3 i2 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



In progression of this type, the middle digits of course 

 leave the ground last and strike it first. Thus the 

 middle toes have been stimulated at the expense of the 

 lateral ones, so that in the Diplarthra, either the mid- 



dle one (Perissodactyla) alone remains, or the middle 

 two (Artiodactyla). In the kangaroos the external toes 

 have been chiefly used, so that the fourth and fifth 

 digits have been principally developed. In man, who 



