hecking 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, {404} &c., 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 distinct species, and some as varieties; these doubtful
forms showing us the steps in the process of modification.

This relation between the power and extent of migration of a species,
either at the present time or at some former period under different
physical conditions, and the existence at remote points of the world of
other species allied to it, is shown in another and more general way. Mr.
Gould remarked to me long ago, that in those genera of birds which range
over the world, many