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. Bs 
“ 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, 
