130 SCIENCE SKETCHES. 



middle of Sweden was submerged, and the Baltic 

 was a great gulf of the Glacial Ocean, and not con- 

 nected with the German Ocean. By the gradual 

 elevation of the Scandinavian Continent, the Baltic 

 became disconnected from the Glacial Ocean, and 

 the great lakes separated from the Baltic. In 

 consequence of the gradual change of the salt 

 water into fresh, the marine fauna became gradu- 

 ally extinct, with the exception of the glacial forms 

 mentioned above." 



It is possible that the presence of marine types 

 in our Great Lakes is to be regarded as due to 

 some depression of the land which would connect 

 their waters with those of the Gulf of St. Law- 

 rence. On this point, however, our data are still 

 incomplete. 



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,! very closely related 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* White-fish 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, 



1 Sahno fario L., in Europe ; Salmo labrax Pallas, etc., in 

 Asia; Salmo gairdneri 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. 



