invasion would, of course, have been greatly favoured by high land, and
perhaps
by a dry climate; for Dr. Falconer informs me that it is the damp with
the heat of the tropics which is so destructive to perennial plants
from a temperate climate. On the other hand, the most humid and hottest
districts will have afforded an asylum to the tropical natives. The
mountain-ranges north-west of the Himalaya, and the long line of the
Cordillera, seem to have afforded two great lines of invasion: and it
is a striking fact, lately communicated to me by Dr. Hooker, that all
the flowering plants, about forty-six in number, common to Tierra del
Fuego and to Europe still exist in North America, which must have lain
on the line of march. But I do not doubt that some temperate
productions entered and crossed even the _lowlands_ of the tropics at
the period when the cold was most intense, when arctic forms had
migrated some twenty-five degrees of latitude from their native country
and covered the land at the foot of the Pyrenees. At this period of
extreme cold, I believe that the climate under the equator at the level
of the sea was about the same with that now felt there at the height of
six or seven thousand feet. During this the coldest period, I suppose
that large spaces of the tropical lowlands were clothed with a mingled
tropical and temperate vegetation, like that now 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
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