CHAP. IX.] 
MILD ARCTIC CLIMATES. 
189 
period, that land-communications have at times existed between 
Europe or Asia and North America, either by way of Behring’s 
Straits, or by Iceland, Greenland, and Labrador. But the same 
evidence shows that these land-communications were the excep- 
tion rather than the rule, and that they occurred only at long 
intervals and for short periods, so as at no time to bring about 
anything like a complete interchange of the productions of the 
two continents. 1 We may therefore admit that the com- 
munication between the tropical and Arctic oceans was occa- 
sionally interrupted in one or other direction ; but if we look 
at a globe instead of a Mercator’s chart of the world, we shall 
see that the disproportion between the extent of the polar and 
tropical seas is so enormous that a single wide opening, with an 
adequate impulse to carry in a considerable stream of warm 
water, would be amply sufficient for the complete abolition of 
polar snow and ice, when aided by the absence of any great 
areas of high land within the polar circle, such high land 
being, as we have seen, essential to the production of perpetual 
snow even at the present time. 
Those who wish to understand the effect of oceanic currents 
in conveying heat to the north temperate and polar regions, 
should study the papers of Dr. Croll already referred to. But 
the same thing is equally well shown by the facts of the actual 
distribution of heat due to the Gulf Stream. The difference 
between the mean annual temperatures of the opposite coasts 
of Europe and America is well known and has been already 
quoted, but the difference of their mean winter temperature is 
still more striking, and it is this which concerns us as more 
especially affecting the distribution of vegetable and animal 
life. Our mean winter temperature in the west of England is 
the same as that of the Southern United States, as well as that 
of Shanghae in China, both about twenty degrees of latitude 
further south ; and as we go northward the difference increases, 
so that the winter climate of Nova Scotia in Lat. 45° is found 
within the Arctic circle on the coast of Norway ; and if the latter 
1 For an account of the resemblances and differences of the mammalia 
of the two continents during the Tertiary epoch, see my Geographical 
Distribution of Animals, Vol. I. pp. 140 — 156. 
