a ea 
CHAPTER VIII 
BARRIERS TO DISPERSION OF RIVER FISHES 
E Process of Natural Selection.— We can say, in 
general, that in all waters not absolutely uninhabit- 
able 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 stand- 
ing; and the fauna of any single stream has as a rule been 
produced by immigration from other regions or from other 
streams. Each species has an ascertainable range of distribu- 
tion, and within this range we may be reasonably certain to 
find it in any suitable waters. 
Fic 74.—Slippery-dick or Doncella, Halicheres bivittatus Bloch, a fish of the 
coral reefs, Key West. Family Labride. 
But every species has beyond question some sort of limit to 
its distribution, some sort of barrier which it has never passed 
in all the years of its existence. That this is true becomes 
evident when we compare the fish fauna of widely separated 
rivers. Thus the Sacramento, Connecticut, Rio Grande, and 
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