180 



EVOLUTION AND GENETICS 



In the earlier species the cuboid bone received only the meta- 

 tarsal of the fourth digit. As the third digit enlarges the cuboid 

 articulates in part with it, and so gives the functional toe greater 

 lateral support and tends to prevent rocking. This change first 

 appeared in Miohippus. 



Environmental Conditions. This epoch was a time of increas- 

 ing aridity due partly to continental uplift. Such conditions re- 



FiG. 106. — Upper teeth of Mesohippus. Three premolars Hke the molars. 



(From Lull, after Matthew.) 



suited in the decrease of bodies of water and the extension of 

 prairie areas, although the persistence of forests and meadows, as 

 well as dryer areas, favored the development of several types of 

 primitive horses which have since disappeared. The continuation 

 of climatic change in the Miocene gave rise to 

 the great prairies of North America and was 

 accompanied by the development of large 

 numbers of grazing species. These animals 

 must necessarily have had the characters 

 which we see in the modern horse, viz., teeth 

 adapted to harsh grasses and legs adapted 

 for rapid locomotion over hard open ground. 



Miocene Horses. The divergence which be- 

 gan in the Oligocene culminated in two lines 

 during the Miocene in North America. These 

 were Parahippus and Hypohippus. Both had 

 low crowned teeth, obviously developed for 

 browsing on soft herbage rather than for graz- 

 ing, and spreading feet which must have been 

 fitted for walking on soft ground. They were 

 undoubtedly forest animals. Most of the 

 species were fairly large, a little more than three feet in height. 

 Migrants from the divergent types also gave rise to an Asiatic 

 genus, Anchitherium, which likewise became extinct. 



In the direct ancestry of the modern horses, the period produced 



Fig. 107.— Feet of 

 Mesohippus. A, 

 anterior; B, pos- 

 terior. (From Lull, 

 after Marsh.) 



