96 DOCTRINE OF EVOLUTION 



tive anatomical study, the odd-toed forms arranged 

 themselves in a series beginning with an animal like an 

 elephant with the full number of five digits on each 

 foot and ending at the opposite extreme with the horse. 

 A reasonable interpretation of these facts was that 

 the animals with fewer toes had evolved from ancestors 

 with five digits, of which the outer ones had progres- 

 sively disappeared during successive geological periods, 

 while the middle one enlarged correspondingly. The 

 facts provided by palaeontology sustain this contention 

 with absolutely independent testimony. Disregarding 

 some problematical five-toed forms like Plienacodus, the 

 first type of undoubted relationship to modern horses 

 is Hyracotherium, a little animal about three feet long 

 that lived during the Eocene period of the Cenozoic 

 epoch. Its forefeet had four toes each, and its hinder 

 limbs ended with three toes armed with small hoofs, 

 but one of its relatives of the same time has a vestige 

 of another digit on the hind foot. By the geological 

 time mentioned, therefore, the earliest true horses had 

 already lost some of the toes that their progenitors 

 possessed. In the Miocene the extinct species, ob- 

 viously descended from the Eocene forms, had lost 

 more of their toes; still higher, that is, in the rocks 

 formed during succeeding periods of time, the animals of 

 this division are much larger and each of their feet has 

 only three toes, of which the middle one is the largest 

 while the ones on the sides are small and withdrawn 

 from the ground so as to appear as useless vestiges. 

 To produce modern horses and zebras from these 

 nearer ancestors, few additional changes in the structure 

 of the feet are necessary, for the lateral toes need only 



