THE MUTUA^ RELATIONS OF ORGANISMS 237 



make new starch, sugar, proteids, etc. ' The ingredients which 

 feed one generation of plants may, after combination in the 

 plant body, nourish a generation of animals, eventually return- 

 ing to the same conditions as those from which they started. 

 The cycle is thus complete, and there need be no danger of 

 exhaustion of the food supply as long as it is possible for the 

 same materials to be used over and over again by green plants, 

 animals, and fungi 



