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 1,800 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 which 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 
x2 
