Seite - 425 - in The Origin of Species

ALTERNATE GLACIAL rKKIOUS 425 the southern hemisphere was in its turn subjected to a glacial cHmate and permitted their further progress; in nearly the same manner as, according to Forbes, isolated spaces inhab- ited by Arctic productions exist to the present day in the deeper parts of the northern temperate seas. 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, whilst 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 remark- able case. Some of these species are so distinct, that we cannot suppose that there has been time since the commence- ment of the last Glacial period for their migration and sub- sequent 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 genera; 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 .Vnt- arctic 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