Barriers to Dispersion of River Fishes 107 
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 peculiar 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 faunas of no two 
distinct hydrographic basins are wholly identical, while on 
the other hand there are very few species confined 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 con- 
firmation of further exploration. It is certain that in no case 
has an entire river faunay originated independently from the 
divergence into separate species of the descendants of a single 
type. 
The existence of boundaries to the range of species implies, 
therefore, the existence of barriers to their diffusion. We may 
now consider these barriers and in the same connection the 
degree to which they may be overcome. 
Local Barriers.—Least important to these are the barriers 
which may exist within the limits of any single basin, and 
which tend to prevent a free diffusion through its waters of 
species inhabiting any portion of it. In streams flowing south- 
ward, or across different parallels of latitude, the difference in 
climate becomes a matter of importance. The distribution of 
species is governed very largely by the temperature of the water. 
Each species has its range in this respect,—the free-swimming 
fishes, notably the trout, being most affected by it; the mud- 
loving or bottom fishes, like the catfishes, least. The latter can 
reach the cool bottoms in hot weather, or the warm bottoms in 
cold weather, thus keeping their own temperature more even than 
that of the surface of the water. Although water communica- 
tion is perfectly free for most of the length of the Mississippi, 
there is a material difference between the faunz of the stream 
in Minnesota and in Louisiana. This difference is caused chiefly 
by the difference in temperature occupying the difference in 
latitude. That a similar difference in longitude, with free 
* Except possibly the Sacramento. 
+ Unless the fauna of certain cave streams in the United States and Cuba 
be regarded as forming an exception. 
