18 



These are all species whose general distribution throughout the 

 country would lead us to expect to find them anywhere in Illinois. 



Even more interesting is another series of limitations upon the local 

 distribution of our Illinois fishes, because it seems to be clearly explain- 

 able as due to an ecological factor of geological origin to the physical 

 character of the surface soils of a large part of southern Illinois cor- 

 responding to the area known as the lower Illinoisan glaciation. This 

 area is notable for the extremely fine division of its soil particles, due 

 to its geological history, and for the consequent persistent and even 

 permanent muddiness of its waters, such that the suspended particles 

 cannot be completely separated by repeated filtering with the finest filter 

 paper, and do not subside even after long intervals of stagnation. This 

 persistent turbidity of the waters might well be expected to have an 

 effect to repel or exclude certain kinds of fishes, particularly those having 

 a special preference for the clean water and hard bottom of the lakes or 

 streams which they inhabit. Other species, on the other hand, which are 

 found in muddy situations elsewhere, might be expected to tolerate the 

 persistently muddy waters of this southern Illinois district. An analysis 

 of our data bears out this assumption in a remarkable way, a fact most 

 clearly shown by examples of the distribution of species selected from 

 our lists of those tolerant, and those intolerant, of muddy waters gen- 

 erally. Compare, for example, our Illinois distribution maps of the stone- 

 cat (Map LVII), the common sucker (XVIII), the hogsucker (XIX), 

 the stone-roller (XXIII), the common shiner (XLI), and the river chub 

 (LI), all rare or wanting in the lower Illinois glaciation, with the fol- 

 lowing six other species freely distributed there, namely : the black bull- 

 head (LV), the tadpole cat (LVIII), the chub-sucker (XVI), the blunt- 

 nosed minnow (XXVIII), the golden shiner (XXXI), and the long- 

 eared sunfish (LXXVI) ; and also the fact that our statistics of ecological 

 distribution, crude as they are, serve to distinguish these two groups 

 strongly with respect to their relation to muddy situations. The fishes of 

 the first group, for example, have occurred over muddy bottom only once 

 to nearly four times over a bottom of mud and sand, while those of the 

 second group have occurred with about equal frequency in the two situa- 

 tions. 



With these merely miscellaneous illustrations of method and prod- 

 uct, I must leave this subject, much too large and too complex for any 

 fairly comprehensive treatment, at least by me, within an hour's lecture. 

 1 am the less disturbed by the fragmentary character of this discussion 

 because I know that you have in charge of your ecological studies a 



