96 
defensive structures or powers of escape sufficient for its protec. 
tion, or a reproductive capacity which will compensate for large 
losses, or it may become adapted to some place of refuge where 
other fishes will not follow. What better refuge could a harassed 
fish desire than the hiding-places among stones in the shallows of 
a stream, where the water dashes ceaselessly by with a swiftness 
few fish can stem? And if, at the same time, the refugee develops 
a swimming power which enables it to dart like a flash against the 
strongest current, its safety would seem to be insured. But what 
food could it find in such a place? Let us turn over the stones 
in such a stream, sweeping the roiled water at the same time with 
a smal! cloth net, and we shall find—larve 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 answer- 
ing very well if our immigrant can keep down his size. Here the 
principles of natural selection assert their power. The limited sup- 
ply of food early arrests the growth of the young; while every fish 
which passes the allowable maximum is fore d fer 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 appetite of the small gar or other gueriila which may occasion: 
ally raid their retreat, and the more easily will they slip about 
under stones in search of their micioscopic gume.* 
Like other fishes, the darters must have their periods of repose, 
all the more urgent because of the conttent struggle with the 
swift current which their habitat imposes. Shut out from the’ deez 
still pools and slow eddies where the larger species luk, they are 
forced to spend their leisure on or beneath the boitom 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 bo!tom. 
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 ol 
our fresh-water fishes are ccmpetitcrs for food, feeding almost 
entirely on Entomostraca and the larve of minute diptera.t As 4a 
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 fod. 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 KE heostomatide as a 
group are explained, in a word, by the hypothesis of the progressive 
adaptation of the young of certain Percide io* a peculiar place ol 
refuge and a peculiarly situated food supply. | 
. 
“In, Boleosoma, which is normuly sealed in fron! of the dorsal fin, we often find the 
skin of this region bare in large specimens, and showing «vid ‘nt signs of rubbing. 
tSeveral of the Catostomide (snekers) are au exception to this rule, feeding when 
young chiefly on Algw and Protuzoa. ; 
odaad < ro 
