284 PRINCIPLES OF ANIMAL BIOLOGY 



3. Flood-plain series. 



(a) Terrigenous river-margin associations. 



(b) Stream-margin thicket associations. 



(c) Elm and river maple associations. 



4. Clay series. 



(a) Bare clay association. 



(b) Sweet clover association. 



(c) High forest-margin associations. 



5. Rock series. 



(a) Bare rock associations. 



(b) Thicket association. 



6. Sand series. 



(a) Lake-margin association. 

 (6) Cottonwood association. 



(c) Pine association. 



(d) Black oak association. 



7. Climatic forest formation of the deciduous forest climate. 



(a) Birch-maple association of the tamarack-forest series. 



(b) Oak-elm -basswood association. 



(c) Black oak, red oak association. 



(d) Red oak, hickory association. 



(e) Beech-maple association. 



It is not to be inferred that physiological dependence upon or prefer- 

 ence for certain physical conditions is the only relation which has brought 

 about the above listed associations, for some animals are there because 

 others are, or because of the presence of certain plants. However, 

 the fact that the associations correspond in distribution to the physical con- 

 ditions indicates that the relations between the animals and the physical 

 conditions are in considerable part responsible for them. It has been 

 demonstrated by physiologists that animals react differently to different 

 stimuli, that a variation from the optimum in any one of the physical 

 conditions essential to life constitutes a stimulus, and that the optimum 

 may differ with different species. This has led to the conclusion that 

 organisms differ in the nature of their protoplasm as well as in their 

 structure and gives a basis for the interpretation of the phenomena of 

 animal associations. The power of spontaneous movement makes it 

 possible within certain limits for a species to find preferred conditions; 

 animals with the same general physiological requirements drift together; 

 and an animal community, based at least in part upon physiological 

 dependence upon the physical environmental conditions, results. 



Animal Reactions and Habitat. — The reference made in the preceding 

 paragraph to preferred conditions and physiological requirements, as 

 factors in the choice of habitats, and hence in the formation of animal 

 associations, may easily prove misleading unless restricted in its ap- 

 plication. In higher animals, those with some degree of intelligence, 

 there may be such things as preferred conditions. Such animals may 

 actually seek places where their physiological requirements may be met, 



