Barriers to Dispersion of River Fishes LII 
realized in the Washita River in Arkansas, and in various trib- 
utaries of the Tennessee, Cuniberland, and Ohio; and in these, 
among American streams, the greatest number of species has 
been. recorded. 
The isolation and the low temperature of the rivers of New 
England have given to them a very scanty fish fauna as com- 
pared with the rivers of the South and West. This fact has 
been noticed by Professor Agassiz, who has called New England 
a “zoological island.”’ * 
In spite of the fact that barriers of every sort are some- 
times crossed by fresh-water fishes, we must still regard the 
matter of freedom of water communication as the essential one 
in determining the range of most species. The larger the river 
basin, the greater the variety of conditions likely to be offered 
in it, and the greater the number of its species. In case of the 
divergence of new forms by the processes called “natural selec- 
tion,” the greater the number of such forms which may have 
spread through its waters; the more extended any river basin, 
the greater are the chances that any given species may some- 
times find its way into it; hence the greater the number of 
species that actually occur in it, and, freedom of movement 
being assumed, the greater the number of species to be found 
in any one of its affluents. 
Of the six hundred species of fishes found in the rivers of the 
United States, about two hundred ,have been recorded from 
the basin of the Mississippi. From fifty to one hundred of 
these species can be found in any one of the tributary streams 
of the size, say, of the Housatonic River or the Charles. In 
the Connecticut River there are but about eighteen species per- 
manently resident; and the number found in the streams of 
Texas is not much larger, the best known of these, the Rio 
Colorado, having yielded but twenty-four species. 
The waters of the Great Basin are not rich in fishes, the 
* “Tn this isolated region of North America, in this zoological island of 
New England, as we may call it, we find neither Lepidosteus, nor Amia, nor 
Polyodon, nor Amblodon (A plodinotus) , nor Grystes (Micropterus), nor Centrar- 
chus, nor Pomoxis, nor Ambloplites, nor Calliurus (Chenobryttus), nor Carpiodes, 
nor Hyodon, nor indeed any of the characteristic forms of North American 
fishes so common everywhere elsc, with the exception of two Pomotis (Lepomis), 
one Boleosoma, and a few Catostomus.’’—AcGassiz, Amer. Journ. Sct, Arts, 1854. 
