108 SCIENCE SKETCHES. 
the Darters occupy in the waters of the Mississippi 
Valley. 
By the same process of ‘analogous variation” 
the Cichlide of South America parallel the Sun- 
fishes of the United States, although in structure 
and in origin the two groups are diverse. 
The single species each of Drum,! Surf-fish,? and 
Cod? found in our fresh waters are evidently immi- 
grants from the sea, although not of recent origin. 
The several species of Sculpin have apparently 
come from two separate marine stocks, — the one 
(Cottus) comparatively ancient and probably origi- 
nating in the Pacific, the other (Zrzglopsis) more 
modern and descended from an Atlantic species 
(Acanthocottus quadricornis, L.). The former type 
is now diffused in all cold waters of North Amer- 
ica, Europe, and northern Asia. The latter be- 
longs only to the depths of the Great Lakes. 
The Flounders and Soles when found in fresh 
waters are merely temporary sojourners from the 
sea. 
We can say, in general, that in all waters not 
absolutely uninhabitable there are fishes. The 
processes of natural selection have given to each 
kind of river or lake species of fishes adapted to 
the conditions of life which obtain there. There 
is no condition of water, of bottom, of depth, of 
speed of current, but finds some species with 
characters adjusted to it. These adjustments are, 
for the most part, of long standing; and the fauna 
1 Aplodinotus grunniens Rafinesque. 
2 Hysterocarpus traski Gibbons. 
3 Lota lota Linnzus. 
