433 
remainder are very rare in our territory, most of them coming from 
the west and south, and they are extremely insignificant elements of 
our fish fauna. 
3. If the ten stream systems of the state be brought into com- 
parison one with another, it appears that the six larger areas, con- 
taining the largest streams and presenting the greatest variety of 
situations, are much more closely affiliated ichthyologically than are 
the four smaller areas. The least closely affiliated with each other 
and with all the rest are the Michigan district of northeastern Illinois 
and the Big Muddy basin in the southwest. The closest relations are 
those between the Illinois, the Rock, and the Mississipp1. 
4. Inthe absence, in Illinois, of geographical barriers to the dis- 
persal of fishes, the causes influencing their distribution are climatic, 
geologic, and ecological. As [Illinois extends through 5.5° of lati- 
tude, differences of climate between the northern and the southern 
sections of the state are sufficient to affect, in considerable measure, 
the distribution of its plant and animal species—differences which, 
in its ichthyology, express themselves in the presence in northern 
Illinois, but not in southern, of 17 species of general northward 
range; and in southern Illinois, but not in northern, of 14 species of 
general southward range. These two groups of species meet and 
mingle in the great north and south rivers of the western half of the 
state, in an area of common occupation about fifty miles in width, 
from the latitude of Springfield northward; while on the eastern 
boundary of the state, occupied by small streams of various direc- 
tion, 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 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 re- 
