surrounded by strangers will have had to compete with many new forms of
life; and it is probable that selected modifications in their
structure, habits, and constitutions will have profited them. Thus many
of these wanderers, though still plainly related by inheritance to
their brethren of the northern or southern hemispheres, now exist in
their new homes as well-marked varieties or as distinct species.
It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many
more identical plants and allied forms have apparently migrated from
the north to the south, than in a reversed direction. We see, however,
a few southern vegetable forms on the mountains of Borneo and
Abyssinia. I suspect that this preponderant migration from north to
south is due to the greater extent of land in the north, and to the
northern forms having existed in their own homes in greater numbers,
and having consequently been advanced through natural selection and
competition to a higher stage of perfection or dominating power, than
the southern forms. And thus, when they became commingled during the
Glacial period, the northern forms were enabled to beat the less
powerful southern forms. Just in the same manner as we see at the
present day,
that very many European productions cover the ground in La Plata, and
in a lesser degree in Australia, and have to a certain extent beaten
the natives; whereas extremely few southern forms have become
naturalised in any part of Europe, though hides, wool, and other
objects likely to carry seeds have been largely imported into Europe
during the last two or three centuries from La Plata, and during the
last thirty or forty years from Australia. Something of the same kind
must have occurred on the intertropical mountains: no doubt before the
Glacial period they were stocked with endemic Alpine forms; but these
have almost everywhere largely yielded to the more dominant forms,
generated in the larger areas and more efficient workshops of the
north. In many islands the native productions are nearly equalled or
even outnumbered by the naturalised; and if the natives have not been
actually exterminated, their numbers have been greatly reduced, and
this is the first stage towards extinction. A mountain is an island on
the land; and the intertropical mountains before the Glacial period
must have been completely isolated; and I believe that the productions
of these islands on the land yielded to those produced within the
larger areas of the north, just in the same way as the productions of
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