EVIDENCE FROM PALEONTOLOGY 103 



with low crowns, which were incapable of growth 

 after they had cut the gum, and have no cement on 

 the crown, but only on the roots. 



It is very interesting to note that this adoption of 

 a high-crowned, persistently growing type of grind- 

 ing teeth was by no means confined to the horses, 

 but was repeated in several other families of hoofed 

 animals and gnawers (Rodentia), and came about 

 during the Miocene epoch. This transformation of 

 the teeth indicates a change from the habit of 

 browsing upon leaves and other soft vegetable sub- 

 stances to grazing, or feeding upon the hard, abrasive 

 grasses, and it is highly suggestive that this coincided 

 with, or quickly followed the spread of grassy plains, 

 which afforded a new and abundant supply of nu- 

 tritious food. 



Again omitting intermediate stages and taking 

 another long backward step, we come to the horses 

 of the lower Oligocene, in which are notable changes 

 in all parts of the skeleton, as well as a distinct reduc- 

 tion in size, for these animals are no larger than sheep. 

 The teeth are low-crowned and devoid of cement, 

 save on the roots, and the sharp-edged incisors have 

 no mark. The facial portion of the skull is shorter 

 and the eye-socket farther forward, placed over 

 the teeth, and is partially open behind; the jaws are 

 shallow and slender, as there is no need of providing 

 for very deep sockets. The neck is only moderately 

 elongate and its vertebrae are much less specialized. 

 In the forearm the ulna is already much reduced in 



