Remains of horses indistinguishable from _E. caballus_ occur in the Pleistocene deposits of Europe and Asia; and it is from them that the dun-coloured small horses of northern Europe and Asia are probably derived. The ancestor of these Pleistocene horses is probably _E. stenonis_, of the Upper Pliocene of Europe, which has a small depression in front of the orbit, while the skull is relatively larger, the feet are rather shorter, and the splint-bones somewhat more developed. In India a nearly allied species (_E. sivalensis_), occurs in the Lower Pliocene, and may have been the ancestor of the Arab stock, which shows traces of the depression in front of the orbit characteristic of the earlier forms. In North America species of _Equus_ occur in the Pleistocene and from that continent others reached South America during the same epoch. In the latter country occurs _Hippidium_, in which the cheek-teeth are shorter and simpler, and the nasal bones very long and slender, with elongated slits at the side. The limbs, especially the cannon-bones, are relatively short, and the splint-bones large. The allied Argentine _Onohippidium_, which is also Pleistocene, has still longer nasal bones and slits, and a deep double cavity in front of the orbit, part of which probably contained a gland. _Onohippidium_ is certainly off the direct line of descent of the modern horses, and, on account of the length of the nasals and their slits, the same probably holds good for _Hippidium_. [Illustration: FIG. 2.--a, Grinding surface of unworn right upper molar tooth of _Anchitherium_; b, corresponding surface of unworn molar of young horse; c, the same tooth after it has been some time in use. The uncoloured portions are the dentine or ivory, the shaded parts the cement filling the cavities and surrounding the exterior. The black line separating these two structures is the enamel or hardest constituent of the tooth.] Species from the Pliocene of Texas and the Upper Miocene (Loup Fork) of Oregon were at one time assigned to _Hippidium_, but this is incorrect, that genus being exclusively South American. The name _Pliohippus_ has been applied to species from the same two formations on the supposition that the foot-structure was similar to that of _Hippidium_, but Mr J.W. Gidley is of opinion that the lateral digits may have been fully developed. Apparently there is here some gap in the line of descent of the horse, and it may be suggested that the evolution took place, not as commonly supposed, in North America, but in eastern central Asia, of which the palaeontology is practically unknown; some support is given to this theory by the fact that the earliest species with which we are acquainted occur in northern India. [Illustration: FIG. 3.--Successive stages of modification of the left fore-feet of extinct forms of horse-like animals, showing gradual reduction of the outer and enlargement of the middle toe (III).