PRIMER OF PRIMERS. 207 



in the atmosphere and in the ocean, which effect 

 an interchange between the heat of the equator 

 and the cold of the poles. 



Any change in the distribution of the land and 

 water areas of the earth, either as regards their 

 relative amount, or as regards their distribution, 

 would seriously affect the life of the earth. 



If the greatest proportion of land existed at the 

 equator, such changes would be produced in the 

 earth's climate as to sweep its present life out of 

 existence. 



Any change in the elevation of the present land 

 areas would produce a marked change in the 

 earth's climate. It was probably an increase in 

 the elevation of the polar lands that caused the 

 severe cold of the glacial epoch, when so much 

 of the northern continents were covered with 

 ice. 



In order to preserve the present relative propor- 

 tions of oxygen and carbonic acid in the atmos- 

 phere, the present animal and plant life of the 

 earth must be preserved. 



Animals are absolutely dependent on plants for 

 their existence. The plants can take their food 

 directly from the air and the soil. Animals re- 

 quire their food to be prepared for them by the 



