THE QUATERNARY PERIOD 



461 



and for the United States that meant in general southward. 

 On the other hand, as the ice retreated during the milder 

 times between glaciations, the same forms of life would be 

 invited by the amelioration of conditions to press northward. 

 In this way probably several backward and forward migrations 

 were induced. 



Southward advance of Arctic life. In the strictly glacial 

 times musk-oxen, such as now live in the Arctic regions, came 

 as far south as Kentucky, and herds of reindeer ranged over 

 the treeless hills of France. Elephants, such as the mammoth 



FIG. 478. The American mastodon. (Painted by Gleeson, in the U.S. 



Nat. Mus.) 



and the mastodon (Fig. 478), and rhinoceroses, both covered 

 with long, woolly hair, were among these Arctic types. Even 

 their bodies with flesh and hide intact have been found pre- 

 served in the frozen gravels of certain Siberian rivers. 



Southern forms come north. During the genial intervals 

 in which the glaciers disappeared, many southern animals 

 B. & B. GEOL. 27 



