3 1 o PR I MAR Y FA CTORS OF OR GANIC E VOL UTION. 



itive character. The same is true of the kangaroos. 

 In digging genera, the fore limbs experience the modi- 



Fig. 85. A, Right posterior foot of Prothippus sejimctus Cope, from Colo- 

 rado, about one-half natural size. From U.S. Geological Survey of Territo- 

 ries, F. V. Hayden, IV. B, Right posterior foot of Poebrotherium labiatum 

 Cope, from Colorado, three-fifths natural size. From Hayden's Report, IV., 

 Plate CXV. 



fications, while the hind limbs are more normal, as in 

 Chrysochloris and various Edentata. 



