Origin of Species Variorum

↑ 5 blocks not present in 1866 1869 1872; present in 1859 1860 1861

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 surrounded by strangers will have had to compete with many new forms of life; and it is probable that selected 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 of the northern or southern hemispheres, now exist in their new homes as well-marked varieties or as distinct species.

→ plants and allied forms have 1866

plants and allied forms have apparently 1859 1860 1861

or now slightly modified species have 1869

or slightly modified species have 1872

→ the north to the 1866 1869 1872

north to 1859 1860 1861

→ became commingled 1859 1860 1861 1866

two sets be- came commingled in the equatorial regions, 1869

two sets became commingled in the equatorial regions, 1872

→ Glacial 1859 1860 1861 1866

alternations of the Glacial 1869 1872

→ will have been 1866

were 1859 1860 1861 1869 1872

→ enabled to beat the less powerful 1859 1860 1861 1866

the more powerful and were able to hold their places on the mountains, and afterwards to migrate southward with the 1869 1872

→ forms. 1859 1860 1861 1866

forms; but not so the southern in regard to the northern forms. 1869 1872

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

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~~

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,

perfection

or dominating power, than the southern forms. And thus, when

the

they

→became commingled during the →Glacial

periods,

period,

the northern forms →will have been →enabled to beat the less powerful southern →forms.