48 



abundant of these insect larvse was that known as Chironomus. 

 The suckers and minnows differ from our other fishes by being 

 toothless while very young, as well as when adult, while our 

 other toothless fishes, gizzard shad, whitefish, etc., have in 

 youth a set of evanescent teeth. These toothless young I 

 found feeding in part on still smaller prey than the others, 

 taking the smallest animal forms (wheel animalcules), various 

 Protozoa, and Algae so minute that the whole plant consists of 

 a single vegetable cell. The food of the whitefish fry was 

 determined by keeping several hundred of them in a large 

 aquarium kept constantly supplied with all the living objects 

 which a fine gauze net would separate from the waters of Lake 

 Michigan.* 



While small fishes of all sorts are evidently competitors 

 for food, this competition is relieved to some extent by differ- 

 ences of breeding season, the species dropping in successively 

 to the banquet, some commencing in very early spring, or even, 

 like the whitefish, depositing their eggs in fall, that their young 

 may be the first at the board, while others delay until June or 

 July. The most active breeding period coincides, however, 

 with that of the greatest evolution of Entomostraca in the back- 

 waters of our streams ; that is, the early spring. That large 

 adult fishes with fine and numerous rakers on the gills — like 

 the shovel fish and the river carp — may compete directly with 

 the young of all other species, and tend to keep their numbers 

 down by diminishing their food supply — especially in times of 

 scarcity — is very probable, but is not certainly true as a general 

 thing ; for these larger fishes have other food resources, also, 

 and may resort to Entomostraca only when these are super- 

 abundant, thus appropriating the mere excess above what are 

 required for the young of other groups. Of the fishes which 

 emerge from this earliest stage through increase in size with 

 failure to develop alimentary structures especially fitted to the 

 appropriation of minute animal forms, some become mud-eaters, 

 like the Campostoma and the gizzard shad ; a few apparently 

 become vegetarians at once ; but most pass into or through an 

 insectivorous stage. After this a few become nearly omnivorous, 



* See note following this paper. 



