THE VERTEBRATES 



439 



admirably fitted to swift running on a hard plain, but the 

 three-toed ancestors would have been better adapted to 

 marshy land. It is well known that our present horses can- 

 not travel on very marshy ground because the single hoof is 

 difficult to withdraw from mud. This is why oxen are used 



FIG. 148. Fore foot of ancestral forms of the horse. Toes are numbered in 

 comparison with a five-toed animal in which I is the inner toe and V 

 the outermost. Only III remains in the present-day horse (6) ; but there 

 were four toes on the earliest known ancestors. (From Wiedersheim.) 



in plowing and otherwise working on very soft soil, for their 

 two-toed feet are easily withdrawn from deep mud. This 

 leads us to think that the three-toed horses of the past ages 

 could have traveled over marshy land as oxen and rhinoce- 

 roses now do. 



The two-toed animals appear to have special advantages 

 in rough and mountainous countries as well as in low and 

 swampy regions. A deer or an antelope can run swiftly on a 

 hard plain, but its feet are also adapted to other conditions 

 not suited to the horse feet. The feet of camels are specially 

 suited to the sandy regions where they occur, for the two-toed 

 condition allows considerable speed even on yielding sands. 



Students who are specially interested in study of adapta- 

 tions of ungulate feet should examine the illustrations in 

 such books on evolution as Romanes, "Darwin and after 

 Darwin,' 7 Vol. I, Figs. 73-85. 



(4) There are numerous adaptations of feet and jaws in 

 carnivors. Especially noteworthy are the feet with four or 



