BACTERIA IN NATURAL PROCESSES. 97 



face of the earth fresh and unencumbered so that 

 life can continue. 



BACTERIA AS AGENTS IN NATURE'S FOOD 

 CYCLE. 



But the matter by no means ends here. When 

 we come to think of it, it is a matter of consider- 

 able surprise that the surface of the earth has 

 been able to continue producing animals and 

 plants for the many millions of years during 

 which life has been in existence. Plants and ani- 

 mals both require food, animals depending wholly 

 upon plants therefor. Plants, however, equally 

 with animals, require food, and although they ob- 

 tain a considerable portion of their food from the 

 air, yet no inconsiderable part of it is obtained 

 from the soil. The question is forced upon us, 

 therefore, as to why the soil has not long since 

 become exhausted of food. How could the soil 

 continue to support plants year after year for 

 millions of years, and yet remain as fertile as 

 ever? 



The explanation of this phenomenon is in the 

 simple fact that the processes of Nature are such 

 that the same food is used over and over again, 

 first by the plant, then by the animal, and then 

 again by the plant, and there is no necessity for 

 any end of the process so long as the sun fur- 

 nishes energy to keep the circulation continuous. 

 One phase of this transference of food from 

 animal to plant and from plant to animal is 

 familiar to nearly every one. It is a well-known 

 fact that animals in their respiration consume 

 oxygen, but exhale it again in combination with 

 carbon as carbonic dioxide. On the other hand, 



