13 



clear water ; a moderate current ; a bottom of gravel (prefer- 

 ably covered by a growth of weeds) ; little fluctuation during 

 the year in the volume of the stream or in the character of the 

 water. 



Limestone streams usually yield more species than streams 

 flowing over sandstone, and either more than the streams of 

 regions having metamorphic rocks. Sandy bottoms usually are 

 not favorable to fishes. In general, glacial drift makes a suita- 

 ble river bottom, but the higher temperature usual in regions 

 beyond the limits of the drift gives to certain Southern streams 

 conditions still more favorable. These conditions are all well 

 realized in the Washita river in Arkansas, and in various trib- 

 utaries of the 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 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 sometimes 

 crossed by fresh-water fishes, we must still regard the matter of 

 freedom of water communication as the essential one in deter- 

 mining 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- 

 time 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. 



* " In this isolated region of North America, in this zoological island of New England, as we may 

 call it, we find neither Lep dosteus, nor Amia, nor Polyodon, nor Amblodon [A^lodinotus], nor 

 Grystes [Mio opterus \ , nor Centrarchus, nor Pomoxis, nor Ambloplites, nor Calliurus [Chanohryi- 

 tus), nor C:>rpiodes, nor HyoJon, nor indeed any of the characteristic forms of North American 

 fishes so common everywhere else, with the exception of two Pomotis [L ■pamis) , one Boleosoma, 

 and a few Catostomus." — Agassiz, Amer. Joiirn. Sci. Arts, 1S54. 



