28 
The Food of Fishes. 
the dangers of the deeper waters, where the chances are 
that it falls a prey. On the other hand, the smaller the 
size of those which escape this alternative, the less likely 
will they be to attract the appetite of the small gar or 
other guerilla which may occasionally raid their retreat, 
and the more easily will they slip about under stones in 
search of their microscopic game/ 
Like other fishes, the darters must have their periods 
of repose, all the more urgent because of the constant 
struggle with the swift current which their habitat im- 
poses. Shut out from the deep, still pools and slow ed- 
dies where the larger species lurk, they are forced to 
spend their leisure on or beneath the bottom of the 
stream, resting on their extended pectorals and anal, or 
wholly buried in the sand. Possibly this fact is correlat- 
ed with the absence or rudimentary condition of the air- 
bladder; as it is a rule with many exceptions — but still, 
probably, a rule — that this organ is wanting in fishes 
which live chiefly at the bottom. 
Doubtless the search for food has much to do with this 
selection of a habitat. I have found that the young of 
nearly all species of our fresh-water fishes are competi- 
tors for food, feeding almost entirely on Entomostraca 
and the larvae of minute Diptera.f As a tree sends out its 
roots in all directions in search of nourishment, so each 
of the larger divisions of animals extends its various 
groups into every place where available food occurs, each 
group becoming adapted to the special features of its 
situation. Given this supply of certain kinds of food, 
nearly inaccessible to the ordinary fish, it is to be expect- 
ed that some fishes would become especially fitted to its 
utilization. Thus the Etheostomatidae as a group are ex- 
plained, in a word, by the hypothesis of the progressive 
adaptation of the young of certain Percidae to a peculiar 
place of refuge and a peculiarly situated food supply. 
Perhaps we may, without violence, call these the moun- 
taineers among fishes. Forced from the populous and fer- 
tile valleys of the river beds and lake bottoms, they have 
• In Boleosoma, which is normally scaled in front of the dorsal fin. we 
often find the skin of this region bare in large specimens, and showing 
evident signs of rubbing. 
t Several of the Catostomidae (suckers) are an exception to this rule, 
feeding when young chiefly on algae and Protozoa. 
