28 The Food of Fishes. 



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 imposes. Shut 

 out from the deep still pools and slow eddies 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 correlated with the absence or rudi- 

 mentary 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 near- 

 ly all species of our fresh-water fishes are competitors for 

 food, feeding almost entirely on Entomostraca and the lar- 

 vae of minute Diptera.f As a tree sends out its roots in all 

 directions in search of nourishment, so eacli 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 expected that some fishes 

 would become especially fitted to its utilization. Thus the 

 Etheostomatidae as a group are explained, in a word, by 

 the hypothesis of the progressive adaptation of the young 

 of certain Percidse to a peculiar place of refuge and a pe- 

 culiarly 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 

 taken refuge from their enemies in the rocky highlands 



*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. 



■^Several of the Catostomidae (suckers) are an exception to this rule, 

 feeding when young chiefly on algae and Protozoa. 



