THE TERTIARY PERIOD 



321 



large and functional, with the other two small toes not long enough 

 to reach the ground. This form was about the size of a pony. 

 During the Pliocene and Quaternary, Equus, or the modern Horse, 

 had one toe only on front and hind feet with the two side toes of 

 Protohippus reduced to splints (the fetlock of the present-day 

 Horse). Thus we see that the middle toe of the original five-toed 



Fig. 202 

 Primitive or ancestral Horses, Eohippus,, of the Eocene 



Knight under the direction of H. F. Osborn. 

 Museum of Natural History.) 



Restored by C. R. 

 (Permission of American 



ancestor has developed, to the exclusion of the others, and it is 

 thought that this has tended toward greater fleetness of foot. 

 While these evolutionary changes took place, there was also gradu- 

 ally developed longer and more complex teeth; the two entirely 

 separate bones (radius and ulna) of the fore limb gradually became 

 consolidated into a single strong bone; and the brain steadily 

 increased in relative size. 



Artiodactyls (e.g. Camel). These even-toed, hoofed Mam- 



