275 



simply and more directly, and, if the data are sufficient, reaches 

 results more immediately and obviously significant. 



I have used both these methods in the present paper, com- 

 paring the results of the two in a way to make the one set ac- 

 count for and explain the other. This paper is thus to be taken 

 as a contribution to an answer to. the following questions: 

 What Illinois fishes are habitually found in each others' society, 

 and what is the relative frequency of their associations? How 

 are Illinois fishes grouped and distributed according to location 

 and situation, and in each ecological assemblage so formed what 

 is the proportionate representation of its various constituent 

 species? How far are the two classes of data, those of associa- 

 tive affiliation and of ecological relationship, comparable, and 

 to what extent may the one be used to explain the other? 

 An answer to these inquiries would enable us to recognize, de- 

 fine, and account for associate groups among our fresh-water 

 fishes, and also to distinguish those members of each group 

 which, being most frequently and most strictly associated, are 

 most characteristic of it. It has, in fact, been a part of my un- 

 dertaking to find a method of distinguishing clearly these cen- 

 tral or typical members of an ecological assemblage, and to ex- 

 press numerically the intensity of the influence — the strength of 

 the bond — which holds them to the local situation, as compared 

 with the more lax or less continuous forces influencing what 

 we may call the outlying members of the group. 



Studies of this description may be expected to give us 

 significant information, also, concerning the competitions of 

 associated species, and concerning the evasions of competition, 

 and the escape from its consequences, by those closely related 

 and similarly endowed, and concerning the niceties of adapta- 

 tion, psychological, physiological, and structural, exhibited by 

 fishes inhabiting a notably uniform area. 



Associative Relationships among the Etheostomin^e. 



For a preliminary and sample study of this description, I 

 have chosen first a subfamily of our fishes, the Etheostomince — 

 or darters, as they are commonly called — and have endeavored 



