59 



ON THE FOOD RELATIONS OF FRESH-WATEU FISHES. 



A SUM3IAKV AND DISCUSSION.*— £(/ *S'. .1. Forbe.S. 



The principal object of the research reported in the series of papersf 

 of which this is the conchiding numl)er. is to determine more precisely 

 than has hitherto been done the relations to nature ot the various genera 

 and families of the fishes of an interior region. This purpose has led 

 especially to a study of the food relallons of tlae groups, for through these, 

 chiefly, fishes exert their influence on the outer world, and are themselves 

 impressed in turn : and thus have appeared a number of subordinate con- 

 siderations having a bearing, more or less direct, on the main intention 

 of the study. 



An examination of the special relations of their food and feeding struct- 

 ures gives us clues, not only to the present significance of fishes, but also 

 to their past effect on life at large, showing how they must have modified 

 the course of evolution: and the occasional occurence in a fish of food 

 prehensile structures out of present relation to its feeding habits, may 

 throw light on the history of its group, indicating conditions of existence 

 once normal to it but now outgrown. Evidence of similar application may 

 also be obtained bv a comparison of the food of the voung and of the 

 adult. 



The feeding apparatus exhibits some of the most significunt examples 

 (if correlation of structure, important to an acquaintance with the course 

 of development in fishes, but not comprehensible without a Icnowledge of 

 the food for whose appropriation it is adapted. I need hardly recall the 

 fact that the defensive apparatus of one species may have its explanation 

 only in the offensive structures of another. 



We shall find also in a study of the food evidence of the indirect but 

 powerful action of a number of external conditions which take effect only 

 through the food relation, and are incomprehensible or perhaps unnoticed 

 unless this is understood— conditions of climate, season, locality, and the 

 like: and especially may we hope tor this when we remember that the 

 distrilnition and abundance of a species may be determined, not so much 

 by ordinary conditions, as by those prevailing at critical intervals, periods 

 of stress, when a slight advantage or a trivial disability may luive pro- 

 longed and multiplied effects. As the range of a plant is often limited, 

 not by the average temperature of the year, but by the extremes of cold 

 or heat, so the existence of an animal may be tlecided by tlie presence or 

 a])sence of some structural modification adapted to carry it safely through 

 a single brief period of unusual scarcity or of extraordinary competition. 



* Reprint of Article VIII. Vol. II. Bull. 111. State Lab. Nat. Hist. 



I Published at intervals from 1877 to 1888, in the first and second volumes of the Bulletin 

 of this Laboratory, as follows: "The Food of Illinois Fishes" (Vol. I, No. 2. i>i). 71-S;0, "The 

 Food of Fishe.s" (No. 3, pp. 18-6.5: reprinted in report 11!. State Fish Commission, 1884. pp. 

 90-127), "On the Food of Young Fishes" (No. 3, pp. tl(l-7!»), "The Food of the Smaller Fresh- 

 water Fishes" (No. 0, pp. tiS-iU), The First Food of the Common White-fish, (No. 6. pp. !tr>-109). 

 and "Studies of the Food of Fresh- Water Fishes' (Vol. II, Art. VII, pp. 4;3:j-473). 



