DISPERSION OF FRESH-WATER FISHES. 1 09 



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 distribution, and within this range we may 

 be reasonably certain to find it in any suitable 

 waters. 



But every species has beyond question some 

 sort of limit to its distribution, some sort of bar- 

 rier which it has never passed in all the years of 

 its existence. That this is true becomes evident 

 when we compare the fish-faunae of widely sepa- 

 rated rivers. Thus the Sacramento, Connecticut, 

 Rio Grande, and St. John's Rivers have not a 

 single species in common ; and with one or two 

 exceptions, not a species is common to any two 

 of them. None of these 1 has any species pecu- 

 liar to itself, and each shares a large part of its 

 fish-fauna with the water-basin next to it. It is 

 probably true that the faunae of no two distinct 

 hydrographic basins are wholly identical, while on 

 the other hand there are very few species con- 

 fined to a single one. The supposed cases of this 

 character, some twenty in number, occur chiefly 

 in the streams of the South Atlantic States and of 

 Arizona. All of these need, however, the confir- 

 mation of further exploration. It is certain that 

 in no case has an entire river-fauna 2 originated 

 independently from the divergence into separate 

 species of the descendants of a single type. 



The existence of boundaries to the range of 



1 Except possibly the Sacramento. 



2 Unless the fauna of certain cave-streams in the United States 

 and Cuba be regarded as forming an exception. 



