real islands have everywhere lately yielded to continental forms,
naturalised by man s agency.
I am far from supposing that all difficulties are removed on the view
here given in regard to the range and affinities of the allied species
which live in the northern and southern temperate zones and on the
mountains of the intertropical regions. Very many difficulties remain
to be solved. I do not pretend to indicate the exact lines and means of
migration, or the reason why certain species and not others have
migrated;
why certain species have been modified and have given rise to new
groups of forms, and others have remained unaltered. We cannot hope to
explain such facts, until we can say why one species and not another
becomes naturalised by man s agency in a foreign land; why one ranges
twice or thrice as far, and is twice or thrice as common, as another
species within their own homes.
I have said that many difficulties remain to be solved: some of the
most remarkable are stated with admirable clearness by Dr. Hooker in
his botanical works on the antarctic regions. These cannot be here
discussed. I will only say that as far as regards the occurrence of
identical species at points so enormously remote as Kerguelen Land, New
Zealand, and Fuegia, I believe that towards the close of the Glacial
period, icebergs, as suggested by Lyell, have been largely concerned in
their dispersal. But the existence of several quite distinct species,
belonging to genera exclusively confined to the south, at these and
other distant points of the southern hemisphere, is, on my theory of
descent with modification, a far more remarkable case of difficulty.
For some of these species are so distinct, that we cannot suppose that
there has been time since the commencement of the Glacial period for
their migration, and for their subsequent modification to the necessary
degree. The facts seem to me to indicate that peculiar and very
distinct species have migrated in radiating lines from some 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 Glacial period, when the antarctic lands, now covered with ice,
supported a highly peculiar and isolated flora. I suspect that before
this flora was exterminated by the Glacial epoch, a few forms were
widely dispersed to various points of the southern hemisphere by
occasional means of transport, and by the aid, as halting-places, of
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