What it was we have now to inquire.
CHAPTER XIX. MAN AND THE GREAT ICE-AGE
In discussing the development of plants and animals during the
Tertiary Era we have already perceived the shadow of the
approaching Ice-Age. We found that in the course of the Tertiary
the types which were more sensitive to cold gradually receded
southward, and before its close Europe, Asia, and North America
presented a distinctly temperate aspect. This is but the penumbra
of the eclipse. When we pass the limits of the Tertiary Era, and
enter the Quaternary, the refrigeration steadily proceeds, and,
from temperate, the aspect of much of Europe and North America
becomes arctic. From six to eight million square miles of the
northern hemisphere are buried under fields of snow and ice, and
even in the southern regions smaller glacial sheets spread from
the foot of the higher ranges of mountains.
It is unnecessary to-day to explain at any length the evidences
by which geologists trace this enormous glaciation of the
northern hemisphere. There are a few works still in circulation
in which popular writers, relying on the obstinacy of a few older
geologists, speak lightly of the "nightmare" of the Ice-Age.
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