i8 9 i.J 
313 
[Upham" 
passed, tlie southern warmer temperate species spread northward 
along both sides of the Atlantic to boreal and even Arctic regions, 
where they are no longer able to live excepting in isolated colo- 
nies that are preserved here and there in sheltered shallow bays. 
Looking lor causes of these changes of temperature in the 
North Atlantic and the adjoining countries, it seems to me very 
probable that they were due mainly to a formerly larger volume 
of the warm oceanic current which is named the Gulf Stream 
because a considerable part of it issues from the Gulf of Mexico, 
flowing through the Strait of Florida, while perhaps a larger part 
leaves the tropics east of Cuba and the Bahamas. This very 
broad current pours northward to the Arctic regions and there 
enters an otherwise almost completely enclosed ocean, from which 
counter currents nearly at the temperature of melting ice flow 
back along the Labrador coast and in the depths of the Atlantic 
under its warmer surface. But within the Recent epoch, during 
which these climatal changes have taken place, an elevation of a 
large region of Alaska and eastern Siberia has been in progress, 
slowly diminishing the depth and width of Bering Strait. 1 The 
recency of this uplifting, probably still going on, is shown, like 
that of the basin of Hudson Bay, by drift-wood on the sea shores, 
lying far above the level now reached by storm waves at the high- 
est tides. Mr. Hall reports that the current of the shallow Ber- 
ing Strait, which has a maximum depth of only 180 feet and is 
about thirty-six miles wide, passes north into the Arctic Ocean 2 ; 
but this may have been reversed when the strait was formerly much 
larger, being thus an outlet for a part of the waters carried north 
by the Gulf Stream. The North Atlantic and the Arctic Ocean 
could then have received more of its northward warm current, 
giving a milder climate to northeastern North America and 
northwestern Europe and adjacent Arctic lands. On the other 
hand, an outflow from the polar sea through Bering Strait would 
be a frigid current, carrying greater cold to Alaska, British Co- 
lumbia, and the Pacific Coast of the United States. 
So nicely balanced are the conditions on which variations of 
climate depend, that the former depression of Bering Strait, 
through resulting changes in the oceanic circulation, may have 
1 William H. Dali, Alaska and its Resources, pp. 462-466, 
2 Ibid., p. 285, 
