I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which now
live so widely separated in the north and south, and sometimes on the
intermediate mountain ranges, are removed on the views above given. The
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.
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