386 DARWINISM chap. 



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 hard- 

 ness. 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 Equidse 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 Equida3 ; and 

 Ave 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 knoAvn 

 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 rudi- 

 ment of another on the forefeet, and three toes behind. Li 

 the structure of the feet and teeth, the Eohippus unmistak- 

 ^ American Addresses, pp. 73-76, 



J 



