CONDITIONS OF EXISTENCE 57 



A given animal is classified primarily with the stratum in which 

 it breeds, as being most important to it, and secondarily with the 

 stratum in which it feeds and lives, as in many cases most im- 

 portant to other animals. The migration of animals from one 

 stratum to another makes the division line difficult to draw in 

 some cases. Still, the recognition of strata is essential even though 

 a rigid classification is undesirable. 



Associations are groups of strata uniform over a considerable 

 area. The majority of mores, consocies, and strata are different in 

 different associations. A minority of strata may be similar. The 

 term is appUed in particular to stages of formation development 

 of this ranking. The unity of association is dependent upon the 

 migration of the same individual and the same mores from one 

 stratum to another at different times of day or at different periods 

 of their Hfe histories. Such migration is far more frequent than 

 from one association to another. 



Formations are groups of associations. Formations differ from 

 one another in all strata, no two being closely similar. The num- 

 ber of species common to two formations is usually small {e.g., 

 5 per cent). Migrations of individuals from one formation to 

 another are relatively rare. 



The following is a list of the commoner fresh-water commu- 

 nities: 



I. Communities of ice, snow, and glacier pools (Moore). ' 

 They live at o° C. or below throughout the year (worms, insects, and 



crustaceans) . 



II. Stream Communities (Shelf ord). 



1. Communities of snow and ice fed streams. They live at a little above 

 the freezing point most of the year. Insects are the chief inhabitants. 



2. Intermittent Stream Communities 



a. Intermittent rapids — variable conditions and fauna 



b. Intermittent pool — variable conditions and fauna 



c. Permanent pool — variable aquatic conditions and hardy animals 



3. Permanent Stream Communities 



a. Spring dominated stages 



(i) Spring consocies — often few or no animals on account of 



water conditions 

 (2) Spring brook associations 



