11 



food, consisting mainly of insects, crustaceans, algae, and fragments of 

 aquatic plants, is as different from that of the preceding species as is its 

 distribution. The little heterodon, on the other hand about two inches 

 long is essentially a lake and pond species, and its abundance, not only 

 in the lake region of northeastern Illinois but also along the larger rivers, 

 is explained by the fact that it is mainly in the lowlands of the river bot- 

 toms that lakes and ponds are to be found in Illinois. It is in such sit- 

 uations that it finds the bottoms of mud and sand which seem to attract 

 it, its frequency ratio there being 71 as compared with 22 over rock and 

 sand, and 7 over mud. The food of this species is consistent with this 

 preference of location, being mainly Entomostraca and small larvae of 

 gnats. It is, indeed, essentially a plankton-eater, being of the size to 

 make the plankton of its favorite resorts its most convenient and abundant 

 food. 



I know well that these specific details are hardly fit for a general 

 lecture, but they are the materials of my generalizations, and I must ask 

 you to indulge me to the extent of two more examples, chosen from an- 

 other family of fishes that most interesting division of the perches 

 commonly known as the darters. These are the johnny darter (Fig. 

 19; Map XC), and another species, Cottogaster shumardi (Fig. 20; Map 

 LXXXVIII), which has no English name. They are particularly inter- 

 esting, because the johnny darter, although very abundant all over the 

 state, seems to avoid the larger streams, having a frequency there of 

 only 3 as against 53 for creeks, while the Cottogaster, although compara- 

 tively rare, occurs almost wholly in the larger rivers and in the bottom- 

 land lakes in their immediate neighborhood. Whether there are differ- 

 ences in food corresponding to their distribution we cannot tell, be- 

 cause the food of the rarer species has not been studied. 



Many other instances might be given of the fact that fishes can be 

 separated into groups according to their habitats as well as by differences 

 in their food, but that the groups so formed are of very unequal scope. 

 It is as if, in classifying fishes structurally, we should find that there 

 were some families which combined the characteristics of nearly all the 

 others ; that other kinds present many such common characters, but a 

 smaller number; and that only a few had differentiated so far from the 

 common mass as to have fixed distinguishing characters of their own. 

 An ecological classification, while quite possible, and indispensable also, 

 ought not to be framed in imitation of the classifications of the taxonomist, 

 but must have objects and methods of its own. 



