OF EVOLUTION. 



59 



Horse. 



Pliohippus. 



(Upper Pliocene.) 



Hipparion. 

 (LowerPliocene.) 



Anchitherium . 

 (Miocene.) 



Mesohippus. 

 (Oligocene.) 



Hyracotheriurn. 

 (Eocene.) 



Successive forms of the American horse type. (After Marsh . ) 



