26 EVOLUTION 



Also in the Eocene there was Protorohippus, 

 with four toes in front and three behind, the 

 side ones behind touching the ground. It 

 seems to have been about fourteen inches 

 high, and the evolution of speed had begun. 

 During the Eocene times North America was 

 in great part forest-clad, but " the moist 

 climate gave rise to many streams and lakes, 

 along the shores of which grew sedgy meadows 

 that in turn gave rise to grassy plains. These 

 were the conditions under which the horses 

 made their first appearance, and the increasing 

 development of grass lands gave the initial 

 trend to their evolution." 



Somewhat later, in the Oligocene, Meso- 

 hippus makes its appearance, the hind-foot 

 with three toes as before, but the fore-foot 

 with the little toe reduced to a splint, so that 

 only three remain, the side ones just touching 

 the ground. The middle or third toe is now 

 much larger than the side toes, which no 

 longer bear much of the weight of the animal, 

 save on marshy ground. The grinding teeth 

 have become more complex. One of the 

 species of Mesohippus was about the size of 

 a sheep, and one of the treasures in the Yale 

 Museum that brings the past very vividly back 

 to us is the nearly perfect skull of its new-born 

 foal. Of the physical conditions of the Oli- 



