The facts seem to indicate that
distinct species belonging to the same genera have migrated in radiating
lines from a common centre; and I am inclined to look in the southern, as
in the northern hemisphere, to a former and warmer period, before the
commencement of the last Glacial period, when the Antarctic lands, now
covered with ice, supported a highly peculiar and isolated flora. It may
be suspected that before this flora was exterminated during the last
Glacial epoch, a few forms had been already widely dispersed to various
points of the southern hemisphere by occasional means of transport, and by
the aid, as halting-places, of now sunken islands. Thus the southern
shores of America, Australia, and New Zealand may have become slightly
tinted by the same peculiar forms of life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
throughout the world on geographical distribution. And we have now seen
that Mr. Croll's conclusion that successive Glacial periods in the one
hemisphere coincide with warmer periods in the opposite hemisphere,
together with the admission of the slow modification of species, explains a
multitude of facts in the distribution of the same and of the allied forms
of life in all parts of the globe. The living waters have flowed during
one period from the north and during another from the south, and in both
cases have reached the equator; but the stream of life has flowed with
greater force from the north than in the opposite direction, and has
consequently more freely inundated the south.
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