As the tide leaves its drift
in horizontal lines, 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
latitude 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 serves as a record, full of interest
to us, of the former inhabitants of the surrounding lowlands.
CHAPTER XIII.
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.
FRESH-WATER PRODUCTIONS.
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 formidable barrier, that they would never have extended to
distant countries. But the case is exactly the reverse. Not only have
many fresh-water species, belonging to different classes, an enormous
range, but allied species prevail in a remarkable manner throughout the
world.
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