| ↑ 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.
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| ↑ 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.
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| → or now slightly modified species have 1869 |
| plants and allied forms have apparently 1859 1860 1861 |
| plants and allied forms have 1866 |
| or slightly modified species have 1872 |
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| → the north to the 1866 1869 1872 |
| north to 1859 1860 1861 |
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| → two sets be- came commingled in the equatorial regions, 1869 |
| became commingled 1859 1860 1861 1866 |
| two sets became commingled in the equatorial regions, 1872 |
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