CXX FISHES OF ILLINOIS 



2. The Illinois basin may be taken as typical, in its fish 

 population, of the ichthyology of the whole state — occupying, as 

 it does, a central position, including more than half the area of 

 the state, and containing a great variety of waters and situations 

 fit for the habitation of fishes, and more than four fifths of the 

 species found anywhere in Illinois. The more important fishes 

 of the state not known from this basin are a few distinctively 

 northern species, most of which are peculiar to the Great Lakes, 

 and a few southern species which do not range as far north, in 

 this state, as the mouth of the Illinois. The remainder are very 

 rare in our territory, most of them coming from the west and 

 south, and they are extremely insignificant elements of our fish 

 fauna. 



3. If the ten stream systems of the state be brought into 

 comparison one with another, it appears that the six larger 

 areas, containing the largest streams and presenting the greatest 

 variety of situations, are much more closely affiliated ichthyo- 

 logically than are the four smaller areas. The least closely 

 affiliated with each other and with all the rest are the Michigan 

 district of northeastern Illinois and the Big Muddy basin in the 

 southwest. The closest relations are those between the Illinois, 

 the Rock, and the Mississippi. 



4. In the absence, in Illinois, of geographical barriers to 

 the dispersal of fishes, the causes influencing their distribution 

 are climatic, geologic, and ecological. As Illinois extends 

 through 5.5° of latitude, differences of climate between the 

 northern and the southern sections of the state are sufficient to 

 affect, in considerable measure, the distribution of its plant and 

 .animal species — differences which, in its ichthyology, express 

 themselves in the presence in northern Illinois, but not in 

 southern, of 17 species of general northward range; and in 

 southern Illinois, but not in northern, of 14 species of general 

 southward range. These two groups of species meet and mingle 

 in the great north and south rivers of the western half of the 

 state, in an area of common occupation about fifty miles in 

 width, from the latitude of Springfield northward, while on the 

 eastern boundary of the state, occupied by small streams of 

 various direction, these groups are separated by an interval of 

 about a hundred and seventy-five miles over which no repre- 

 sentative of either group has been taken. 



5. Geological limitations to the dispersal of fishes are 

 (illustrated by peculiarities of distribution in southern Illinois as 



