108 SCIENCE SKETCHES. 



the Darters occupy in the waters of the Mississippi 

 Valley. 



By the same process of " analogous variation " 

 the Cichlidce 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 Dmm} 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 

 (Coitus] comparatively ancient and probably origi- 

 nating in the Pacific, the other {Triglopsis} 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 Aplodi 'notus grunnien s Rafinesque. 



2 Hysterocarpus traski Gibbons. 



3 Lota lota Linnaeus. 



