THE ALLIES OF THE SQUIRREL: MAMMALIA 429 



are able to trace its evolution from an ancestor (ProtoroTiip 1 - 

 pus, Fig. 226) a little larger than a cat, with four toes on 

 the front feet and three on the hind feet. The figure of 

 Protorohippus is photographed from a water-color by Mr. 

 Charles II. Knight, based on skeletal material at the Amer- 

 ican Museum of Natural History. The markings are drawn 

 as they are supposed to have existed on the animal. There is 



FIG. 226. Protorohippus 

 (American Museum of Natural History) 



reason to believe that the undiscovered ancestors of this early 

 form had five toes on each foot. The transition to the horse 

 of to-day has been accomplished by a gradual increase of size, 

 a reduction in the number of toes, and a reduction in number 

 and an increase of complexity in the teeth. The main steps 

 in the evolution of the bones of the feet are shown in Fig. 227. 

 The changes in the limbs are in the nature of adaptations 

 fitting the animal for rapid locomotion over level, grassy 



