DISPERSION OF FRESH-WATER FISHES. 15 
scanty fish-fauna as compared with the rivers of 
the South and West. This fact has been noticed 
by Professor Agassiz, who has called New England 
a “ zoélogical island.” ! 
In spite of the fact that barriers of every sort 
are sometimes 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 selection,” 
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 sometime 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 hun- 
dred have been recorded from the basin of the 
Mississippi. From fifty to one hundred of these 
1 “Jn. this isolated region of North America, in this zodlogical 
island of New England, as we may call it, we find neither Lepidos- 
teus, nor Amia, nor Polyodon, nor Amblodon (Aflodinotus), nor 
Grystes (Micropterus), nor Centrarchus, nor Pomoxis, nor Am- 
bloplites, nor Calliurus (Chexobryttus), nor Carpiodes, nor Hyodon, 
nor indeed any of the characteristic forms of North American 
fishes so common everywhere else, with the exception of two Po- 
motis (Lepomis), one Boleosoma, and a few Catostomus.” ~ 
Acassiz, Amer. Journ. Sci. Arts, 1854. 
