THEORIES AND MECHANISM OF EVOLUTION 



657 



Eohippus 



Miohippus 



Merychippus 



Pliohippus 



Equus (modern) 



Fig. 26-4. The evolution of the horse illustrates orthogenesis. Probably these animals did not progress along a direct 

 path from the tiny Eohippus to Equus but rather there were many deviations along the way. Progress was in a 

 general way toward the present-day horse, but there were many paths that led to extinction. 



succulent leaves and avoiding contact with 

 the huge mammals of this period. 



In the Oligocene we find a somewhat 

 larger animal, Miohippus, with only three 

 toes on the forefeet, although a rudiment 

 of a fourth remains. The middle toe on each 

 foot apparently bore the brunt of the weight 

 of the animal because it was much larger 

 than those on either side. Otherwise, there 

 were few changes from EoJiippiis. 



During the Miocene Menjcliippus came 

 into prominence, still with three toes but 

 with added emphasis on the middle one 

 which bore all of the body weight. The 

 other two toes, while externally visible, 

 appear to have been of little or no use to the 

 animal. The molar teeth possessed rather 



high crowns and the flat nature of the upper 

 surfaces indicates that these animals were 

 grazers rather than browsers, that is, they 

 fed on grasses in the open prairie lands 

 rather than on leaves in the forests. The 

 face was longer, thus displacing the orbit 

 posteriorly. Merychippus was larger than 

 any of the horses up to that time, standing 

 about 3 or 4 feet in height. 



This gradual tendency toward greater 

 size and reduction of the number of toes 

 continued into the Pliocene, where Plio- 

 hippus was without external lateral toes, 

 altliough retaining splints indicating the 

 historic loss. Thus, in Pliohippus we see 

 only a single toe or hoof, a character which 

 has remained unchanged up to the present. 



