324 PRINCIPLES OF ANIMAL BIOLOGY 



and chewing habits. Whereas the ancestral forms whose molars bore 

 prominent elevations lived on twigs and tender herbage which they 

 crushed in mastication, the mammoths with their flattened tooth sur- 

 faces devom-ed grasses, sedges, and other harsh vegetation which they 

 ground with lateral motion of the teeth upon one another. In this 

 respect modern elephants are like the mammoths. 



In the changes described above is found one of the most beautiful 

 and best established evolutionary series with which the paleontologist 

 is acquainted. Only a few others equal or approach it in clearness and 

 completeness. 



Evolution of the Horse. — An excellent evolutionary series is that of 

 horse-like forms of America and the Old World. Practically the oldest 

 recognizable equine animal is Eohippus of Eocene time in western North 

 America, though Europe had a very slightly simpler animal of the same 

 general period. Most of the development of the line of descent from 

 Eohippus took place in North America, where, to select only some of the 

 forms that seem to be in the direct line, there appeared Mesohippus 

 in the lower Oligocene, Merychippus in the Miocene, Pliohippus of 

 Pliocene time, and Equus which includes a number of Pliocene and 

 Pleistocene fossil forms, as well as the living horses. These animals 

 underwent changes in the feet from a three- or four-toed condition to 

 the modern one-toed animal; the teeth, in the early forms short-crowned 

 and capped with conical prominences on the upper surfaces, became high- 

 crowned and wore down as they grew so as to have flat grinding surfaces; 

 the skull changed in proportions and in certain features of the orbit of the 

 eye; and the body as a whole increased greatly in size. 



Eohippus. — ^Eohippus stood about twelve inches high, and had a 

 short head and neck. The forefoot bore four well-developed toes, but 

 the skeleton shows a splint bone on the inner side of the hand which 

 indicates a five-toed ancestry (Fig. 221). The hind foot has three toes 

 with a splint-bone representing a fourth, and in one specimen another 

 small splint-bone remaining from a fifth toe. If the ancestors of Eohippus 

 had five toes, as the splint-bones seem to show, the hind foot lost its 

 toes more rapidly than did the fore foot. It is also worthy of note that, 

 as shown by the size of the splint-bones, the inner digit of each foot was 

 the first to disappear, the outer toe next in succession. The inner digit 

 of vertebrate animals is customarily designated the first, and the others 

 in order outward. Thus as portrayed in Fig. 221, the first digit in the 

 horse was the first to be lost, the fifth digit next. As is pointed out 

 below, in later members of the horse series, the second toe degenerated 

 next, followed soon by the fourth. The teeth of Eohippus were short of 

 crown and relatively long of root. The upper surface bore several 

 conical cusps which, however, showed some sign of fusing to form trans- 

 verse crests (Fig. 222). The skull (Fig. 223) was small, the lower jaw 



