CHAPTER XVII 



BARRIERS TO DISPERSION OF RIVER FISHES 



(HE 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. 



Fig. 189.— Slippery-dick or Doncella, Halichreres hivitlatus Bloch, a fish of the 

 coral reefs. Key West. Family Lohridm. 



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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