good cutting instruments and powerful and lasting crushers are needful. Accordingly, the twelve cutting teeth of a horse are close-set and concentrated in the forepart of its mouth, like so many adzes or chisels. The grinders or molars are large, and have an extremely complicated structure, being composed of a number of different substances of unequal hardness. The consequence of this is that they wear away at different rates; and, hence, the surface of each grinder is always as uneven as that of a good millstone."
We thus see that the Equidae differ very widely in structure from most other mammals. Assuming the truth of the theory of evolution, we should expect to find traces among extinct animals of the steps by which this great modification has been effected; and we do really find traces of these steps, imperfectly among European fossils, but far more completely among those of America.
It is a singular fact that, although no horse inhabited America when discovered by Europeans, yet abundance of remains of extinct horses have been found both in North and South America in Post-Tertiary and Upper Pliocene deposits; and from these an almost continuous series of modified forms can be traced in the Tertiary formation, till we reach, at the very base of the series, a primitive form so unlike our perfected animal, that, had we not the intermediate links, few persons would believe that the one was the ancestor of the other. The tracing out of this marvellous history we owe chiefly to Professor Marsh of Yale College, who has himself discovered no less than thirty species of fossil Equidae; and we will allow him to tell the story of the development of the horse from a humble progenitor in his own words.
"The oldest representative of the horse at present known is the diminutive Eohippus from the Lower Eocene. Several species have been found, all about the size of a fox. Like most of the early mammals, these ungulates had forty-four teeth, the molars with short crowns and quite distinct in form from the premolars. The ulna and fibula were entire and distinct, and there were four well-developed toes and a rudiment of another on the forefeet, and three toes behind. In the structure of the feet and teeth, the Eohippus unmistak-
- American Addresses, pp. 73-76.