142 ANIMALS OF THE PAST 



were thrust upward, cutting off the moist winds from 

 the Pacific, making great changes in the rainfall and 

 climate to the eastward 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 Eohippus 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 

 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 terri- 

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

 rushes gave place to fine grasses, often browned and 

 withered beneath the summer's sun, the complex tooth 

 had an advantage over that of simpler structure, while 

 the cutting-teeth, so completely developed in the horse 

 family, enabled their possessors to crop the grass as 

 closely as one could do it with scissors. Likewise, up to 

 a certain point, the largest, most powerful animal will 

 not only conquer, or escape from, his enemies, but pre- 

 vail over rivals of his own kind as well, and thus it came 

 to pass that those early members of the horse family 

 who were preeminent in speed and stature, and harmon- 

 ized best with their surroundings, outstripped their 

 fellows and transmitted these qualities to their progeny, 

 until, as a result of long ages of natural selection, there 

 was developed the modern horse. The rest man has 

 done: the heavy, slow-paced dray horse, the fleet 

 trotter, the huge Percheron, and the diminutive pony 

 are one and all the recent products of artificial selection. 



