174 ANIMALS OF THE PAST 



horse. They were adapted to their environ- 

 ment as it was ; but when the western moun- 

 tains were thrust upward, cutting off the 

 moist winds from the Pacific, making great 

 changes in the rainfall and climate to the east- 

 ward of the Rocky Mountains, these big 

 beasts, slow of foot and dull of brain, could 

 not keep pace with the change, and their race 

 vanished from the face of the earth. The day 

 of the little Hyracothere was at the beginning 

 of the great series of changes by which the 

 lake country of the West, with its marshy 

 flats and rank vegetation, became transformed 

 into dry uplands sparsely clad with fine 

 grasses. On these dry plains the more nimble- 

 footed animals would have the advantage in 

 the struggle for existence ; and while the four- 

 toed foot would keep its owner from sinking 

 in soft ground, he was handicapped when it 

 became a question of speed, for not only is a 

 fleet animal better able to flee from danger 

 than his slower fellows, but in time of drouth 

 he can cover the greater extent of territory 

 in search of food or water. So, too, as the 

 rank rushes gave place to fine grasses, often 



