GENERAL AND INTERIOR DISTRIBUTION CXxi 



related to the area of the lower Illinoisan glaciation, which 34 

 species evidently avoid while 35 other species enter upon it 

 freely and inhabit it successfully. A comparison of the ecologi- 

 cal relations of these two groups of species as represented by our 

 collection records, shows that they are strongly distinguished 

 by the repugnance of the first group, and the indifference of the 

 second, to waters with a muddy bottom, collections of the first 

 group having been made from such situations in an average 

 ratio more than three times as great as that for the second. 

 The waters of this region, on the other hand, are remarkably 

 and persistently turbid, never clearing themselves spontaneously. 

 This is owing in part to the extremely fine division of the soil, 

 and in part to its generally acid character and the consequent 

 lack of " granulation, " or cohesion of its ultimate particles in 

 granules, such as occurs in the alkaline soils of the other geological 

 areas of the state. The surface waters of the district are soft 

 and slightly alkaline, but contain much silica, and much solid 

 matter in suspension which it is extremely difficult to remove 

 completely by any ordinary filtering or precipitation process. 

 The inference is plain that it is to this condition of the waters 

 due to the geological history of the soil of this region that the 

 unequal distribution of these fishes is largely to be attributed. 

 6. In consequence of another clearly recognizable ine- 

 quality of distribution, partly coincident with the two preceding 

 and partly independent of them, two additional groups may be 

 distinguished; one of 8 species, distributed in this state mainly 

 through the Ohio and Wabash drainage, and the other of 27 

 species, distributed through the Mississippi and its more north- 

 erly tributaries. The general distribution throughout the 

 country at large of each of these two groups of species is quite 

 varied, and offers no hint of a reason for these differences in 

 Illinois Two hypothetical explanations are suggested the 

 first presupposing different centers of population outside the 

 state, from and towards which these species move, into and out 

 of Illinois streams, with the spring rise, summer recession, and 

 winter cooling of the waters, one of these centers to the west and 

 north, and one to the east and south; and the second presuppos- 

 ing an organization of the fish population into more or less 

 distinct communities of mutually well-adjusted species, each 

 community so adapted to its environment that members of 

 adjacent communities can not successfully intrude upon its 

 territory. 



