DISPERSION OF FRESH-WATER FISHES. 89 



a period of greater elevation of the land, and of conse- 

 quent greater cold, would congeal the waters and cover 

 their courses with glaciers. The fishes would be driven 

 to the neighborhood of the coast, though no doubt in 

 more southern latitudes a sufficient extent of uncongealed 

 fresh waters would flow by a short course into the ocean, 

 to preserve from destruction many forms of fresh-water 

 fishes. Thus, through many vicissitudes, the fauna of a 

 given system of rivers has had opportunity of uninter- 

 rupted descent, from the time of the elevation of the 

 mountain range, in which it has its sources. . . . 



" As regards the distinction of species in the discon- 

 nected basins of different rivers, which have been sepa- 

 rated from an early geologic period, if species occur 

 which are common to any two or more of them, the sup- 

 porter 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 develop- 

 mentalist, on the other hand, will accept the last propo- 

 sition, or else suppose that time has seen an identical 

 process and similar result of modification in these dis- 

 tinct 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, 



