stacea to Great
Britain, its antipode, than to any other part of the world:" and Mr. Darwin
adds "Sir J. Richardson also speaks of the reappearance on the shores of
New Zealand, Tasmania, &c. of northern forms of fish. Dr. Hooker informs me
that twenty-five species of algae are common to New Zealand and to {150}
Europe, but have not been found in the intermediate tropical seas."

Many more examples of the kind could easily be brought, but these must
suffice. As to the last-mentioned cases Mr. Darwin explains them by the
influence of the glacial epoch, which he would extend actually across the
equator, and thus account, amongst other things, for the appearance in
Chile of frogs having close genetic relations with European forms. But it
is difficult to understand the persistence and preservation of such
exceptional forms with the extirpation of all the others which probably
accompanied them, if so great a migration of northern kinds had been
occasioned by the glacial epoch.

Mr. Darwin candidly says,[158] "I am far from supposing that all
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." ...
"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." Again he adds, "Various 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." Mr.
Darwin goes on to account for these facts by the probable existence of a
rich antarctic flora in a warm period anterior to the last glacial {151}
epoch. There are indeed many reasons for thinking that a southern
continent, rich in living forms, once existed. One such reason is the way
in which struthious