720 TEXTBOOK 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 

 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 the 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 

 seas. Obviously, since old maids are fond of cats, the number of 

 the latter is greater when old maids are numerous. Cats eat field 

 mice, which in turn prey upon the nests of bumblebees. Thus, a 

 large cat population is favorable for the development of clover 

 which is fertilized by the bumblebee. Clover is fed to cattle and 

 it is well known that Britain's sea power is due to "The roast beef 

 of old England." 



Most of the relationships suggested in this series have to do with 

 food. It is often possible to gain a better idea of relations within 

 a community by the use of a diagram indicating the more obvious 

 influences. The accompanying figure, modified from Shelford (Fig. 

 376), represents food relationships only in a hypothetical prairie 

 community and may be called a ''food chain diagram." Many 

 very obvious food chains are omitted in order to avoid a complexity 

 too great for easy reading, and, of course, any real community 

 would include many more kinds of animals. The chart is very in- 

 complete, also, in that no relations other than those directly con- 

 cerned with food are indicated. 



Life exists on the earth only in a relatively very limited space. 

 We might represent the relationship of the various parts of the 

 planet on which we live by a series of concentric circles, the area 

 within the inner circle corresponding to the "solid" portion of the 



