The Food of Younc/ Fishes. 77 



lection, by secondary influences, but manifesting itself 

 where these are not brought into play. 



This species is in marked contrast with the darters, not 

 only in the rapidity of its growth and the ultimate size 

 attained, but in the form and size of the head, which in the 

 darters is small and pointed, but in these fishes is unusu- 

 ally large, square and strong. 



The principle of adaptation has here resulted in a diifer- 

 ent line of development. While the little Etheostomatidge 

 have become fitted to slip and pry about beneath the stones 

 for their food, Hypentelium has acquired the power of 

 rolling the stones before it. As it grows larger, it resorts, 

 of course, to deeper water, but always prefers the rocky 

 reaches of the stream. The moulding power of natural 

 selection could scarcely have a better illustration than 

 that afforded by the adaptive characters, both similar and 

 dissimilar, of these two widely separated groups of fishes. 



A single specimen of hlack sucker' {Minytrema melan- 

 ops) was too large properly to come within this group ; but, 

 although six inches long, most of its food was Cyclops 

 (eighty per cent.). Other items were Alona, Difflugia, 

 Closterium and very young Uniones. 



Four chub-suckers {Frhnyzon sticetta), two of which 

 were three-fourths of an inch, and two an inch and a quar- 

 ter long, differed greatly in food from the foregoing. The 

 two smaller specimens, from Long L., near Pekin, taken 

 June 2, 1880, had eaten only Cladocera, with a trace of 

 water mites. Ohydorus was the principal element of their 

 food (eighty per cent.), but Pleuroxus, Alona and Scapho- 

 leheris mucronata were also present. In the two larger 

 specimens, locality and date unknown, a surprising num- 

 ber and variety of the minutest animal and vegetable 

 forms were found. Squamella, Anurt^a of several species. 

 Rotifer vulgaris and other Rotifera ; Difflugia and Arcella* 

 among the Protozoa ; Chroococcus, Closterium, Oosmarium, 



♦Slides of the food of this genus and Myxostoma were submitted to Dr. 

 Jos. Leidy, of Philadelphia, and Prof. W. S. Barnard, of Cornell Univer- 

 sity, N. Y., and these gentlemen kindly sent me the following names of 

 Rhizopoda as occurring therein : From Prof. Barnard, Difflugia acumi- 

 nata, pyriformis, constricta and globosa ; from Dr. Leidy, D. pyrifonnis, 

 acuminata, globiilosa, lobostoma and A rcella vulgaris and discoides. 



