As the cold became more and more intense, we know
that Arctic forms invaded the temperate regions; and from the facts just
given, there can hardly be a doubt that some of the more vigorous, dominant
and widest-spreading temperate forms invaded the equatorial lowlands. The
inhabitants of these hot lowlands would at the same time have migrated to
the tropical and subtropical regions of the south, for the southern
hemisphere was at this period warmer. On the decline of the Glacial
period, as both hemispheres gradually recovered their former temperature,
the northern temperate forms living on the lowlands under the equator,
would have been driven to their former homes or have been destroyed, being
replaced by the equatorial forms returning from the south. Some, however,
of the northern temperate forms would almost certainly have ascended any
adjoining high land, where, if sufficiently lofty, they would have long
survived like the Arctic forms on the mountains of Europe. They might have
survived, even if the climate was not perfectly fitted for them, for the
change of temperature must have been very slow, and plants undoubtedly
possess a certain capacity for acclimatisation, as shown by their
transmitting to their offspring different constitutional powers of
resisting heat and cold.
In the regular course of events the southern hemisphere would in its turn
be subjected to a severe Glacial period, with the northern hemisphere
rendered warmer; and then the southern temperate forms would invade the
equatorial lowlands.
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