CVl FISHES OF ILLINOIS 



divisions, geological distribution merges into ecological relation, the 

 distribution of species being no longer by geological areas, but by 

 ecological situations. In this sense two species may occupy pre- 

 cisely the same territory without ever coming into any effective con- 

 tact with each other, because they are differently related to certain 

 features of their environment. 



As an explanation of the more general facts of distribution re- 

 quires an analysis and interpretation of continental, terrestrial, and 

 even cosmic agencies affecting it, so an understanding of what we 

 may call the ecological distribution of a species, requires a corre- 

 sponding analysis of the ecological features of the region. Such an 

 analysis can here be carried but a little way, since the ecological data 

 borne by our collections are only of a very general type ; but such as 

 they are, they may, if used with discretion, add definiteness and de- 

 tail and some degree of satistical precision to our knowledge of this 

 part of the subject. 



The attention of the reader is called especially to the interesting 

 manner in which our statistics of associate occurrence exhibit the 

 frequent tendency of closely allied species inhabiting the same terri- 

 tory to avoid each other's company, and thus to evade competition 

 with one another, by the choice of different haunts and situations 

 within the area of their common habitation. In consequence of 

 this tendency, we sometimes find widely unlike species more closely 

 and commonly associated in our collections than like, the ecological 

 repulsion of each for its similars bringing dissimilars together in 

 more or less definite associate groups. Apparent examples of this 

 reaction may be found in the body of this report in the discussion 

 of the suckers, the minnows, the catfishes (especially the bullheads), 

 the top-minnows, and the sunfishes. 



Ninety-seven of our species have been collected in large enough 

 numbers, and from a sufficient variety of locations, to give us data 

 for comparison with reference to the general character and size of the 

 water bodies which they prefer; 62 species furnish available data 

 concerning the bottom or substratum of these water bodies ; and 49 

 species, data concerning current and rate of fiow. The numbers of 

 collections for the various species covered by these figures vary 

 greatly from a minimum of 10 collections of a species to a maximum 

 of 376. Unfortunately, the larger and more important fishes are 

 commonly represented by the smaller numbers of collections, and 

 statements made concerning these are less likely to be found fairly 

 accurate and generally correct than are those concerning the smaller 

 fishes, represented by larger numbers of collections. 



