ECOLOGICAL SUCCESSION. 337 



The study of the mould into which the organism fits becomes a 

 legitimate method as soon as the selection of habitat and the adjust- 

 ment of behavior and physiological makeup, to the environment are 

 shown to be general facts . The study of the environment must be 

 accompanied by studies of the effects of the various factors upon 

 the organisms concerned. This is necessary if important factors 

 are to be emphasized (Shelford, '12*, p. 90; Shelford and Allee, '12). 



2 . Ecological Specificity and Specificity of Behavior. 



We have stated that ecological classification is dependent upon 

 similarities and differences in mores (physiological life histories, 

 behavior, modes of life). We have noted also (Shelford, 'I2 1 ) 

 that there is similarity of mores within the same animal com- 

 munity and that the limits of animal communities are based upon 

 similarities and differences of mores. Several questions at once 

 arise. Is the same environment ever the same to different 

 species? How much and what kind of similarity is to be ex- 

 pected? It is possible for two very different species to live under 

 practically identical conditions, but probably this is rarely true 

 in the same community, such cases usually being separated 

 geographically. If the organs for the reception of stimuli, on the 

 bodies of two animals living side by side, are differently placed, 

 there must be obvious differences in reception of mechanical stim- 

 uli, light, etc. Indeed different species living under similar condi- 

 tions may be sufficiently different physiologically to be differently 

 affected by the same single stimulus, but ecologically, classifica- 

 tion is based upon the complete physiological life history, mode of 

 life, and behavior, so that differences in the effect of single stimuli 

 cannot be too much emphasized. 



It is quite clear to every naturalist, that within a given area, 

 nearly every race or species possesses certain special peculiarities 

 of structure and also of behavior, physiology, and mode of life. 

 There is a large amount of specificity in the behavior of a species 

 and as a rule, students of behavior have been unduly impressed 

 by it. For example fishes (Shelford and Allee, '12) show a general 

 community or similarity of reaction to such factors as carbon 

 dioxide and other differences in water. The fishes turn back 

 when they encounter increased carbon dioxide or other differ- 



