lxxii FISHES OF ILLINOIS 



On the General and Interior Distribution 

 of Illinois Fishes 



The geography of Illinois is, in its most obvious features, so 

 simple and so monotonous that one naturally expects a similar 

 simplicity and monotony in the geographic distribution of its 

 plants and animals. The plan of its hydrography is as little 

 complicated as the geography of its land areas. Surrounded on 

 more than two thirds of its circumference by three large rivers, 

 the Mississippi, the Ohio, and the Wabash, with Lake Michigan 

 covering a narrow strip at its northeast corner and draining a 

 bordering region of scarcely greater area, its other waters flow 

 southwestward into the Mississippi and southward into the 

 Wabash and the Ohio, all mingling finally opposite its southern- 

 most extremity for their journey to the Gulf. Its principal 

 watersheds are inconspicuous ridges or slightly elevated plains, 

 most of them originally more or less marshy, and the head- 

 waters and tributaries of its various stream systems so approach 

 and intermingle that in times of flood they formed an interlacing 

 network, through which it would seem that a wandering fish 

 might have found its way in almost any direction and to almost 

 any place. 



Its climate varies considerably, of course, within the five 



and a half degrees of its length from north to south, but by 



nsensible gradations, with no lines of abrupt transition anywhere 



to set definite boundaries to the range of its aquatic species. 



Its surface geology is more diversified than its topography, 

 and its soils, although uniformly fertile throughout most of the 

 state, differ notably in their origin and physical constitution, 

 some of these differences being such as to affect more or less 

 the surface waters and, through them, to influence the conditions 

 of aquatic life. The extreme northwestern and the extreme 

 southern parts of the state are bare of drift; but the surface of 

 all the remainder of the state, excepting a small area above 

 the mouth of the Illinois, has been repeatedly worked over by 

 ice in the course of the successive divisions of the glacial period. 

 The oldest glaciated area, known as the lower Illinoisan glacia- 



