95 
Six specimens of Htheostoma jlabellare var. lineolata, from four 
ocalities, had eaten sixty-one per cent. of Chironomus_ larva, 
wenty-seven per cent. larve of small ephemerids, and twelve per 
rent. of Copepoda (Cyclops). . 
- Boleichthys elegans, found only in the southern part of the State 
three specimens examined), had eaten only dipterous larve (thirty- 
‘even per cent.) and ephemerid larve (sixty-three per cent.). This 
s a larger, heavier species than most of the others, and, therefore, 
ike Alvordius, prefers ephemerids to gnats. 
Last and least comes Microperca punctulata, represented by nine 
specimens from four localities in Northern Illinois. This smallest of 
the darters shares with Percina, the largest, the peculiarity of a 
arge ratio of crustacean food, which made up sixty-four per cent. of 
he total. The principal kinds were Cyclops, Chydorus, young Gam- 
narus fasciatus, Say, and young Crangonyx gracilis, Smith. The re- 
naining elements were Chironomus larve (thirty-four per cent.) and 
i trace of ephemerids (two per cent.). 
It will be seen that the family, taken as a whole, divides into two 
sections, distinguished by the abundance or deficiency of crustacean 
‘ood. Thisis easily explained by the fact that Percina and Microperca 
‘ange much more freely than the other genera—being frequently 
ound among weeds and Alge in comparatively slow water with 
nuddy bottom, while the others are rather closely contined to swift 
md rocky shallows. | 
In discussing the food of the whole group, taken as a unit, it may 
yest be compared with the food of the young of other percoids. It is 
thus seen to be remarkable for the predominance of the larve 
of Chironomus and small Ephemeride—the former of these 
somprising forty-four per cent. and the latter twenty-three per cent. 
of the whole food of the seventy specimens. In young black bass 
Micropterus pallidus), on the other hand, the averages of nine 
specimens, ranging from five-eighths inch to one and a half inches 
n length, were, in general terms, as follows: Cladocera forty-two 
ger cent., Copepoda seven per cent., young fishes twenty per cent., 
Jorixa and young Notonecta twenty-nine per cent., and larval Chi- 
ronomus only two per cent. The search for the cause of this 
lifference leads naturally to an examination of the whole economy 
of these little fishes, and opens up the question of their origin as a 
zroup. 
The close relation of the Etheostomatide to the Percide requires 
as to believe that the two groups have but recently diverged, if, 
ndeed, they are yet distinctly separate. 
We must inquire, therefore, into the causes which have operated 
ipon a group of percoids to limit their range to such apparently 
anfavorable situations, to diminish their size, to develop unduly the 
daired fins and reduce the air-bladder, to remove the scales of sev- 
ral 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 can not, 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 
