GENERAL AND INTERIOR DISTRIBUTION Ixvii 



On the General and Interior Distribution of Illinois Fishes 



The geography of IlHnois is, in its most obvious features, so sim- 

 ple and so monotonous that one naturally expects a similar sim- 

 plicity 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 Oho, all 

 mingling finally opposite its southernmost 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 headwaters 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 insensible 

 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, dif- 

 fer notably in their origin and physical constitution, some of these dif- 

 ferences 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, and their soil is derived immediately from the underlying 

 rock; 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 lUinoisan glaciation, 

 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 



