GENERAL AND INTERIOR DISTRIBUTION XCV 



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 in- 

 cludes 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 dis- 

 trict, with an average affiliation ratio of 37 . 6. 



If we average separately, for these groups, the ratios of each dis- 

 trict 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 individual and peculiar — least 

 closely affiliated among themselves and each with all the others — 

 are the Michigan, the Galena, the Saline, and the Big Muddy dis- 

 tricts, 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 our 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 

 accompanying set. 



The fishes of these three divisions number 119 species for 

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

 ively. Fourteen species have been found by us only in the northern 

 division, 9 only in the southern, and 5 only in the central, and 89 spe- 

 cies are found in all three sections. Twelve species occur in both 

 northern and central Illinois, but not in southern, 17 in both south- 

 ern and central Illinois, but not in northern, and 4 in both the north- 

 ern and southern divisions of the state, but not in the central. 



