ORIGIN OF THE DIVERSE FORMS OF ANIMAL LIFE 349 



toes as an encumbrance fell prey to 

 their pursuers. This process of de- 

 veloping new species is called evolu- 

 tion by variation and natural selection. 

 The ancestor of the three-toed horses 

 were four-toed animals whose remains 

 are found in the layers of rock be- 

 neath those containing the three-toed 

 ones. By this process of variation 

 and natural selection the four-toed 

 forms were derived from a five-toed 

 mammal with such a generalized FIG. 390. -Photograph of 



t foot of four-toed horse 



Structure that Other Of itS progeny in American Museum of 



Natural History. 



developed into sheep, cows, and deer 



in accordance with the various foods and changing factors 



FIG. 391. After Matthews in Guide Leaflet of American Museum of Natural History. 



