320 



HISTORICAL GEOLOGY 



about the size of a large cat (Fig. 202). On the forefoot it had 

 four functional toes (one larger than the others) and a splint or 

 imperfectly developed fifth toe. The hind foot had three func- 

 tional toes and a splint. Doubtless this early member of the Horse 

 family was derived from an original five-toed ancestor 1 whose 

 general structure was something like Phenacodus. In the later 



THE EVOLUTION OF THE HORSE. 



in Western United Slates and Characteristic Type of Horse in Each 



Fore Foot 



One Toe 



Sri.n, «f 



I 



ft*"*" 



Hmd Foot 



O ne To e 



V*m4 **d.(:i. 



T hree To ts 

 1*M * 5*difil. 



U* 



$1 





Hypothetical Ancestors with Five Toes on Each Fo*« 

 and Teeth like those of Hookers ac. 



Fig. 201 



Chart to illustrate the evolution of the Horse family. (After W. D. Matthew, 



Amer. Mus. Nat. Hist. Journal.) 



Eocene Protorohippus had four distinct toes on the front foot and 

 three on the hind foot, but with no sign of splints. This form was 

 but little larger than Eohippus. During the Oligocene Mesohippus 

 had three functional toes (the middle one being distinctly larger) , 

 with the former fourth toe reduced to a splint on the front foot, 

 while the three functional toes continued on the hind foot. It was 

 about the size of a sheep. In the Miocene Protohippus had three 

 toes on both fore and hind feet, but in each case only one was 



1 Very recently there has been reported the discovery of a still more 

 primitive form, even more closely resembling the five-toed ancestor. 



