GENERAL AND INTERIOR DISTRIBUTION Clll 



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 OP 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, dis- 

 tribution in the Mississippi drainage in the western and northern 

 parts of the state, while 8 species, on the other hand, are very defi- 

 nitely 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 dis- 

 •tributed freely through this southern Illinois district. .We have evi- 

 dence here of another influence strongly affecting distribution, coin- 

 cident in part with that already discussed, but independent of it also 

 in part, the two causes, or sets of causes, operating together to deter- 

 mine the actual range of most of the species of limited distribution in 

 this state. 



The impression produced by an examination of the two sets of 

 maps for the fishes above mentioned, is that of a small group of spe- 

 cies, on the one hand, which enter the state from the south and east 

 by way of the Wabash and the smaller tributaries of the Ohio, and, 

 on the other hand, of a much larger group, most of which have en- 

 tered the state from the west and north, making their way to its in- 

 terior mainly by the Illinois' and the Rock, but sometimes by the 



