(72) 



We ought also to gain, by this means, some addition to our knowledge 

 of the causes of variation, of the origin and increase, the decline and 

 extinction of species, and of the remarkable persistence of such forms as the 

 river gar. What groups crowd upon each other in the struggle for sub- 

 sistence ? Do closely allied species, living side by side, ever compete for 

 food 1 What relation, if any, do specific and generic diflferences bear to 

 differences of food 1 These, and many similar questions, may not improba- 

 bly be helped toward a solution. 



Several structures not now fully understood, ought to receive their ex- 

 planation. The variously developed grinding surfaces on the pharyngeal 

 teeth of some cyprinoids, the differences in the structure of the gill-rakers 

 among sun-fishes and of the lips among suckers, are cases in point. 



It seems likely, however, that the food habits of fishes will be found, 

 like their structure, much less highly differentiated than those of birds. 

 This is what we should expect a priori, and it is indicated by the observa- 

 tions I have made upon both classes. * Prominent peculiarities, having appar- 

 ently an important bearing upon the taking of food, will probably be found 

 merely to extend a little the capacities of the species, or to enable it to take 

 those slight advantages of its competitors when the struggle for existence 

 comes to the death grapple, which after all are sufficient to decide the con- 

 test. To bring out such facts as this, a great number of observations will 

 be necessary, covering all varieties of circumstance, and made with refer- 

 ence to the relative proportions of the different elements in the food of each 

 species. The Top Minnows, for example, will probably be found to take 

 the surface-swimming insects inore frequently/ than the C^prinidae do, but 

 not by any means to depend on them chiefly. 



Really intelligent fish-culture, on any large scale, implies a full ac- 

 quaintance with the food of the native species. It is a matter of especial 

 importance that the predaceous fishes should all be known, as well as the 

 kinds of fishes on which each chiefly preys. A knowledge of the food of all 

 species worth saving is, of course, indispensable, in order that proper meas- 

 ures may be taken to preserve their food supplies. It will also be of inter- 

 est to know what fishes there are at once worthless for human food and 

 harmless in their habits, and therefore worth encouraging, or perhaps even 

 hatching, as food for the more valuable "game fishes." The gizzard shad 

 (Dorosoma cepediamwi), seems to be a fish of this character, as it lives 

 chiefly on vegetable food and minute Crustacea, and contributes largely to 

 the food of the marketable fishes. Apparently ignorant of this fact, the 

 fishermen often leave long lines of this species to rot on the bank where the 

 seines are hauled. 



* See "The Food of Birds," in Trans, ill. Hort. Soc, Vol. X, p. 37, 1876. 



