o.->o 



THE AMERICAN NATURALIST [Vol. XLVII1 



larly to two or more stimuli not 

 differing greatly in intensity from 

 their optimum, i. e., the percent- 

 age of positive or negative trials is 

 essentially the same for standard 

 intensities. There is also probably 

 similarity in the rates of metabol- 



(6) The specificities of behavior 

 such as the mode of moving the 

 organs, e. g., of locomotion, and 



of different behavior reactions are 

 similar and hence are ecologically 

 equivalent. The size and efficiency 

 of the organs are also involved. 



A testing, for example, of the rheotaxis of a large num- 

 ber of brook-rapids animals has shown them to be 

 strongly positive, and when active individuals only are 

 considered the percentage of positive trials is very 

 similar for the entire rapids community. Likewise they 

 are in accord in their avoidance of sand bottom. Many 

 of the animals have special means of attachment which 

 may be brought into play with speed. 



As has already been pointed out elsewhere, ecological 

 equivalence is illustrated here. The darters (fish) are 

 strong swimmers and are able to live in rapids by virtue 

 of their swimming powers and positive reaction, while 

 snails meet the same general conditions through positive 

 rheotaxis and the strong foot which enables them to hold 

 to rocks. 



3. Sessile and Motile Organisms in Ecological 

 Succession 



(a) Ecological succession is succession of ecological 

 (physiological) types over a given area, due to changes of 

 conditions which both cause migration of physiological 

 types and transformation of such types as remain (Shel- 

 ford, '11a, 'lib, y lld, '12a, '126 and citations). Changes 

 of conditions are geographic, i. e., physiographic, climatic. 



