r course of events the southern hemisphere would in
its turn be subjected to a severe Glacial period, with the northern
hemisphere rendered warmer; and then the southern temperate forms would
invade the equatorial lowlands. The northern forms which had before been
left on the mountains would now descend and mingle with the southern
forms. These latter, when the warmth returned, would return to their
former homes, leaving some few species on the mountains, and carrying
southward with them some of the northern temperate forms which had
descended from their mountain fastnesses. Thus, we should have some
few species identically the same in the northern and southern temperate
zones and on the mountains of the intermediate tropical regions. But
the species left during a long time on these mountains, or in opposite
hemispheres, would have to compete with many new forms and would be
exposed to somewhat different physical conditions; hence, they would
be eminently liable to modification, and would generally now exist as
varieties or as representative species; and this is the case. We must,
also, bear in mind the occurrence in both hemispheres of former Glacial
periods; for these will account, in accordance with the same principles,
for the many quite distinct species inhabiting the same widely separated
areas, and belonging to genera not now found in the intermediate torrid
zones.

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 or slightly modified species have migrated from the north to
the south, than in a reversed direction. We see, however, a few southern
forms on the mountains of Borneo and Abyssinia. I suspect that this
preponderant migration from the north to the 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
the two sets became commingled in the equatorial regions, during the
alternations of the Glacial periods, the northern forms were the more
powerful and were able to hold their places on the mountains, and
afterwards migrate southward with the southern forms; but not so the
southern in regard to the northern forms. In the same manner, at the
present day, w