In the regular 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.