CONDITIONS OF EXISTENCE 



53 



animals differ in their relations to bottom and are in disagreement 

 with reference to their vertical distribution in nature. Turning 

 to reactions to light one finds a comparable difference. Animals 

 living beneath stones show a preference for weak light; those living 

 on stones, medium light; those among stones, strong light. If one 

 were to study the community in full one would find that reactions 

 to many other factors are of importance. Associative memory no 

 doubt plays a role. Thus there is agreement in reaction to factors 

 of prime importance in the community habitat as a whole and 

 disagreement in respect to factors differing strikingly in the levels 

 in which the animals live within the community habitat. These 



FIG. 6. 



Food relations of aquatic animals. Arrows point to animal doing the eating. For explanation see text. 

 (Original.) 



levels are called strata. The pool community shows a striking 

 difference from the rapids community in the presence of a strong 

 preference for sand bottom and in the presence of the burrowing 

 habit, both of which are wanting among the animals of the rapids 

 community. The non-burrowing pool species are positive to cur- 

 rent but the burrowing species do not respond within ordinary 

 lengths of time. 



Forbes has devised a method by which the frequency ot associa- 

 tion may be determined for any two or more species. Data re- 

 garding such frequency may be obtained from collections made so 



