110 THE STORY OF GERM LIFE. 



is brought back again to the circle through 

 the agency of bacterial life. And so the food 

 material of animals and plants continues in this 

 never-ceasing circulation. It is the sunlight that 

 furnishes the energy for the motion. It is the 

 sunlight that forces the food around the circle 

 and keeps up the endless change ; and so long as 

 the sun continues to shine upon the earth there 

 seems to be no reason why the process should 

 ever cease. It is this repeated circulation that 

 has made the continuation of life possible for the 

 millions and millions of years of the earth's his- 

 tory. It is this continued circulation that makes 

 life possible still, and it is only this fact that the 

 food is thus capable of ever circulating from ani- 

 mal to plant and from plant to animal that makes 

 it possible for the living world to continue its 

 existence. But, as we have seen, one half of this 

 great circle of food change is dependent upon 

 bacterial life. Without the bacterial life the ani- 

 mal body and the animal excretion could never 

 be brought back again within the reach of the 

 plant; and thus, were it not for the action of 

 these micro-organisms the food cycle would be 

 incomplete and life could not continue indefi- 

 nitely upon the surface of the earth. At the 

 very foundation, the continuation of the present 

 condition of Nature and the existence of life 

 during the past history of the world has been 

 fundamentally based upon the ubiquitous pres- 

 ence of bacteria and upon their continual action 

 in connection with both destructive and con- 

 structive processes. 



