202 OUTLINES OF FORESTRY. 



acid gas and a variable quantity of the vapor of 

 water. 



The oxygen of the air is necessary for animal 

 life. The carbonic acid gas is necessary for plant 

 life. The moisture is necessary for both animal 

 and plant life. 



Animals take in oxygen and give out carbonic 

 acid gas. During growth, when exposed to sun- 

 shine, plants take in carbonic acid gas and give out 

 oxygen. 



The presence of both animal and plant life, 

 therefore, is necessary to keep the composition of 

 the atmosphere the same. 



In the geological past the earth's atmosphere 

 was vaster than at present. It contained more 

 oxygen and more carbonic acid than it does now. 

 Much of the oxygen, which then existed in a free 

 state in the air, is now combined with various ma- 

 terials that form the earth's crust. 



The excess of carbonic acid which existed in the 

 earth's atmosphere during the geological past was 

 removed from it mainly by the plants of the car- 

 boniferous period, and now exists in the earth as 

 beds of coal. 



In order to avoid any disturbance in the balance 

 between plant and animal life of the earth, forests, 



