GENERAL AND INTERIOR DISTKIRI TION lxxiii 



tion, covers the greater part of southern Illinois and a narrow 

 belt of the southeast part of the central section of the state. 

 Next to this at the 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 Wisconsin boundary. 

 East of the last three mentioned, and north of the southern 

 Illinois district, the Wisconsin glaciation, the most recent of the 

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

 features of the lower Illinoisan glaciation 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 

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

 and there is little to suggest the possibility of anj^thing in the 

 least peculiar in the general constitution or the relat ons of its 

 fauna, or anything problematical or especially interesting in 

 the details of the distribution of its native fishes. We shall find 

 reason to believe, however, that this appearance is misleading, 

 and that the subject, studied in detail, contains matter of 

 unusual interest, and presents problems of considerable difficult}^ 

 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 features of the topography of the state. A few species 

 occurring ; n Lake Michigan and characteristic of the Great 

 Lakes are, in fact, the only Illinois fishes which are definitely 

 and permanently separated from their fellows in other Illinois 

 waters b}^ what ma} T be caled geographical conditions, and 

 these conditions are 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 

 overlapping 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. 



