THE BALANCE OF NATURE. 167 



than one thousand feet above the sea level, large 

 parts of its interior are as yet absolutely unknown 

 to the white man. 



What, then, would be the effect on the earth's 

 present life if, instead of the present excess of 

 water surface at the equator, there existed an ex- 

 cess of land surface ? Beyond doubt the present 

 life of the earth would be swept out of existence. 



In the same manner any marked increase in 

 either the elevation or the extent of the land in 

 the polar regions would be followed by such an in- 

 crease in the severity of the cold as to sweep out 

 of existence much of the present life of the earth. 

 It was, in the opinion of most geologists, a change 

 in the elevation of the polar lands that caused the 

 severe cold of the glacial epoch, when most of the 

 northern continents were covered with enormous 

 ice-fields. 



The Air. Any change in the composition of the 

 earth's atmosphere, such, for example, as in the 

 amount of its oxygen or its carbonic acid gas, 

 would be followed by a change in its animal and 

 plant life. 



The existence of animal life tends to decrease 

 the amount of oxygen in the atmosphere, and to 

 increase the amount of carbonic acid. The ex- 



