382 ELEMENTS OF BIOLOGY 



of aquatic green plants and small animals, for example small fish, 

 snails, planarian worms, hydra, or other aquatic animals, the com- 

 munity may become balanced so that the metabolism of the green 

 plants present directly and indirectly provides just enough material 

 and energy for the maintenance of the entire community. But a 

 perfectly balanced community that persists without change prob- 

 ably does not exist in Nature. 



Man himself, whether savage or civilized, is a member of a 

 food chain. In the savage state the human body forms a link in a 

 natural food chain, taking its share of the kills and itself falling 

 victim to other animals as well as to disease and old age. Man in 

 the civilized state arranges artificial food chains. In so doing he 

 does not develop new principles but merely puts into wider execu- 

 tion the biological principles that govern all animal communities. 

 Human food supply is at once diversified and stabilized by the 

 introduction of long-distance food transportation, with the corre- 

 sponding development of mechanical transport systems and the 

 industrial efiforts that such systems require. By selection in breeding 

 and the application of the laws of heredity human foods are in- 

 creased and improved. By suppression of species that have no value 

 as human food, numbers and qualities are encouraged in domesti- 

 cated plants and animals; pests and parasites that in Nature consti- 

 tute links in the food chain are reduced or eradicated. Moreover, 

 by technical methods recently developed, nitrogen is artificially 

 fixed, that is, chemically combined into forms available for plant 

 subsistence. So Man is able to dispense to some extent with nitro- 

 gen-fixing bacteria and thus to take a short cut to protein synthesis 

 by green plants. Thus the human civilized community tends to in- 

 clude more and more of the habitable portion of the earth's surface 

 and to establish therein food chains, that, while fundamentally 

 grounded on the facts and relations of natural food chains, are arti- 

 ficially balanced. The food balance in civilized human communities 

 is maintained only by special industrial effort. When such effort is 



