ble 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 {380} 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 th