CVlll PISHES OF ILLINOIS 



sayanus, on the other hand, was found sixty-two t'mes on a 

 muddy bottom to nineteen times in each of the other situations. 



By tabulating data of this description separately for each of 



the two lists of species referred to — thirty-four species in the one 



list and thirty-five in the other — and averaging the ratios for 



^each group separately, significant evidence was obtained of the 



factors which affect the distribution of these fishes. 



The species which distribute themselves freel}^ over southern 

 Illinois are those which are generally tolerant of turbid waters, 

 as shown by the fact that 32 per cent, of all our collections of 

 this group came from muddy streams and ponds, 34 per cent, 

 from situations where the bottom was composed largely of rock 

 and sand, and 24 per cent, from a bottom of sand and mud. 

 The species avoiding the central area of southern Illinois, on 

 the other hand, are, as a rule, intolerant of muddy waters, only 

 10 per cent, of all our data-bearing collections of this group 

 coming from such situations, while 61 per cent, of them were 

 from bottoms of rock and sand, and 29 per cent, from those of 

 sand and mud. It is consequently clear that the suspended 

 detritus of the streams of southern Illinois and the clay and mud 

 of which their banks and bottoms are commonly composed, are 

 an important part, at least, of the cause of the smaller variety of 

 fishes in these waters; and these conditions trace back through 

 the character of the soil to the geological history of the central 

 part of southern Illinois. 



FISHES OF THE OHIO AND OF THE MISSISSIPPI DRAINAGE 



A comparison and classification of our distribution maps 

 from another point of view enables us further to distinguish two 

 rather definite groups of species coincident in great measure, 

 but not wholly so, with the two groups which we have found in 

 an opposite relation to the lower Illinoisan glaciation. No less 

 than 27 of our species have either an exclusive, or at least a 

 strongly preponderant, distribution in the Mississippi drainage 

 in the western and northern parts of the state, while 8 species, 

 on the other hand, are very definitely preponderant in the Ohio 

 drainage in the southern and eastern parts. Nineteen of the 27 

 species of the first list are also on the list of species excluded from 

 the region of the lower Illinoisan glaciation, while 6 of the 8 

 species of the second list are also on that of species distributed 

 freely through this southern Illinois district. We have evidence 

 here of another influence strongl}^ affecting distribution, coin- 



