Origin of Species Variorum

Comparison with 1859

even crossed the equator. The invasion would, of course, have been greatly favoured by high land, and perhaps by a dry climate; for Dr. Falconer informs me that it is the damp with the heat of the tropics which is so destructive to perennial plants from a temperate climate. On the other hand, the most humid and hottest districts will have afforded an asylum to the tropical natives. The mountain-ranges north-west of the Himalaya, and the long line of the Cordillera, seem to have afforded two great lines of invasion: and it is a striking fact, lately communicated to me by Dr. Hooker, that all the flowering plants, about forty-six in number, common to Tierra del Fuego and to Europe still exist in North America, which must have lain on the line of march. ↑ But I do not doubt that some temperate productions entered and crossed even the lowlands of the tropics at the period when the cold was most intense,— when arctic forms had migrated some twenty-five degrees of latitude from their native country and covered the land at the foot of the Pyrenees. At this period of extreme cold, I believe that the climate under the equator at the level of the sea was about the same with that now felt there at the height of six or seven thousand feet. During this the coldest period, I suppose that large spaces of the tropical lowlands were clothed with a mingled tropical and temperate vegetation, like that now growing with strange luxuriance at the base of the Himalaya, as graphically described by Hooker. ↑ ↑ ↑

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.

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;