I 



have acted since the glacial epoch, when the whole Utah Basin 

 was part of the drainage of the Columbia. 



To certain species of upland or mountain fishes, the 

 depression of the Mississippi basin itself forms a barrier which 

 cannot be passed. The black-spotted trout {Sahno fario L. , 

 in Europe ; Sahno labrax Dallas, etc., in Asia ; Salmo gaird- 

 neri Richardson, in streams of the Pacific Coast. Sahno mykiss 

 Walbaum, in Kamtschatka, Alaska, and throughout the Rocky 

 Mountain range to the Mexican boundary, and the head- 

 waters of the Kansas, Platte, and Missouri), very closely re- 

 lated species of which abound in all waters of northern Asia, 

 Europe, and western North America, has nowhere crossed the 

 basin of the Mississippi, although one of its species finds no 

 difficulty in passing Behring Strait. The trout and whitefish of 

 the Rocky Mountain region are all species different from those 

 of the Great Lakes or the streams of the Alleghany system. 

 To the grayling, the trout, the whitefish, the pike, and to 

 arctic and sub-arctic species generally, Behring Strait has evi- 

 dently proved no serious obstacle to diffusion ; and it is not 

 unlikely that much of the close resemblance of the fresh-water 

 faunjE of northern Europe, Asia and North America is due to 

 this fact. To attempt to decide from which side the first 

 migration came in regard to each group of fishes might be 

 interesting ; but without a wider range of facts than is now in 

 our possession, such attempts would be mere guesswork and 

 without value. The interlocking of the fish-faunas of Asia and 

 North America presents, however, a number of interesting 

 problems, for numerous migrations in both directions have 

 doubtless taken place. 



I could go on indefinitely with the discussion of special 

 cases, each more or less interesting or suggestive in itself, but 

 the general conclusion is in all cases the same. 



The present distribution of fishes is the result of long-con- 

 tinued action of forces still in operation. The species have 

 entered our waters in many invasions from the Old World, or 

 from the sea. Each species has been subjected to the various 

 influences implied in the term natural selection, and under 

 varying conditions, its representatives have undergone many 

 different modifications. 



