On the same continent, also,
pre-occupation has probably played an important part in checking the
commingling of the species which inhabit different districts with nearly
the same physical conditions. Thus, the south-east and south-west corners
of Australia have nearly the same physical conditions, and are united by
continuous land, yet they are inhabited by a vast number of distinct
mammals, birds, and plants; so it is, according to Mr. Bates, with the
butterflies and other animals inhabiting the great, open, and continuous
valley of the Amazons.
The same principle which governs the general character of the inhabitants
of oceanic islands, namely, the relation to the source whence colonists
could have been most easily derived, together with their subsequent
modification, is of the widest application throughout nature. We see this
on every mountain-summit, in every lake and marsh. For Alpine species,
excepting in as far as the same species have become widely spread during
the Glacial epoch, are related to those of the surrounding lowlands; thus
we have in South America, Alpine humming-birds, Alpine rodents, Alpine
plants, etc., all strictly belonging to American forms; and it is obvious
that a mountain, as it became slowly upheaved, would be colonised from the
surrounding lowlands. So it is with the inhabitants of lakes and marshes,
excepting in so far as great facility of transport has allowed the same
forms to prevail throughout large portions of the world.
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