ot 
looks one wheel; and by the time he has studied all these sufti- 
siently, he will find that he has run through the whole complicated 
mechanism of the aquatic life of the locality, both animal and veg- 
atable, of which his species forms but a single element.* 
In such a general survey of the plants and animals of a region, 
the study of their food relations will be found to afford an admirable 
ybjective point. Doubtless, of all the features of the environment 
of an individual, none affect it at the same time so powerfully, so 
variously and so intimately as the elements of its food. Even eli- 
mate, season, soil and the inorganic circumstances generally, influ- 
ence an animal through its food quite as much as by their direct 
action. It is through the food relation that animals touch each 
other and the surrounding world at the greatest number of points, 
here they crowd upon each other the most closely, at this point the 
struggle for existence becomes sharpest and most deadly; and, 
finally, it is through the food relation almost entirely that animals 
are brought in contact with the material interests of man. Both for 
the student of science and for the economist, therefore, we find this 
subject of peculiar interest and value. It includes many of the 
most important relations of a species, and may properly ke made 
the nucleus about which all the facts of its natural history are 
gathered. 
In a paper on the food of Mlinois fishes, published in the second 
bulletin of this Laboratory, the subject was treated in.a general and 
cursory way, the amount of material upon which that paper was 
based being insufficient for exact or detailed description. ‘Lhe favor 
with which that preliminary notice was received, has made it pos- 
sible to undertake a more serious investigation; and this paper con- 
tains an account of the food of the Acanthoptera of the State which 
I believe to be nearly or quite sufficient for the student of science 
and for the practical fish culturist. It is still necessary only to 
study the food of specimens under a half-inch in length, and to test 
the value of the general conclusions here reached, by occasional ex- 
aminations of fishes taken from other waters at other seasons of the 
year. Among the results of this study, those retating to the food of 
the young are especially worthy of attention, and these have there- 
fore 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 correspondences between structure and 
food will be made out, which will enable us to tell with considerable 
accuracy 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 
*I can not too strongly emphasize the fact—frequently illustrated, I venture to hope, by 
‘the papers of this series—that a comprehensive survey of our entire natural history is 
‘absolutely essential to a good working knowledge of those parts of it which chiefly attract 
popular attention,—that is, its edible fishes, its injurious and beneficial insects, and its 
parasitic plants. Such a survey, however, should not stop with a study of the dead forms 
of Nature, ending iu mere lists and descriptions. To have an applicable value, it must 
treat the life of the region as an organic unit, must study it in action, and direct principal 
attention to the laws of its activity. 
