due to the capacity of snow and ice for storing up cold,
and its singular power (when in large masses) of preserving itself unmelted
under a hot sun by itself causing the interposition of a protective
covering of cloud and vapour. But mobile currents of water have no such
power of {88} accumulating and storing up heat or cold from one year to
another, though they do in a pre-eminent degree possess the power of
equalising the temperature of winter and summer and of conveying the
superabundant heat of the tropics to ameliorate the rigour of the Arctic
winters. However great was the difference between the amount of heat
received from the sun in winter and summer in the Arctic zone during a
period of high excentricity and winter in _aphelion_, the inequality would
be greatly diminished by the free ingress of warm currents to the polar
area; and if this was sufficient to prevent any accumulation of ice, the
summers would be warmed to the full extent of the powers of the sun during
the long polar day, which is such as to give the pole at midsummer actually
more heat during the twenty-four hours than the equator receives during its
day of twelve hours. The only difference, then, that would be directly
produced by the changes of excentricity and precession would be, that the
summers would be at one period almost tropical, at the other of a more mild
and uniform temperate character; while the winters would be at one time
somewhat longer and colder, but never, probably, more severe than they are
now in the west of Scotland.

But though high excentricity would not directly modify the mild climates
produced by the state of the northern hemisphere which prevailed during
Cretaceous, Eocene, and Miocene times, it might indirectly affect it by
increasing the mass of Antarctic ice, and thus increasing the force of the
trade-winds and the resulting northward-flowing warm currents. Now there
are many peculiarities in the distribution of plants and of some groups of
animals in the southern hemisphere, which render it almost certain that
there has sometimes been a greater extension of the Antarctic lands during
Tertiary times; and it is therefore not improbable that a more or less
glaciated condition may have been a long persistent feature of the southern
hemisphere, due to the peculiar distribution of land and sea which favours
the production of ice-fields and glaciers. And as we have seen that during
the last three million years the excent