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societies, and such society groups as can be clearly recognized 

 rarely have any precise boundaries. For a full knowledge of 

 the intricate web of the relations to their physical environment, 

 and through that to each other, of the animals of any com- 

 posite area, it is necessary that the entire assemblage of the in- 

 habitants of that area should be studied as a compound unit, 

 and for this, of course, extensive and comprehensive collections 

 must be made, such as will fairly represent the entire animal 

 life of their region. 



The possession of a miscellaneous but very large collection 

 of Illinois fishes, obtained during various seasons of a long pe- 

 riod of years, from all kinds of waters and in all parts of the 

 state (see Map I.), each lot still bearing, as a rule, the original 

 collector's data giving both the time of collection and the ex- 

 act locality, has suggested to me a trial study, intended to show 

 what may be learned with regard to the ecology of fishes by a 

 critical analysis of the local data of such a collection. 



These data may be organized and generalized for ecologic- 

 al study in two ways. They may be treated in one mass, with- 

 out local subdivision, and in such a way as to bring out the 

 facts concerning the association of the different species of fishes 

 with each other, without reference, in the first instance, to the 

 localities and situations from which the specimens have been 

 taken; or they may be first divided and arranged according to 

 location and surroundings, the assemblage of species from each 

 geographical unit and from each kind of ecological situation 

 being studied separately, as a local animal society. The first 

 method has the advantage over the second, that it gives 

 us much larger numbers of specimens and collections from 

 which to generalize, and thus enables us to enter further into 

 the details of the associate relationship without danger of 

 error from unsafe generalization; and it also enables us to dis- 

 tinguish similarities and differences of ecological relationship 

 among the species, uninfluenced by any previous discrimination 

 or classification of ecological situations. The second method has 

 the advantage over the first, that it attacks the problem more 



