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ESSENTIALS OF ZOOLOGY 



mentioned. The characteristics of such communities vary from time 

 to time and from place to place. A difference in climate may be 

 sufficient to change almost every component of the community. The 

 domestic animals and plants associated with man in the tropics are 

 quite different from those in arctic regions. Perhaps one species, 

 the dog, almost as adaptable as man himself, might be considered 

 as a member of both communities. The removal of a single species 

 or the addition of a new one may alter profoundly the aspect of 

 the community. Consider, for example, a human society from which 



Fig. 196. — Diagram to show food relations in a hypothetical prairie community. 

 (Redrawn and modified from Shelf ord. Animal Communities in Temperate America, 

 University of Chicago Press.) 



all cows were removed, or the changes made in the life of certain 

 sections of the United States with the introduction of the cotton 

 boll weevil. 



Some idea of 4:he complexity of the relationships involved in a 

 community of organisms may be gained by citing Charles Darwin's 

 example of the dependence of clover on cats, or Thomas Huxley's 

 extension of the chain of cause and effect to the responsibility of 

 the old maids of England for the supremacy of that nation on the 



