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, T 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- 



