Origin of Species Variorum

↑ 15 blocks not present in 1859 1860 1861 1869 1872; present in 1866

At one time I had hoped to find evidence that the tropics in some part of the world had escaped the chilling effects of the Glacial period, and had afforded a safe refuge for the suffering tropical productions. We cannot look to the peninsula of India for such a refuge, as temperate forms have reached nearly all its isolated mountain-ranges, as well as Ceylon; we cannot look to the Malay archipelago, for on the volcanic cones of Java we see European forms, and on the heights of Borneo temperate Australian productions. If we look to Africa, we find that not only some temperate European forms have passed through Abyssinia along the eastern side of the continent to its southern extremity; but we now know that temperate forms have likewise travelled in a transverse direction from the mountains of Abyssinia to Fernando Po, aided perhaps in their march by east and west ranges, which there is some reason to believe traverse the continent. But even granting that some one large tropical region had retained during the Glacial period its full warmth, the supposition would be of no avail, for the tropical forms therein preserved could not have travelled to the other great tropical regions within so short a period as has elapsed since the Glacial epoch. Nor are the tropical productions of the whole world by any means of so uniform a character as to appear to have proceeded from any one harbour of refuge. The eastern plains of tropical South America apparently have suffered least from the Glacial period; yet even here there are on the mountains of Brazil a few southern and northern temperate and some Andean forms, which it appears must have crossed the continent from the Cordillera; and some forms on the Silla of Caraccas, which must have migrated from the same great mountain-chain. But Mr. Bates, who has studied with such care the insect-fauna of the Guiano-Amazonian region, has argued with much force against any recent refrigeration in this great region; for he shows that it abounds with highly peculiar endemic Lepidopterous forms, thus apparently contradicting the belief in much recent extinction near the equator. How far his facts can be explained on the supposition of the almost entire annihilation during the Glacial period of a pleistocene equatorial fauna adapted for greater heat than any now prevailing, and the formation of the present equatorial fauna by the commingling of two former sub-tropical faunas, I will not pretend to say. Notwithstanding these several difficulties, we are led to believe that a considerable number of plants, a few terrestrial animals, and some marine productions, migrated during the Glacial period both from the northern and from the southern temperate zones into the intertropical regions, and that some of them even crossed the equator. When the heat returned, these temperate forms will naturally have ascended the higher mountains, being exterminated on the lowlands; and the greater number will have re-migrated northward or southward towards their former homes. 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.

↑ 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.

→ or slightly modified species have 1872

plants and allied forms have apparently 1859 1860 1861

plants and allied forms have 1866

or now slightly modified species have 1869

→ the north to the 1866 1869 1872

north to 1859 1860 1861

→ two sets became commingled in the equatorial regions, 1872

became commingled 1859 1860 1861 1866

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

→ alternations of the Glacial 1869 1872

Glacial 1859 1860 1861 1866

→ were 1859 1860 1861 1869 1872

will have been 1866

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

enabled to beat the less powerful 1859 1860 1861 1866

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

forms. 1859 1860 1861 1866

→ OMIT 1872

as we see 1859 1860 1861 1866

we see 1869

→ we see that 1872

that 1859 1860 1861 1866 1869

→ New Zealand, and to 1869 1872

and in 1859 1860 1861 1866

→ OMIT 1869 1872

to a certain extent 1859 1860 1861 1866

→ the northern hemisphere, 1866 1869 1872

Europe, 1859 1860 1861

→ Before the last great Glacial period, no doubt 1869 1872

Something of the same kind must have occurred on 1859 1860 1861

Before the Glacial period, no doubt 1866

→ OMIT 1866 1869 1872

no doubt before the Glacial period they 1859 1860 1861

→ those which have become naturalised; 1869 1872

the naturalised; 1859 1860 1861

those which have become naturalised there; 1866

→ OMIT 1869 1872

if the natives have not been actually exterminated, their numbers have been greatly reduced, and 1859 1860 1861 1866

→ their inhabitants have 1869 1872

the intertropical mountains before the Glacial period must have been completely isolated; and I believe that the productions of these islands on the land 1859 1860 1861

the intertropical mountains before the Glacial period must have been completely isolated; and I believe that the inhabitants of these islands on the land 1866

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,

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

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

they

the

→two sets became commingled in the equatorial regions, during the →alternations of the Glacial

period,

periods,

the northern forms →were →the more powerful and were able to hold their places on the mountains, and afterwards to migrate southward with the southern →forms; but not so the southern in regard to the northern forms.

Just in

the same manner →OMIT at the present day, →we see that very many European productions cover the ground in La Plata, →New Zealand, and to a lesser degree in Australia, and have →OMIT beaten the natives; whereas extremely few southern forms have become naturalised in any part of →the northern hemisphere, though hides, wool, and other objects likely to carry seeds have been largely imported into Europe during the last two or three centuries from La Plata, and during the last

thirty

forty

fifty

years from Australia. The Neilgherrie mountains in India, however, offer a partial exception; for here, as I hear from Dr. Hooker, Australian forms are rapidly sowing themselves and becoming naturalised. →Before the last great Glacial period, no doubt the intertropical

mountains:

mountains

→OMIT were stocked with endemic Alpine forms; but these have almost

every-where

everywhere

largely

....

yielded to the more dominant forms, generated in the larger areas and more efficient workshops of the north. In many islands the native productions are nearly

equalled

equalled,

or even

outnumbered

out-numbered,

outnumbered,

by →those which have become naturalised; and →OMIT this is the first stage towards

extinction.

their extinction.