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.