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.^ 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 



^ 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. 



