ve vegetation. The only other opening than that between Iceland and
Britain by which warm water penetrates within the Arctic circle, is through
Behring's Straits; but this is both shallow and limited in width, and the
consequence is that the larger part of the warm currents of the Pacific
turns back along the shores of the Aleutian Islands and North-west America,
while a very small quantity enters the icy ocean.

But if there were other and wider openings into the Arctic Ocean, a vast
quantity of the heated water which is now turned backward would enter it,
and would produce an amelioration of the climate of which we can hardly
form a conception. A great amelioration of climate would also be caused by
the breaking up or the lowering of such {80} Arctic highlands as now favour
the accumulation of ice; while the interpenetration of the sea into any
part of the great continents in the tropical or temperate zones would again
tend to raise the winter temperature, and render any long continuance of
snow in their vicinity almost impossible.

Now geologists have proved, quite independently of any such questions as we
are here discussing, that changes of the very kinds above referred to have
occurred during the Tertiary period; and that there has been, speaking
broadly, a steady change from a comparatively fragmentary and insular
condition of the great north temperate lands in early Tertiary times, to
that more compact and continental condition which now prevails. It is, no
doubt, difficult and often impossible to determine how long any particular
geographical condition lasted, or whether the changes in one country were
exactly coincident with those in another; but it will be sufficient for our
purpose briefly to indicate those more important changes of land and sea
during the Tertiary period, which must have produced a decided effect on
the climate of the northern hemisphere.

_Geographical Changes Favouring Mild Northern Climates in Tertiary
Times._--The distribution of the Eocene and Miocene formations shows, that
during a considerable portion of the Tertiary period, an inland sea, more
or less occupied by an archipelago of islands, extended across Central
Europe between the Baltic and the Black and Caspian Seas, and thence by
narrower channels south-eastward to the valley of the Euphrates and the
Persian Gulf, thus opening a communication between the North Atlantic and
the Indian Oceans. From the Caspian also a wide arm of t