DISPERSION OF FRESH-WATER FISHES. 109 
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-faunz 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! 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 faunz 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? 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. 
