THE EVOLUTION OF THE HORSE 



are known as Pliohippus, Protohippus, and Hipparion, each 

 probably representing adaptation to life on a certain type of 

 plain — the grassy plain, the brushy plain, and the desert 

 plain. 



At the beginning of the next geological epoch, the Plio- 

 cene, the three types just named were living in America. 



Fig. 2. — Principal stages in the evolution of the teeth 

 and the fore foot of the horse. Showing the increase in 

 the complexity of the grinding teeth and the gradual 

 loss of toes on the front foot. 



1. Four-toed horse {Eohippus) . Eocene epoch. 



2. Early three-toed horse {Mesohippus) . OHgocene epoch. 



3. Later three-toed horse {Merychippus) . Miocene epoch. 



4. One-toed horse (Equus) . Pleistocene and Recent epochs. 



During this epoch America .was again united to Asia by a 

 stretch of land that extended across Bering Sea. Over this 

 land horses migrated from America to Asia and from Asia to 

 Europe, where they became abundant and were differentiated 

 into several species. Horses also found their way from 

 North America to South America across the Isthmus of 

 Panama, then recently emerged. 



The Pliocene horses were all plains horses of the three 



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