FISHES OF ILLINOIS 



two groups, the first of six districts, with relatively high ratios, 

 and the second of four, with relatively low ratios. The first 

 group comprises the basins of the larger rivers — the Mississippi, 

 the Rock, the Illinois, the Kaskaskia, the Wabash, and the Ohio, 

 each with its more or less complex system of tributaries. The 

 average ratio for this group is 52.7 per cent. The second group 

 is made up of small, widely separated districts, containing only 

 small streams and lakes, except that one of them includes a little 

 of the shallow southwestern border of Lake Michigan. In this 

 group are the northwestern driftless area, the Saline River and 

 its tributaries, the Big Muddy district, and the Michigan district, 

 with an average affiliation ratio of 37.6. 



If we average separately, for these groups, the ratios of 

 each district to all the other districts of its group, we obtain for 

 the first and higher group a ratio of mutual affiliation of 63 per 

 cent., and for the lower group a similar ratio of 33 per cent. 

 It is thus made clear that the districts most typical of our Illinois 

 fauna are the first six above mentioned, while those most indi- 

 vidual and peculiar — least closely affiliated among themselves 

 and each with all the others — are the Michigan, the Galena, the 

 Sahne, and the Big Muddy districts, excepting only the relation 

 of the two last mentioned which, as already said, is unusually close. 



THE FISHES OF NORTHERN, CENTRAL, AND 

 SOUTHERN ILLINOIS 



If mere difference in latitude, involving a climatic difference 

 within a range of five and a half degrees, limits the distribution 

 of any of oiir fishes, the fact should appear upon a comparison of 

 the species list of the northern, central, and southern sections of 

 the state, although due caution must, of course, be exercised 

 that other and more local causes are not confused with climatic 

 ones. The division of the state here adopted is shown on Map 

 II. of the accompanjdng atlas. 



The fishes of these three divisions number 119 species for 

 northern, 123 for central, and 119 for southern Illinois, respec- 

 tivel3^ Fourteen species have been found by us only in the 

 northern division, 9 only in the southern, and 5 only in the cen- 

 tral, and 89 species are found in all three sections. Twelve 

 species occur in both northern and central Illinois, but not in 

 southern, 17 in both southern and central Illinois, but not in 

 northern, and 4 in both the northern and southern divisions of 

 the state, but not in the central. 



