DISPERSION OF FRESH-WATER FISHES. 99 
a larger size than the same species in smaller 
streams or ponds. Fishes from foul or sediment- 
laden waters are paler in color and slenderer in 
form than those from waters which are clear and 
pure. Again, it is often true that specimens from 
northern waters are less slender in body than those 
from farther south; and soon. Other things be- 
ing equal, the more remote the localities from each 
other, the greater are these differences. 
In our fresh-water fishes each species on an 
average has been described as new from three to 
four times, on account of minor variations, real or 
supposed. In Europe, where the fishes have been 
studied longer and by more different men, upwards 
of six or eight nominal species have been described 
for each one that is now considered distinct. 
It is evident, from these and other facts, that the 
idea of a separate creation for each species of fishes 
in each river basin, as entertained by Agassiz, is 
wholly incompatible with our present knowledge 
of the specific distinctions or of the geographical 
distribution of fishes. This is an unbroken grada- 
tion in the variations from the least to the greatest, 
— from the peculiarities of the individual, through 
local varieties, geographical sub-species, species, 
sub-genera, genera, families, super-families, and so 
on, until all fish-like vertebrates are included in a 
single bond of union. 
It is, however, evident that not all American 
types of fishes had their origin in America, or even 
first assumed in America their present forms. 
Some of these are perhaps immigrants from 
northern Asia, where they still have their nearest 
