2 
in the end as containing food for aquatic plants and animals, and so 
finally for the support of man. Even the sewage of great cities is to be 
classed with the rest as an available resource, capable of being arrested, 
redeemed, and returned to us in acceptable form, provided that certain 
conditions are observed necessary to the protection of these organs of 
digestion against chemical poisoning and against a mechanical overload- 
ing with more food stuffs than they can continuously assimilate. From 
this point of view we may say that as the land loses fertility, the waters 
should gain; and if they do not, it is because of faulty management. 
We may be helped to an analysis and understanding of the con- 
struction and operation of this aquatic apparatus of appropriation and 
assimilation if we so arrange the principal organisms of our Illinois 
waters in the form of a table of feeders and their food, of eaters and 
things eaten, that we may see at a glance for each group of fresh water 
animals both what it feeds upon and what feeds upon it in turn. (Fig. 1.) 
This table, I need hardly say, might be indefinitely complicated and 
enlarged; but I am intending it only to show the main features of the 
relationship. If I had made it to include details and exceptions and 
organisms of secondary importance, or even internal parasites, it would 
have been too complex for our present purpose. 
Notice especially the evident predominance of fishes in this scheme 
of vital relationship, shown by the fact that they feed upon everything 
in the bill of fare from terrestrial wastes to frogs, while they are, on 
the other hand, their own worst enemies, more fishes falling a prey to 
other fishes than to all other aquatic enemies combined. Further, if we 
take account not only of the food of fishes, but also of the food of their 
food, we shall see that it covers every item on our table excepting a few 
at the lower right-hand corner relating to turtles, serpents, birds, and 
mammals, including, of course, man; and that even these exceptional 
groups themselves all feed on fishes. It is thus graphically evident to us 
that to understand the ecology of fishes completely we must study also 
the ecology of every class of living things in the midst of which they 
live. We must even go outside the aquatic environment and analyze the 
relations of fishes to their terrestrial enemies, and to ‘many terrestrial 
sources of their food. To handle anything so complex, we must have 
the aid of such groupings and classifications as our materials will per- 
