168 OUTLINES OF FORESTRY. 



istence of plant life tends to increase the amount 

 of oxygen and to decrease the amount of carbonic 

 acid gas. 



A wonderful balance is maintained in nature as 

 to the composition of the atmosphere, from the 

 fact that what plants reject, animals require for 

 their existence, and what the animals reject, plants 

 require. 



Minerals, Plants, and Animals. The mutual in- 

 terdependence of the mineral, the plant, and the 

 animal affords another illustration of the balance 

 of nature. Animals obtain their food either from 

 other animals or from plants. Plants, as a rule, 

 live on minerals. They are so constituted as to be 

 able to take the various substances directly from 

 the soil, and to change them into forms that can 

 be readily assimilated by animals. The continued 

 existence of animals depends on the continued 

 existence of plants. 



Mature has very carefully insured the presence of 

 those germs or seeds that are absolutely necessary 

 for the birth of either animals or plants. To in- 

 sure the presence of the germs in all cases, the 

 number of such germs produced is always vastly 

 in excess of the number that can possibly live. In 

 the case of nearly all plants and animals the num- 



