e north, just in the same way as the productions of
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 {381} 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 {382} 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 h