CHANGES IN PHYSICAL GEOGRAPHY. 323 



absence of crayfishes (if they are really absent) in the 

 rivers Obi, Yenisei, and Lena, and in the great lake 

 Baikal, which lies more than 1,300 feet above the sea, 

 and is frozen over from November to May. Moreover, 

 there can be no doubt that, at a comparatively recent 

 period, the whole of this region, from the Baltic to the 

 mouth of the Lena, was submerged beneath a southward 

 extension of the waters of the Arctic ocean to the Aralo- 

 Caspian Sea and Lake Baikal, and a westward extension 

 to the Gulf of Finland. 



The great lakes and inland seas which stretch, at 

 intervals, from Baikal, on the east, to Wenner in Sweden, 

 on the west, are simply pools, isolated partly by the rising 

 of the ancient sea-bottom and partly by evaporation ; and 

 often completely converted into fresh water by the inflow 

 of the surrounding land-drainage. But the population 

 of these pools was originally the same as that of the 

 Northern Ocean, and a few species of marine crustaceans, 

 mollusks, and fish, besides seals, remain in them as 

 living evidences of the great change wliich has taken place. 

 The same process which, as we shall see, has isolated 

 the Mysis of the Arctic seas in the lakes of Sweden and 

 Finland, has shut up with it other arctic marine Crustacea, 

 such as species of Gammarus and Idothea. And the very 

 same species of Gammarus is imprisoned, along with 

 arctic seals, in the waters of Lake Baikal. 



The distribution of the American crayfishes agrees 

 equally well with the hypothesis of the northern origin of 



T 2 



