growing with
strange luxuriance at the base of the Himalaya, as graphically described
by Hooker.

Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial period
from the northern and southern temperate zones into the intertropical
regions, and some even crossed the equator. As the warmth returned,
these temperate forms would naturally ascend the higher mountains, being
exterminated on the lowlands; those which had not reached the equator,
would re-migrate northward or southward towards their former homes; but
the forms, chiefly northern, which had crossed the equator, would travel
still further from their homes into the more temperate latitudes of the
opposite hemisphere. Although we have reason to believe from geological
evidence that the whole body of arctic shells underwent scarcely any
modification during their long southern migration and re-migration
northward, the case may have been wholly different with those intruding
forms which settled themselves on the intertropical mountains, and in
the southern hemisphere. These being 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 produc