Origin of Species Variorum

But any temperate forms which had reached and crossed the equator would have travelled still farther from their homes into the more temperate latitudes of the opposite hemisphere. Although we have reason to believe from geological evidence that the arctic shells underwent scarcely any modification during their long southern migration and re-migration northward, the case may have been wholly different with the intruding northern 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 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 in the northern hemisphere, now exist in their new homes as well-marked varieties or as distinct species. So it will have been with intruders from the south. ↑

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 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 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 they became commingled during the Glacial period, the northern forms will have been enabled to beat the less powerful southern forms.