DISPERSION OF FRESH- WA TER FISHES. 1 1 5 



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 " zoological 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 " In this isolated region of North America, in this zoological 

 island of New England, as we may call it, we find neither Lepidos- 

 teus, nor Amia, nor Polyodon, nor Amblodon {Aplodinotus), nor 

 Grystes {Micropterus), nor Centrarchus, nor Pomoxis, nor Am- 

 bloplites, norCalliurus [Chcenobryttics], 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." — 

 Agassiz, Amer. Journ. Set. Arts, 1854. 



