On the Food of Young Fishes. 
r- ~ 
i i 
through natural selection, 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 
unusually large, square and strong. 
The principle of adaptation has here resulted in a dif- 
ferent line of development. While the little Etheostomat- 
idae 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 simi- 
lar and dissimilar, of these two widely separated groups 
of fishes. 
A single specimen of black sucker ( Minytrema melan- 
ops) was too large properly to come within this group; 
but, although six inches long, most of its food was Cy- 
clops (eighty per cent.). Other items were Alona, Dif- 
flugia, Closterium and very young Unios. 
Four chub-suckers (Erimyzon sucetta), two of which 
were three-fourths of an inch, and two an inch and a 
quarter 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. Chydorus was the principal element of 
their food (eighty per cent.), but Pleuroxus, Alona and 
Scapholeberis mucronata were also present. In the two 
larger specimens, locality and date unknown, a surpris- 
ing number and variety of the minutest animal and veg- 
etable forms were found. Squamella, Anursea of several 
species, Rotifer vidgaris and other Rotifera; Difflugia 
and Arcella* among the Protozoa; Chroococcus, Closte- 
* 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. pyriformis, 
acuminata, globulosa, lobostoma and Arcella vulgaris and discoides 
