The Food of Fishes. 
21 
of a species, and may properly be made the nucleus about 
which all the facts of its natural history are gathered. 
In a paper on the food of Illinois fishes published in the 
second bulletin of this Laboratory, the subject was treat- 
ed in a general and cursory way, the amount of material 
upon which that paper was based being insufficient for 
exact or detailed description. The favor with which that 
preliminary notice was received, has made it possible to 
undertake a more serious investigation; and this paper 
contains an account of the food of the Acanthopteri of 
the State which I believe to be nearly or quite sufficient 
for the student of science and for the ymactical fish-cul- 
turist. It is still necessary only to study the food of 
specimens under a half-inch length, and to test the value 
of the general conclusions here reached, by occasional 
examinations of fishes taken from other waters at other 
seasons of the year. Among the results of this study, 
those relating to the food of the young are especially 
worthy of attention, and these have therefore been 
summed up separately. 
The explanation of certain structural conditions about 
the mouth, throat and gills, has proceeded so far as to 
make it very likely that a number of definite general cor- 
respondences between structure and food will be made 
out, which will enable us to tell with considerable ac- 
curacy and detail what the food of an unknown fish must 
be, by a mere inspection of the fish itself ; provided, of 
course, that we know what food is accessible to it in its 
habitat. It seems likely to prove to be a general rule that 
a fish makes scarcely more than a mechanical selection 
from the articles of food accessible to it, taking 
almost indifferently whatever edible things the water 
contains which its habitual range and its peculiar alimen- 
tary apparatus enable it to appropriate, and eating of 
these in about the ratio of their relative abundance and 
the ease with which they can be appropriated at any time 
and place. If this is so, knowing the structure of a fish 
and the contents of a body of water, we shall be able to 
tell, a priori , what the fish will eat if placed therein. 
This is, in fact, the objective point of the present inves- 
tigation — to arrive at a knowledge of the correlations of 
