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 Salmo fario L., in Europe; Salmo labrax Pallas, etc, in 
Asia; Salmo gairdneri Richardson, in streams of the Pacific 
Coast. Salmo 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. 
