160 COPE. OWAoren IK 
later.) A still more important body of evidence which shows 
that the inequality in size and number of digits is due to im- 
pacts and strains unequally distributed, has been brought for- 
ward by Ryder. He points out that definite results are to be 
observed in those limbs of a given type of animal which expe- 
Figure 10.— A, Right posterior foot of Protohippus sejunctus Cope, from Colo- 
rado, about one-half nat. size. From U. S. Geol. Surv. Terrs., F. V. Hayden, IV. 
B, Right posterior foot of Potbrotherium labiatum Cope, from Colorado, three-fifths 
nat. size. From Hayden’s Report, IV., Pl. CXV. 
rience correspondingly definite influences ; while in the limbs 
where the strains are equal, the modifications do not appear. 
Examples of this kind are to be found in the unguiculate Mam- 
malia, and in the Marsupialia. Thus in the jerboas which use 
the hind limbs in leaping, these only display reduced digits, the 
