merged in one. Thus the common channel catfish {Ictahirus 

 piDictatns Rafinesque) of our rivers has been described as a 

 new species not less than twenty-five times, on account of 

 differences, real or imaginary, but comparatively trifling in value. 



Where species can readily migrate, their uniformity is pre- 

 served ; but whenever a form becomes localized its representa- 

 tives assume some characters not shared by the species as a 

 whole. 



Comparing a dozen fresh specimens of almost any kind of 

 fish from any body of water with an equal number from some- 

 where else, one will rarely fail to find some sort of differences 

 — in size, in form, in color. These differences are obviously 

 the reflex of differences in the environment, and the collector 

 of fishes seldom fails to recognize them as such ; often it is not 

 difficult to refer the effect to the conditions. Thus, fishes from 

 grassy bottoms are darker than those taken from over sand, and 

 those from a bottom of muck are darker still, the shade of 

 color being, in some way not well understood, dependent on the 

 color of the surroundings. Fishes in large bodies of water reach 

 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 being equal, the more remote the 

 localities from each other, the greater are these differences. 



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 geo- 

 graphical distribution of fishes. This is an unbroken gradation 

 in the variations from the least to the greatest — from the pecu- 

 liarities 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 



