Biology, namely, those connected with the nature of life itself. For instance, it is allowable to infer, from the statements already made, that the main factor in the evolution of organisms is some inherent impulse—the 'bathmic force' of Cope—which acts with unerring certainty whatever be the conditions of the moment."[{286}]
E. Pedigree of the Horse.
Some recent evidence on this subject certainly does not clear away the difficulties set forth in the text.
From Nature, Sept. 8, 1904, p. 474.
"Professor Osborn (in a lecture before the British Association) mentioned that more than a hundred more or less complete skeletons of horses and horse-like animals had been found in North America. He thought he had established the fact that horses were polyphyletic, there being four or five contemporary series in the Miocene, but that the direct origin of the genus Equus in North America was not established with certainty."
Professor Sedgwick, Student's Text Book of Zoology, p. 599.
"Much has been written on the ancestry of the horse. It has been maintained by many authors that a continuous series of forms connecting it with the four-toed, brachyodont Hyracothoridæ of the Eocene has been discovered, and that here if anywhere a demonstrative historical proof has been obtained of the doctrine of organic evolution. Without desiring in the smallest degree to impugn that doctrine, it may be permitted us here to examine rather closely the view that the series of forms which recent palæontological research has undoubtedly brought to light constitute that historical proof which has been claimed for them."
[After an examination of the structural characters of these intermediate forms, viz., Pliohippus, Protohippus, Desmathippus, Miohippus, Mesohippus, Orohippus, and Hyracotherium, the author proceeds]:
"So far as the characters mentioned are concerned, we have here a very remarkable series of forms which at first sight seem to constitute a linear series with no cross-connections. Whether, however, they really do this is a difficult point to decide. There are flaws in the chain of evidence, which require careful and detailed consideration. For instance, the genus Equus appears in the Upper Siwalik beds, which have been ascribed to the Miocene age. It[{287}] has, however, been maintained that these beds are in reality Lower Pliocene, or even Upper Pliocene. It is clear that the decision of this question is of the utmost importance. If Equus really existed in the Upper Miocene, it was antecedent to some of its supposed ancestors. Again in the series of equine forms, Mesohippus, Miohippus, Desmathippus, Protohippus, which are generally regarded as coming into the direct line of equine descent, Scott[333] points out that each genus is, in some respect or other, less modified than its predecessor. In other words, it would appear that in this succession of North American forms the earlier genera show, in some points, closer resemblance to the modern Equus than to their immediate successors. It is possible that these difficulties and others of the same kind will be overcome with the growth of knowledge, but it is necessary to take note of them, for in the search after truth nothing is gained by ignoring such apparent discrepancies between theory and fact."
Besides the structure of limbs and teeth, another argument for the descent of the horse has been drawn from certain phenomena of colouration. Stripings are found not unfrequently to occur in the legs and withers, which Darwin took for a reversion to the character of a very remote ancestor, the common parent, in fact, of horses and asses, which he supposed to have been striped all over like a zebra. Like other such common ancestors, this hypothetical animal had never been seen, but was thought to be most nearly represented by the Kathiwar horse, with stripes on a dun ground, a specimen of which is exhibited as illustrating the hypothesis in the National Museum of Zoology.