When we consider these facts, and the further circumstance that the Hipparions, the remains of which have been collected in immense numbers, were subject, as M. Gaudry and others have pointed out, to a great range of variation, it appears to me impossible to resist the conclusion that the types of the Anchitherium, of the Hipparion, and of the ancient Horses constitute the lineage of the modern Horses, the Hipparion being the intermediate stage between the other two, and answering to B in my former illustration.

The process by which the Anchitherium has been converted into Equus is one of specialisation, or of more and more complete deviation from what might be called the average form of an ungulate mammal.  In the Horses, the reduction of some parts of the limbs, together with the special modification of those which are left, is carried to a greater extent than in any other hoofed mammals.  The reduction is less and the specialisation is less in the Hipparion, and still less in the Anchitherium; but yet, as compared with other mammals, the reduction and specialisation of parts in the Anchitherium remain great.

Is it not probable then, that, just as in the Miocene epoch, we find an ancestral equine form less modified than Equus, so, if we go back to the Eocene epoch, we shall find some quadruped related to the Anchitherium, as Hipparion is related to Equus, and consequently departing less from the average form?

I think that this desideratum is very nearly, if not quite, supplied by Plagiolophus, remains of which occur abundantly in some parts of the Upper and Middle Eocene formations.  The patterns of the grinding-teeth of Plagiolophus are similar to those of Anchitherium, and their crowns are as thinly covered with cement; but the grinders diminish in size forwards, and the last lower molar has a large hind lobe, convex outwards and concave inwards, as in Palueotherium.  The ulna is complete and much larger than in any of the Equidoe, while it is more slender than in most of the true Paloeotheria; it is fixedly united, but not ankylosed, with the radius.  There are three toes in the fore limb, the outer ones being slender, but less attenuated than in the Equidoe.  The femur is more like that of the Paloeotheria than that of the horse, and has only a small depression above its outer condyle in the place of the great fossa which is so obvious in the Equidoe.  The fibula is distinct, but very slender, and its distal end is ankylosed with the tibia.  There are three toes on the hind foot having similar proportions to those on the fore foot.  The principal metacarpal and metatarsal bones are flatter than they are in any of the Equidoe; and the metacarpal bones are longer than the metatarsals, as in the Paloeotheria.

In its general form, Plagiolophus resembles a very small and slender horse,[4] and is totally unlike the reluctant, pig-like creature depicted in Cuvier’s restoration of his Paloeotherium minus in the “Ossemens Fossiles.”