GENERAL AND INTERIOR DISTRIBUTION CXV 



boundary of the state, occupied bysmall streams of various direction, 

 these groups are separated by an interval of about a hundred and 

 seventy-five miles over which no representative of either group has 

 been taken. 



5 . Geological limitations to the dispersal of fishes are illustrated 

 by peculiarities of distribution in southern Illinois as 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 suc- 

 cessfully. A comparison of the ecological relations of these two 

 groups of species as represented by our collection records, shows that 

 they are strongly distingushed 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 sec- 

 ond. 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 "gran- 

 ulation," or cohesion of its ultimate particles in granules, such as oc- 

 curs in the alkaline soils of the other geological areas of the state. The 

 surface waters of the district are soft and slightly alkaline, but con- 

 tain much silica, and much solid matter in suspension which it is ex- 

 tremely difficult to remove completely by any ordinary filtering or 

 precipitation process. The inference is plain that it is to this condi- 

 tion 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 inequality of 

 distribution, partly coincident with the two preceding and partly in- 

 dependent 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 northerly tributaries. The general dis- 

 tribution 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 dif- 

 ferences 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 presupposing an organiza- 

 tion of the fish population into more or less distinct communities of 



