emisphere by
occasional means of transport, and by the aid, as halting-places, of
existing and now sunken islands: 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.

* * * * *

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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
have ranged widely within the same country, and as the sea is apparently a
still more impassable barrier, that they never would have extended to
distant countries. But the case is exactly the reverse. Not only have many
fresh-water species, belonging to quite different classes, an enormous
range, but allied species prevail in a remarkable manner throughout the
world. I well rem