ANIMAL AND ITS ENVIRONMENT 



459 



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 Shelf ord (Fig. 

 185), 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 



"--Lithosphere"' 

 ---Hydrosphere - - 

 ---Atmosphere - - 



Fig-, 197. — Diagram to show the relationship of the general areas of our planet. 



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 

 earth, the lithosphere. If the lithosphere were uniform in diameter, 

 it would be covered by a layer of water, the hydrosphere, which is 

 in turn surrounded by a gaseous layer, the atmosphere. It is only 

 where the components of these three layers interact that life is pos- 

 sible. Living organisms cannot exist in the lithosphere without the 

 presence also of water, and of oxygen from the atmosphere. Simi- 

 larly life in water is possible only where it contains in solution both 



