REPTILES, BIRDS, AND MAMMALS 469 



but the center one had become much larger and stronger than the side 

 toes, which did not ordinarily touch the ground. Functionally this was 

 a one-toed foot, which, though less well adapted to rough going than the 

 earlier three-toed type, was better for running on the hard surface of the 

 plains. The teeth for the first time showed a heavy deposit of cement on 

 the sides and in the folds. The milk premolars were still low-crowned, but 

 the permanent molars and premolars were now all alike, moderately high- 

 crowned and with an enamel pattern similar to that of modern horses. 

 High crowns imply grass-feeding habits; the silica content of grass is 

 highly abrasive and would rapidly destroy low-crowned teeth. As the 

 cheek teeth enlarged, the face elongated, and the orbit became closed 

 behind for the first time. Merychippus is regarded as a direct ancestor 

 of modern horses. 



Pliohippus, like the preceding, was a widespread genus of many species 

 which occurred during the Pliocene. Some of its members were three-toed 

 horses, but others were the first horses with a single toe. In these, fusion 

 of the lower leg bones was complete, and the two side toes had been 

 reduced to mere splint bones buried in the tissues of the leg. The grinding 

 teeth of both milk and permanent sets were moderately high-crowned, 

 with a more complicated enamel pattern than that of Merychippus. The 

 average size was that of a small pony. 



Equus, the genus to which modern horses belong, appeared in the 

 Pleistocene and represents the last stage in the evolution of the horses. 

 Its characteristics have already been described rather fully above. 



Most of the evolution of the horse group occurred in North America, 

 which was the developmental center throughout Tertiary time. An occa- 

 sional offshoot succeeded in reaching the Old World or South America, 

 but for the most part these soon died out. Pliohippus is evidently the 

 stock from which the modern horses and their relatives have come. Some 

 of its progeny reached South America and there became specialized 

 types, which later disappeared; but the main line of descent was that 

 which led to the genus Equus, which first appeared at the beginning of 

 the Pleistocene. Equus arose in North America and rapidly spread to 

 every continent except Australia. It subdivided to form the zebras, now 

 confined to Africa ; the asses, still found wild in the Old World tropics ; and 

 true horses, of which a wild species still exists in central Asia. Numerous 

 species of zebras, asses, and true horses existed in North America until 

 late in the Pleistocene. It is a strange fact that although North America 

 had been the evolutionary center for so long, the entire horse group 

 became extinct in the New World by the end of the Pleistocene. The 

 horses that now range the Great Plains all came from the horses brought 

 by man from the Old World back to their ancestral home. 



