208 ON THE DISTRIBUTION OF FRESH WATER FISHES 



assume, however, that the predecessors of our present Cyprinidte, Esocidse, etc., may 

 not have been capable of such change. 



As regards the distinction of species in the disconnected basins of different rivers, 

 which have been separated from an early geologic period, if species occur \\hich are 

 common to any two or more of them, the supporter of the theory of distinct creations 

 must suppose that such species have been twice created, once for each hydrographic 

 basin, or that waters flowing into the one basin have been transferred to another. 

 The developmentalist, on the other hand, will accept the last proposition, or suppose 

 that time has seen an identical process and similar result of modification in these 

 distinct regions. 



Facts of distribution in the eastern district of North America, are these. Several 

 species of fresh-water fishes occur at the same time in many Atlantic basins, from the 

 Merrimac or from the Hudson to the James, and throughout the Mississippi Valley, 

 and in the tributaries of the great Lakes. On the other hand, the species of each 

 river may be regarded as pertaining to four classes, whose distribution has direct 

 reference to the character of the water and the food it offers. First those of the tide 

 waters, of the river channels, bayous and sluggish waters near them, or in the flat 

 lands near the coast; second, those of the river channels of its upper course, where 

 the currents are more distinct; third, those of the creeks of the hill country; fourth, 

 those of the elevated mountain streams which are subject to falls and rapids. 



Agassiz has already alluded in " Lake Superior" to the difi'erences between the 

 lower, middle, and upper waters of rivers in this respect. 



To Class I belong :* 



Perca, Cyprinodontidae (many), 



Lepomis, many sp. Esocidte, 

 Enneacanthus, • Umbridse, 



Acantharchus, Stilbe, 



Hololepis, sp., Hybopsis, sp., 



Gasterosteidce, Moxostoma, 

 Aphrodedirus. 



To Class IV ; 



Boleosoma, Poecilichthys, sp., 



Hyostoma, 



Uranidea, • Chrosomus, 



Argyreus, Salmo fontinalis. 



The majority of the species belong to Class III. 



* Vid. Pr. A. N. Sci. 1S65, 274. 



