GENERAL WI> [NTERIOB DISTBIBUTION C1X 



cident in part with that already discussed, but independent of 

 it also in part, the two causes, or sets of causes, operating to- 

 gether to determine 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 species, 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 entered the state from the west and north, making 

 their way to its interior mainly by the Illinois and the Rock, but 

 sometimes by the Kaskaskia and the Big Muddy also. Species 

 of the Ohio group sometimes seem to spread into the headwaters 

 of adjacent streams, especially into the branches of the Kaskaskia 

 where these come nearest to the Embarras, and into those of the 

 Big Vermilion of the Illinois which are nearest to the Little 

 Vermilion of the Wabash. Some species, however, remain care- 

 fully within the tributaries of the Wabash system. 



It seems possible that this appearance of an approach to 

 the state and entrance upon its territory from opposite directions 

 is not altogether deceptive, and that the annual movements of 

 the fishes of the state, up the streams at the time of the spring 

 floods, downwards with the recession of the waters, and still 

 farther downwards, for many species, into deeper water in the 

 winter, may take these two contingents of our fish population 

 in opposite directions, from and towards local centers of popula- 

 tion for the species, situated on opposite sides of the state. 

 Whether and where such local centers of population actually 

 exist, is a question which can not be answered definitely for 

 lack of numerical or statistical data in the faunal lists and other 

 literature of geographical distribution for the surrounding states. 

 If they exist, the Wabash fishes would constitute one such sys- 

 tem, and those of the Mississippi and its tributaries, another. 



If we may speculate still further upon this subject, we may 

 perhaps surmise that a general critical analysis of the fish popu- 

 lation of the larger area of which Illinois forms the central part, 

 would enable us to distinguish fairly well-defined districts, each 

 with its characteristic assemblage of prevalent species, so asso- 

 ciated and ecologically related as to form a balanced assemblage 

 of species, all so adjusted to each other and so advantageously 

 placed in their environment as to constitute a closed system, 



