136 EVOLUTION OF THE EARTH 



tlon became too severe for the native inhabitants to endure. 

 Assimilation of the native stocks by such an invading army is 

 impossible among animals, however it may be with mankind. 

 The archaic mammals, therefore, had but little choice. Some 

 lingered on, enduring the competition until it became greater 

 than they could bear, others may have migrated still farther 

 south to find asylum, which served to postpone their inevitable 

 fate. Yet others, a very few, may have evolved into higher 

 types, such as the family Miacidae of the archaic carnivores, 

 although whether they deserve the stigma of genetic relation- 

 ship with the other archaics in view of this potentiality is 

 somewhat doubtful. 



Rise of grazing mammals. The modernized invaders are 

 now established in their kingdom; they are the early odd- 

 toed ungulates — horses, rhinoceroses, tapirs ; the even-toed un- 

 gulates, such as camels, deer, and swine ; the rodents ; the carni- 

 vores, insectivores, and primates; and in the Old World the 

 proboscideans or elephants and mastodons. Continental ele- 

 vation in Europe and more especially in Asia during the 

 Miocene brought in its train a marked increase in aridity which 

 in turn had a more or less profound effect upon the flora of 

 the temperate zones, for it meant a diminution of shrubby 

 and herbaceous plants and a wide expansion of the harsher 

 grasses, which now become the dominant note in the world's 

 flora. 



This could not but affect the mammals most profoundly, 

 especially the hoofed forms. Floral differentiation during the 

 Oligocene had already made its impress upon certain groups, 

 such as the horses, so that they in turn were differentiating 

 along several lines, some with short-crowned teeth suited to 

 tender herbage, others with grinders whose length and com- 

 plexity forecast the grazing teeth of their successors and whose 

 dietary choice led in the direction of the coming grasses. Then 

 came the floral change of the Miocene and with it a rapid 



