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 so on. 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 



