"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 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. Recently, however, Professor Ridgeway, who has devoted special attention to the problem, has satisfied himself that there is no sufficient foundation for these suppositions. He thus sums up the evidence which he has been able to collect:[334]