GENERAL AND INTERIOR DISTRIBUTION CXI 



widening to a distance of one hundred and seventy-five mi es 

 on the eastern boundary. 



GENERAL FEATURES OF ECOLOGICAL DISTRIBUTION 



In addition to the general distribution of Illinois fishes over 

 the North American continent, their general or partial distribu- 

 tion within the state, and the unevenness of their distribution 

 over the different divisions of the state, hydrographic, climatic, 

 and geological, there are also recognizable differences and in- 

 equalities of distribution corresponding to the size of the water 

 bodies in which the species are found, to the nature of the 

 bottom and the consequent clearness and purity of the waters, 

 and to the existence and rate of current or flow in the waters 

 inhabited by them. In this class of divisions, geological dis- 

 tribution 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 precisely the 

 same territory without ever coming into any effective contact 

 with each other, because they are differently related to certain 

 features of their environment. 



As an explanation of the more general facts of distribution 

 requires 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 corresponding 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 detail and some degree of statistical precision 

 to our knowledge of this part of the subject 



The attention of the reader is called especially to the in- 

 teresting manner in which our statistics of associate occurrence 

 exhibit the frequent tendency of closely allied species inhabiting 

 the same territory 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 habita- 

 tion. 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, 



