and 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