106 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 eep 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 Eztomostraca 
and the larvee 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 Z¢heostomatine [Darters] as a group are to be 
