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 to a
great height under the equator. The various beings thus left stranded
may be compared with savage races of man, driven up and surviving in
the mountain-fastnesses of almost every land, which serve as a record,
full of interest to us, of the former inhabitants of the surrounding
lowlands.
CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION _continued_.
Distribution of fresh-water productions. On the inhabitants of oceanic
islands. Absence of Batrachians and of terrestrial Mammals. On the
relation of the inhabitants of islands to those of the nearest
mainland. On colonisation from the nearest source with subsequent
modification. Summary of the last and present chapters.
As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
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