rvel at the endemic species
which inhabit the several islands of the Galapagos Archipelago not
having all spread from island to island. 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. We see the same
principle in the character of most of the blind animals inhabiting the
caves of America and of Europe. Other analogous facts could be given.
It will, I believe, be found universally 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; and wherever many closely-allied species occur,
there will be found many forms which some naturalists rank as distinct
species, and others as mere varieties; these doubtful forms showing us
the steps in the process of modification.

The relation between the power and extent of migration in certain
species, either at the present or at some former period, and the
existence at remote points of the world of closely allied species, is
shown in another and more general way.