lxviii FISHES OF ILLINOIS 



northwest, and immediately east of the lower half of the Illinois 

 River, is the middle Illinoisan; above this, in the west-central part 

 of the state, between the Illinois River and the Rock, is the upper 

 •Illinoisan; and still farther north, in the Rock River basin, are the 

 Iowan and Preiowan glaciations, reaching northward across the Wis- 

 consin boundary. East of the last three mentioned, and north of the 

 southern Illinois district, the Wisconsin glaeiation, the most recent 

 of the series, covers about a fourth of the state. It is to the peculiar 

 features of the lower Illinoisan glaeiation especially, that we shall 

 presently be compelled to pay particular attention, because of their 

 evident effect on the distribution of a considerable group of our 

 fishes. 



The topographical relations of the state to the surrounding terri- 

 tory are as simple and open as its own interior hydrography, and 

 -there is little to suggest the possibility of anything in the least pecul- 

 iar in the general constitution or the relations of its fauna, or any- 

 thing problematical or especially interesting in the details of the dis- 

 tribution of its native fishes. We shall find reason to believe, how- 

 ever, that this appearance is misleading, and that the subject, stud- 

 ied in detail, contains matter of unusual interest, and presents prob- 

 lems of considerable difficulty, a solution of which will lead us to 

 some novel results. 



It is true, however, generally speaking, that the distribution of 

 "Illinois fishes reflects, in uniformity and relative monotony, the fea- 

 tures of the topography of the state. A few species occurring in Lake 

 Michigan and characteristic of the Great Lakes are, in fact , the only Illi- 

 nois fishes which are definitely and permanently separated from their 

 fellows in other Illinois waters by what may be called geographical 

 conditions, and these conditions arc not physical obstacles to their 

 passage from Lake Michigan to the Illinois River. 



Excluding, for the moment, these fishes special to the Great 

 Lakes, we find elsewhere in Illinois a general commingling and over- 

 lapping of the fish population of the surrounding territory, the limits 

 to whose range are climatic, local, and ecological, but topographic 

 only in a secondary sense. 



THE GENERAL DISTRIBUTION 



Most of the ISO species of the native fishes of Illinois range far 

 and wide in all directions beyond its narrow boundaries, thus illus- 

 trating the breadth and the simplicity of our geographical affiliations 

 with the surrounding territory; but a considerable number, on the 



