area became almost _wholly_ buried in snow and ice--as was certainly the
case with Northern Europe and America during the glacial epoch--then the
glacial conditions would be continued and perhaps even intensified when the
sun approached nearest to the earth in winter, instead of there being at
that time, as Mr. Croll maintains, an almost perpetual spring. This
important result is supported by reference to the existing differences
between the climates of the northern and southern hemispheres, and by what
is known to have occurred during the last glacial epoch; and it is shown to
be in complete harmony with the geological evidence as to interglacial mild
periods.

Discussing next the evidence for glacial epochs in earlier times, it is
shown that Mr. Croll's views are opposed by a vast body of facts, and that
the geological evidence leads irresistibly to the conclusion that during a
large portion of the Secondary and Tertiary periods, uninterrupted warm
climates prevailed in the north temperate zone, and so far ameliorated the
climate of the Arctic regions as to admit of the growth of a luxuriant
vegetation in the highest latitudes yet explored. The geographical
condition of the northern hemisphere at these periods is then investigated,
and it is shown to have been probably such as to admit the warm tropical
waters freely to penetrate the land, and to reach the Arctic seas by
several channels; and, adopting Mr. Croll's calculations as to the enormous
quantity of heat that would thus be conveyed northwards, it is maintained
that the mild Arctic climates are amply accounted for. With such favourable
geographical conditions, it is shown, that changes of excentricity and of
the phases of precession would have no other effect than to cause greater
differences {536} of temperature between summer and winter; but, wherever
there was a considerable extent of very lofty mountains the snow-line would
be lowered, and the snow-collecting area being thus largely increased a
considerable amount of local glaciation might result. Thus may be explained
the presence of enormous ice-borne rocks in Eocene and Miocene times in
Central Europe, while at the very same period all the surrounding country
enjoyed a tropical or sub-tropical climate.

The general conclusion is thus reached, that geographical conditions are
the essential causes of great changes of climate, and that the radically
different distribution of land and sea in the northern and