ic.

It is not of course supposed that all the modifications here indicated
co-existed at the same time. We have good reason to believe, from the known
distribution of animals in the Tertiary 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 exception
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.[24] We may therefore
admit that the {85} communication between the tropical and Arctic oceans
was occasionally 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 Shanghai 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. 45deg is found within the Arctic circle on the coast
of Norway; and if the latter country did not consist almost wholly of
precipitous snow-clad mountains, it would be capable of supporting most of
the vegetable pr