•^0 2 Barriers to Dispersion of River Fishes 



realized in the AA^ashita River in Arkansas, and in various trib- 

 utaries of tlie Tennessee, Cumberland, 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 liavc given to them a very scanty fish fauna as com- 

 pared with the rivers of the South and AA'est. This fact has 

 Ijeen noticed bv 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 riA'cr 

 basin, the greater the variety of conditions likely to hie offered 

 in it, and the greater the number of its species. In ease of the 

 divergence of new forms by the processes called "natural selec- 

 tion," the greater the nund-.er 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 nutnljcr of 

 species that actually occur in it, and, freedom of mox'ement 

 liemg assumed, the gi'eater the number of species to be found 

 in any one of its affluents. 



Of the six litUT.lred s])ecies of fishes found in the rivers of the 

 United States, about tAvo hundred ha\-e been recorded from 

 the basin of the Alississippi. From fifty to one hundred of 

 these species can be found m any one of the tributary streams 

 of the size, say, of the Housatomc River or the Charles. In 

 tlie 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 luit t\vent)'-four species. 



The waters of tlie Crreat Basin are not rich in fishes, the 



* "In this isolated ix-g'um of X(irth America, in this zoological island of 

 New England, as we may call it, we find neither Lepidosteus, nor Amia, nor 

 Polyodon, nor Amblodon (.-! plodinotus) , nor Grystcs (Micropicrns) . nor Centriir- 

 chus, nor Pomoxis, nor Ambloplites, nor CalHurns (Clicciiobryllns) , nor Carpiodes, 

 nt.r Hyodon, nor indeed any of the characteristic forms of North American 

 fishes so common everywhere else, with the exception of two Pomotis {Lcpoiiiis) , 

 one Boleosoma,andafewCatostomus."— Ag.^ssiz, Aiiicr. Joiirn. Sci. .\rls, 1S54! 



