IO6 SCIENCE SKETCHES. 



of Chironomus and small Ephemerids, and other such 

 prey and little else, food too minute and difficult of 

 access to support a large fish, but answering very well if 

 our immigrant can keep down his size. . . . The limited 

 supply of food early arrests the growth of the young; 

 while every fish which passes the allowable maximum is 

 forced for food to brave 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 appe- 

 tite of the small gar or other guerilla, which may occa- 

 sionally 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 

 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 ventrals and anal, or 

 wholly buried in the sand. . . . 



" Doubtless the search for food has much to do with 

 this selection in a habitat. I have found that the young 

 of nearly all species of our fresh-water fishes are com- 

 petitors for food, feeding almost entirely on Entomostraca 

 and the larvae of minute Diptera. 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 inacces- 

 sible to the ordinary fish, it is to be expected that some 

 fishes would become especially fitted to its utilization. 

 Thus the Etheostomatina [Darters] as a group are to be 



