the molar teeth is, too, a little different from that of Equus. Protohippus of the North American Pliocene is also three-toed, but the two additionally-developed toes are smaller than in Hipparion. Other forms are dealt with below in connexion with the ancestry of Perissodactyles. It is a curious fact about Hipparion, which is not now regarded as on the direct line of equine descent, that the edges of the enamel plates of the molars may show a complicated folding very like that presented by that clearly terminal form of Perissodactyle life, the gigantic Elasmotherium. This is indicative of high specialisation, which ended in extinction.
Ancestry of the Horses.—The Lophiodontidae and the Palaeotheriidae are two of the most interesting extinct families of Perissodactyles; for among them we find what would appear to be the ancestral forms of both the existing Tapirs and Horses. The Rhinoceroses also would seem to be derivable from the Palaeotheriidae. The very vagueness of the characters of these creatures, considered from a classificatory point of view, has led to much diversity in their placing. This though gratifying to the evolutionist is tiresome to the writer who wishes to give a methodical account of their various characters. It will be best perhaps not to attempt an accurate placing or to reconcile conflicting opinion, but to give some salient features of osteology which lead to the belief in their relationship to existing groups of Perissodactyles. A book upon the history of mammals would be incomplete without some account of that well-ascertained series of forms which seem to connect these primitive Perissodactyles with the modern Horse. Equus, in fact, is not only the "show horse" of the doctrine of evolution, but also the "stalking horse."
In the Eocene of both Europe and America are met with a number of forms from which we may start. Hyracotherium, regarded on the one hand as the type of a sub-family of the Equidae themselves, and on the other as a member of the family Lophiodontidae, was a small-sized animal, three feet or so in length; it possesses the complete Eutherian dentition with a slight diastema. The orbits are not separated from the temporal fossa; the fore-limbs were four-toed, the hind three-toed, with moderately long metapodia, especially on the hind-feet. The shoulder blade
