e see that very many European productions cover the ground
in La Plata, New Zealand, and to a lesser degree in Australia, and have
beaten the natives; whereas extremely few southern forms have become
naturalised in any part of the northern hemisphere, 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 forty or fifty years from Australia. The Neilgherrie Mountains
in India, however, offer a partial exception; for here, as I hear from
Dr. Hooker, Australian forms are rapidly sowing themselves and becoming
naturalised. Before the last great Glacial period, no doubt the
intertropical mountains were stocked with endemic Alpine forms;
but these have almost everywhere 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 those which have become naturalised; and this is the
first stage towards their extinction. Mountains are islands on the land;
and their inhabitants have yielded to those produced within the larger
areas of the north, just in the same way as the inhabitants of real
islands have everywhere yielded and are still yielding to continental
forms naturalised through man's agency.

The same principles apply to the distribution of terrestrial animals and
of marine productions, in the northern and southern temperate zones, and
on the intertropical mountains. When, during the height of the Glacial
period, the ocean-currents were widely different to what they now are,
some of the inhabitants of the temperate seas might have reached
the equator; of these a few would perhaps at once be able to migrate
southwards, by keeping to the cooler currents, while others might remain
and survive in the colder depths until the southern hemisphere was in
its turn subjected to a glacial climate and permitted their further
progress; in nearly the same manner as, according to Forbes, isolated
spaces inhabited by Arctic productions exist to the present day in the
deeper parts of the northern temperate seas.

I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which
now live so widely separated in the north and south, and sometimes on
the intermediate mountain ranges, are removed on the views above given.
The