378 ELEMENTS OF BIOLOGY 



animal communities, the tropical group, the sub-tropical, the tem- 

 perate, and so on. 



Within these zones on this temperature gradient it is also possible 

 to mark oflf areas, the fauna of which is determined by other fac- 

 tors. Thus aquatic fauna may be set off from land fauna, and 

 marine fauna may be set ofiF from fresh water fauna. The analysis 

 of local groupings may be carried on much farther. Thus two fresh 

 water lakes within a few miles of each other, having the same 

 yearly temperatures, may possess quite distinct fauna. Examination 

 will show that a number of quite local differences may be responsi- 

 ble for the differences in animal communities in the two lakes; for 

 example, the f^oor of one may be stony and sandy, that of the 

 other, muddy. Or the feeding streams of the first lake may flow 

 through territory that contains one type of substance in excess, for 

 example salts, while the waters pouring into the second lake may 

 contain little of such substances. Then, too, the plant life of such 

 lakes may be different, due to one or more of the conditions that 

 have been suggested or will suggest themselves. Without exception 

 all animal communities are superimposed on plant communities, for 

 as has been emphasized, directly or indirectly all animals depend on 

 plants for their source of materials and energy for vital processes. 



If the fauna of a single lake is examined it will be found possible 

 to separate the forms that occur into a number of local communi- 

 ties within the lake boundaries. Thus some types, for example, 

 amphibian and insect larvx, small crustaceans and small fish, will be 

 found near the shore, while large fish and large crustaceans, if pres- 

 ent, will in general be found in the deeper waters. Still more strik- 

 ingly the animal life of the ocean divides itself into shore forms, 

 surface forms, and deep water forms. 



Ecologists in the study of such animal communities and distribu- 

 tions recognize the limitations of the various groups within certain 

 levels or regions as due to the interplay of various environmental 

 factors that occur as gradients in intensity. Thus in passing from the 



