exact lines of migration cannot be indicated. We cannot say why
certain species and not others have migrated; why certain species
have been modified and have given rise to new forms, while 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 species ranges twice or thrice as far, and is
twice or thrice as common, as another species within their own homes.

Various special difficulties also remain to be solved; for instance,
the occurrence, as shown by Dr. Hooker, of the same plants at points
so enormously remote as Kerguelen Land, New Zealand, and Fuegia; but
icebergs, as suggested by Lyell, may have been concerned in their
dispersal. The existence at these and other distant points of the
southern hemisphere, of species, which, though distinct, belong to
genera exclusively confined to the south, is a more remarkable case.
Some of these species are so distinct, that we cannot suppose that there
has been time since the commencement of the last Glacial period for
their migration and subsequent modification to the necessary degree.
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