The Food of Fishes. 
27 
Notonecta twenty-nine per cent., and larval Chironomus 
only two per cent. The search for the cause of this differ- 
ence leads naturally to an examination of the whole econ- 
omy of these little fishes, and opens up the question of 
their origin as a group. 
The close relation of the Etheostomatidse to the Perci- 
dae requires us to believe that the two groups have but re- 
cently diverged, if, indeed, they are yet distinctly sep- 
arate. 
We must inquire, therefore, into the causes which have 
operated upon a group of percoids to limit their range to 
such apparently unfavorable situations, to diminish their 
size, to develop unduly the paired fins and reduce the air- 
bladder, to remove the scales of several species more or 
less completely from the head, breast, neck, and ventral 
region, and to restrict their food chiefly to the few forms 
Mentioned above. 
No species can long maintain itself anywhere which 
cannot, in some way, find a sufficient supply of food, and 
also protect itself against its enemies. In the contest with 
its enemies it may acquire defensive structures or powers 
of escape sufficient for its protection, 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 shal- 
lows of a stream, where the water dashes ceaslessly by 
with a swiftness few fish can stem? And if, at the same 
time, the refugee develops a swimming power which en- 
ables 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 small cloth net, and we shall find larvag 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. Here the principles of natural selection 
assert their power. The limited supply of food early ar- 
rests the growth of the young; while every fish which 
passes the allowable maximum is forced for food to brave 
