464 HISTORICAL GEOLOGY 



Final readjustments in the living world. Since the last 

 retreat of the ice, only slight changes have been wrought 

 in the living world. The animals and plants we have to-day 

 are similar to those of the Glacial epoch. True, certain species 

 have migrated from one region to another. Thus the reindeer 

 has moved north to Lapland and Siberia. Such animals as 

 the mammoth and the cave bear have become extinct. Few, 

 if any, newer types, however, have appeared. 



The event of chief importance, not only to us as human 

 beings, but from a purely geological viewpoint, was the rapid 

 spread and advancement of the races of men. Long before 

 the dawn of historic times man had pushed outward from 

 the place of his origin (itself yet unknown) and had colonized 

 all the larger lands, and eventually even such remote islands 

 as New Zealand and Hawaii, whither no other mammal except 

 bats had ever gone. So long ago were the principal migra- 

 tions made that the inhabitants of different continents have 

 become distinct races through long isolation. Some of these 

 races have since made comparatively little progress, while 

 others have increased and developed with astonishing rapidity. 



The geologic effects of human activities. Probably no 

 other land animal, certainly no other mammal, has equaled 

 the human species in its effect upon the earth and its many 

 living things. By digging canals he has connected seas and 

 lakes hitherto separated. By cutting down the forests he 

 has exposed to rain and wind the soils formerly held firmly 

 upon the hills. Clear streams have thus become muddy, 

 and permanent streams intermittent, while springs have 

 disappeared and shifting sands have buried plains once fertile. 



In an even more striking way man has produced changes in 

 the animal and plant world. Certain kinds he has protected 

 and domesticated, so that they have become abundant in 

 many countries. Others he has hunted almost or quite to 

 extinction. Among the former are the cat, dog, and cattle ; 

 while the auk, the passenger pigeon, and the bison may serve 

 as examples of the latter. A full list of either would be long. 



