Before we consider the biological effect of this great of
refrigeration of the globe, we must endeavour to understand the
occurrence itself. Here we enter a world of controversy, but a
few suggestions at least may be gathered from the large
literature of the subject, which dispel much of the mystery of
the Great Ice-Age.
It was at one time customary to look out beyond the earth itself
for the ultimate causes of this glaciation. Imagine the sheet of
ice, which now spreads widely round the North Pole, shifted to
another position on the surface of the planet, and you have a
simple explanation of the occurrence. In other words, if we
suppose that the axis of the earth does not consistently point in
one direction-- that the great ball does not always present the
same average angle in relation to the sun--the poles will not
always be where they are at present, and the Pleistocene Ice-Age
may represent a time when the north pole was in the latitude of
North Europe and North America. This opinion had to be abandoned.
We have no trace whatever of such a constant shifting of the
polar regions as it supposes, and, especially, we have no trace
that the warm zone correspondingly shifted in the Pleistocene.
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