476 Illinois State Laboratory of Natural History. 



tion it is adapted. I need hardly recall the fact that the 

 defensive apparatus of one species may have its explanation 

 only in the raptatorial 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 ur perhaps unnoticed unless this is under- 

 stood conditions of climate, season, locality, and the like; 

 and especially may we hope for this when we remember that 

 the distribution 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 have prolonged 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 decided by the presence or 

 absence of some structural modification adapted to carry it 

 safely through a single brief period of unusual scarcity or of 

 extraordinary competition. 



That the study here set forth should give us details not 

 to be otherwise obtained of the struggle for existence among 

 fishes themselves, goes without saying; and that it may thus 

 explain some peculiarities of distribution, seems also probable. 

 I have thought it not impossible that by taking into account 

 all the data collected, and the mass of related facts, structural, 

 biological, and other, that materials might be found bearing 

 on the interesting question of the precedence in time and the 

 relative evolutionary importance of desire and effort on the 

 one hand and structural aptitudes on the other. 



Among the purely practical results to be anticipated, are a 

 more accurate knowledge of the conditions favorable to the 

 growth and multiplication of the more important species; the 

 ability to judge intelligently of the fitness of any body of water) 

 to sustain a greater number or a more profitable assemblage of 

 fishes than those occurring there spontaneously; guidance as 

 to the new elements of food and circumstance which it will be 

 necessary to supply to insure the successful introduction into 

 any lake or stream of a fish not native there; jand a clear recog- 

 nition of the fact that intelligent fish culture must take into 



