It would be hopelessly
tedious to discuss all the exceptional cases of the same species, now
living at distant and separated points; nor do I for a moment pretend that
any explanation could be offered of many instances. But, after some
preliminary remarks, I will discuss a few of the most striking classes of
facts, namely, the existence of the same species on the summits of distant
mountain ranges, and at distant points in the Arctic and Antarctic regions;
and secondly (in the following chapter), the wide distribution of fresh
water productions; and thirdly, the occurrence of the same terrestrial
species on islands and on the nearest mainland, though separated by
hundreds of miles of open sea. If the existence of the same species at
distant and isolated points of the earth's surface can in many instances be
explained on the view of each species having migrated from a single
birthplace; then, considering our ignorance with respect to former
climatical and geographical changes, and to the various occasional means of
transport, the belief that a single birthplace is the law seems to me
incomparably the safest.
In discussing this subject we shall be enabled at the same time to consider
a point equally important for us, namely, whether the several species of a
genus which must on our theory all be descended from a common progenitor,
can have migrated, undergoing modification during their migration from some
one area.
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