419 
manent muddiness of its waters as compared with those of the other 
areas in which lime in the soil renders it alkaline. 
The acidity of this southern soil seems not to be of a kind or 
amount to affect the surface waters sensibly and directly, since the 
water samples from this region analyzed by the State Water Survey 
show a soft water, slightly alkaline, and chemically unobjectionable 
as a medium for fishes. 
CLASSIFICATION AND USE OF ECOLOGICAL DATA 
That these conditions are a part, at least, of the cause of the phe- 
nomenal distribution of southern Illinios fishes may be shown by a 
comparison of our ecological data for the fishes of the two lists—one 
composed of those adapted to the conditions of the lower Illinoisan 
glaciation and the other of those avoiding them. In the organiza- 
tion of the data of our collections of Illinois fishes, those concerning 
the character of the water body in which collections were made were 
classified in a way to show the number of collections of each species 
taken from each class of situation. By reducing these numbers to 
ratios of frequency of occurrence, we have a means of exhibiting the 
preference of species with respect to the situations in which each oc- 
curs. Pimephales notatus, for example, was found twenty times 
over a muddy bottom to thirty-four over a bottom of mud and 
sand, and to forty-six over a bottom of rock and sand. Aphredoderus 
sayanus, on the other hand, was found sixty-two times on a muddy 
bottom to nineteen times in each of the other situations. 
By tabulating data of this description separately for each of the 
two lists of species referred to—thirty-four species in the one list and 
thirty-five in the other—and averaging the ratios for each group 
separately, significant evidence was obtained of the factors which 
affect the distribution of these fishes. 
The species which distribute themselves freely over southern Illh- 
nois are those which are generally tolerant of turbid waters, as shown 
by the fact that 32 per cent. of all our collections of this group came 
from muddy streams and ponds, 34 per cent. from situations where 
the bottom was composed largely of rock and sand, and 24 per cent. 
from a bottom of sand and mud. The species avoiding the central 
area of southern Illinois, on the other hand, are, as a rule, intolerant 
