392 
If we look to the further distribution of the northern and south- 
ern elements of our fish population, distinguishing northeastern from 
northwestern species, and southeastern from southwestern, we find 
that the southeastern species largely outnumber the southwestern 
in Illinois, and that the northeastern outnumber the northwestern. 
Thus there are 47 species of the west Gulf and Rio Grande region in 
this state, and 58 species of the east Gulf and Florida districts. 
Further, there are more species known as common to Illinois and 
the far northeast than there are to Illinois and the southwestern dis- 
trict of the west Gulf and the Rio Grande. Notwithstanding the 
much greater distance from us of the Quebec and New England 
district, there are 53 of the fishes of that region known in Illinois to 
47 of those of the west Gulf district. The northeastern fishes have, 
however, been much more carefully collected than the southwest- 
ern, and an equal knowledge of both districts might change these 
relative numbers. 
THE INTERIOR DISTRIBUTION 
The interior distribution of the fishes of the state may best be ex- 
hibited by treating each considerable stream-system as a unit, and 
comparing the fishes of each such system with all the others. The 
state may be conveniently divided into ten such hydrographic 
districts, as follows: 
1. The Galena district, including the streams of the northwest- 
ern unglaciated area, most of which empty into the Mississippi 
through Galena, Apple, and Plum rivers. 2. The Rock River dis- 
trict, extending southward and westward from the northern bound- 
ary of the state to the Mississippi at the mouth of the Rock. 3. The 
Illinois district, including the entire drainage of the Illinois River. 
4. The Michigan district, a narrow strip along the borders of Lake 
Michigan—the Lake Michigan drainage—most of which centers in 
the Chicago and the Calumet rivers. 5. The Mississippi River, and 
an irregular strip adjacent not included in any of the more definite 
river systems and mainly drained by small streams of the bluffs and 
neighboring highlands. This district is divided by the lower end 
of the Illinois basin. 6. The Kaskaskia basin. 7. The Illinois 
drainage of the Wabash, including that stream itself so far as it helps 
to form the boundary line between Illinois and Indiana. 8. The 
basin of the Big Muddy River, in the southwestern part of the state. 
