ArtIcLe III.—On the General and Interior Distribution of 
Illinois Fishes.* By S. A. Forses. 
The geography of Illinois 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 Ohio, 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, toinfluence 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 
* This article is a reprint, with minor ‘changes, of a chapter in the introduction 
to ‘‘ The Fishes of Illinois,” by S. A. Forbes and R. E. Richardson. 
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