stone are annually transported from Porto Santo to Madeira, yet this
latter island has not become colonised by the Porto Santo species:
nevertheless both islands have been colonised by some European
land-shells, which no doubt had some advantage over the indigenous
species. From these considerations I think we need not greatly marvel
at the endemic and representative species, which inhabit the several
islands of the Galapagos Archipelago, not having universally spread
from island to island. In many other instances, as in the several
districts of the same continent, pre-occupation has probably played an
important part in checking the commingling of species under the same
conditions of life. 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.
The principle which determines the general character of the fauna and
flora of oceanic islands, namely, that the inhabitants, when not
identically the same, yet are plainly related to the inhabitants of
that region whence colonists could most readily have been derived, the
colonists having been subsequently modified and better fitted to their
new homes, is of the widest application throughout nature. We see this
on every mountain, in every lake and marsh. For Alpine species,
excepting in so far as the same forms, chiefly of plants, have spread
widely throughout the world during the recent 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
of strictly American forms, and it is obvious
that a mountain, as it became slowly upheaved, would naturally 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 given the same general forms to the whole world. We see
this same principle in the blind animals inhabiting the caves of
America and of Europe. Other analogous facts could be given. And it
will, I believe, be universally found to be true, that wherever in two
regions, let them be ever so distant, many closely allied or
representative species occur, there will likewise be found some
identical species, showing, in accordance with the foregoing view, that
at some former period there has been intercommunication or migration
between the two regions. And wherever many closely-allied species
occur, there will be found many forms which some naturalists rank as
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