tions 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 real islands have
everywhere lately yielded to continental forms, naturalised by man's
agency.

I am far from supposing that all difficulties are removed on the view
here given in regard to the range and affinities of the allied species
which live in the northern and southern temperate zones and on the
mountains of the intertropical regions. Very many difficulties remain
to be solved. I do not pretend to indicate the exact lines and means
of migration, or the reason why certain species and not others have
migrated; why certain species have been modified and have given rise to
new groups of forms, and others have remained unaltered. We cannot hope
to explain such facts, until we can say why one species and not another
becomes naturalised by man's agency in a foreign land; why one ranges
twice or thrice as far, and is twice or thrice as common, as another
species within their own homes.

I have said that many difficulties remain to be solved: some of the
most remarkable are stated with admirable clearness by Dr. Hooker in
his botanical works on the antarctic regions. These cannot be here
discussed. I will only say that as far as regards the occurrence of
identical species a