148 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



With the opening of the Neocene age, we have the 

 descendant of Epihippus in Mesohippus Mar&h, which 

 differs from its predecessor as follows. There are but 

 three toes on all the feet ; three premolars resemble 

 the true molars ; the crests which connect the internal 

 pair of cusps with the external in both jaws, are much 

 more elevated, and soon form on wearing a part of the 

 pattern of the crown. Hyracotherium already walked 

 on the ends of its toes, and the feet of Mesohippus 

 continue the character. The crowns of all the molars 

 are short like those of its ancestors. In the Middle 

 Neocene formations we have the genus Anchitherium 

 Kaup, where the incisor teeth show the addition of a 

 ridge, or cingulum round the inner side, which bounds 

 a cup ; forming the cupped incisors so characteristic 

 of the horses. The species have been all the while 

 growing gradually larger. 



Towards the end of Neocene time important pro- 

 gress was made. In the Loup Fork epoch the three 

 toed horses were very numerous in species, but their 

 lateral toes were all much shortened so that they did 

 not reach the ground. The crests of the molar teeth 

 were much stronger, and in the superior series the 

 conules had assumed a greater importance, extending 

 themselves posteriorly from the transverse crests, and 

 showing crescentic sections, resembling those of the 

 outer cusps, with which they are parallel. The an- 

 terior conule extended so far posteriorly as to join the 

 posterior one, resembling in this respect also the an- 

 terior external cusp. So the crown came to have six 

 modified cusps of which the two inner are the smallest 

 and remain unconnected with each other. The crowns 

 of the molars vary in length in these later three-toed 

 horses. Some are short like the Anchitheriums, and 



