t 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 existing and now sunken
islands, and perhaps at the commencement of the Glacial period, by
icebergs. By these means, as I believe, the southern shores of America,
Australia, New Zealand have become slightly tinted by the same peculiar
forms of vegetable life.

Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate on
geographical distribution. I believe that the world has recently felt
one of his great cycles of change; and that on this view, combined with
modification through natural selection, a multitude of facts in the
present distribution both of the same and of allied forms of life can be
explained. The living waters may be said to have flowed during one short
period from the north and from the south, and to have crossed at the
equator; but to have flowed with greater force from the north so as
to have freely inundated the south. As the tide leaves its drift in
horizontal lines, though rising higher on the shores where the tide
rises highest, so have the living waters left their living drift on our
mountain-summits, in a line gently rising from the arctic lowlands t