CENOZOIC ERA: AGE OF MAMMALS 6ll 



was apparently an immigrant from Europe by way of Asia, but it 

 was in America that the race developed, although from time to time 

 modified representatives migrated back to Europe. During the 

 Eocene the climate was moist, forests covered the lands, and lakes, 

 marshes, and streams were abundant. Under conditions such as 

 these the early horses lived. During the Oligocene the conditions 

 had not greatly changed, but increasing aridity caused a drying up 

 of the streams and lakes and the development of considerable areas 

 of prairie lands. The woodlands, meadows, and dry prairies of the 

 time favored the evolution of several branches adapted to the dif- 

 ferent environments, and the horse remains of the epoch show that 

 branches, fitted for the varied conditions, were developed. Some 

 of these soon became extinct, while others gave rise to the horses of 

 the Miocene. 



The great expansion of the prairies and diminution of the forested 

 areas in the Miocene favored the evolution of horses fitted for rapid 

 motion on the dry, hard plains. Two explanations for the increasing 

 length and complexity of the teeth have been offered : (i) that, as 

 the race changed from a habitat of forest and marsh to one of prairie, 

 the teeth became fitted to grind up the hard, nutritious grasses that 

 covered the plains (Osborn) ; and (2) that on dry, sandy plains where 

 the grass was short, the teeth wore out rapidly because of the sand 

 grains which were necessarily caught up with the grass when it was 

 cropped, and which wore away the teeth even more rapidly than 

 hard vegetation would. (Gidley.) Those who hold the latter view 

 maintain also that the plains grasses were and are actually less hard 

 than the vegetation of the marshes and forests, and that consequently 

 a change to plains vegetation would have been unimportant had it 

 not been for the presence of sand grains in the food. As a result of 

 the above causes, we find the Miocene horses fitted for plains condi- 

 tions increasing, and those with the spreading foot and short-crowned 

 teeth fitted for forest conditions becoming extinct. After the Mio- 

 cene the race became more and more like the modern horse. By 

 the beginning of the Glacial Period they had become extraordinarily 

 abundant, but at its close they had entirely disappeared from the 

 western hemisphere, though the descendants of migrants to the 

 Old World lived on. 



Cause of the Extinction of the Horse in North America. — It is 

 difficult to assign a reason for the extinction of the horses in America. 

 The cold of the Glacial Period has been suggested, but is hardly 



