Origin of Species Variorum

→ came slowly on, all the tropical plants and other productions will have retreated from both sides towards the equator, followed in the rear by 1859 1860 1861

came slowly on, the tropical plants and animals will have retreated from both sides towards the equator, followed in the rear by 1866

became more and more intense, we know that Arctic forms invaded 1869 1872

→ these by the arctic; but with the latter we are not now concerned. 1859 1860 1861 1866

from the facts just given, there can hardly be a doubt that some of the more vigorous, dominant, and widest-spreading temperate forms actually then invaded the equatorial lowlands. 1869

from the facts just given, there can hardly be a doubt that some of the more vigorous, dominant and widest-spreading temperate forms invaded the equatorial lowlands. 1872

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

The whole problem of what will have occurred is excessively complex. The probable existence before the Glacial period of a pleistocene equatorial flora and fauna, fitted for a hotter climate than any now existing, must not be over-looked. This old equatorial flora will have been almost wholly destroyed, and the two pleistocene sub-tropical floras, commingled and reduced in number, will then have formed the equatorial flora. There will also probably have been during the Glacial period great changes in the precise nature of the climate, in the degree of humidity, &c.; and various animals and plants will have migrated in different proportions and at different rates. So that altogether during the Glacial period the inhabitants of the tropics must have been greatly disturbed in all their relations of life.

↑ 4 blocks not present in 1859 1860 1861 1866; present in 1869 1872

The inhabitants of these lowlands would at the same time migrate to the tropical and sub-tropical regions of the south, for the southern hemisphere was at this period warmer. On the decline of the Glacial period, as both hemispheres gradually recovered their former temperatures, the northern temperate forms living on the lowlands under the equator, would be driven to their former homes or be destroyed, being replaced by the equatorial forms returning from the south. Some, however, of the northern temperate forms would almostc ertainly ascend any adjoining high land, where, if sufficiently lofty, they would long survive, like the Arctic forms on the mountains of Europe. They might survive, even if the climate was not perfectly fitted for them, for the change of temperature must have been very slow, and plants undoubtedly possess a certain capacity for acclimatisation, as shown by their transmitting to their offspring different constitutional powers of resisting heat and cold.

→ The tropical plants probably 1859 1860 1861

Hence they will have 1866

→ see 1859 1860 1861

now see crowded together 1866

→ present day crowded together at the 1859 1860 1861

OMIT 1866

→ warmest spots. 1859

lowest, most protected, and warmest districts. 1860 1861 1866

↑ 2 blocks not present in 1859 1860 1861 1869 1872; present in 1866

Nor must it be overlooked that, as the cold will have come on very slowly, it is almost certain that many of the inhabitants of the tropics will have become in some degree acclimatised; in the same manner as the same species of plant when living on lowlands and highlands certainly transmit to their seedlings different constitutional powers of resisting cold. Nevertheless, it cannot be denied that all tropical productions will have greatly suffered, and the chief difficulty is to understand how they can have escaped entire annihilation.

→ their own. 1859 1860 1861

that proper to them. 1866

The Glacial period, as measured by years, must have been very long; and when we remember over what vast spaces some naturalised plants and animals have spread within a few centuries, this period will have been ample for any amount of migration. As the cold →came slowly on, all the tropical plants and other productions will have retreated from both sides towards the equator, followed in the rear by the temperate

regions;

productions,

and,

and

→these by the arctic; but with the latter we are not now concerned. ↑ ↑ →The tropical plants probably suffered much extinction; how much no one can say; perhaps formerly the tropics supported as many species as we →see at the →present day crowded together at the Cape of Good Hope, and in parts of temperate

Australia.

As we know that many tropical plants and animals can withstand a considerable amount of cold, many might have escaped extermination during a moderate fall of temperature, more especially by escaping into the →warmest spots. ↑ But the great fact to bear in mind is, that all tropical productions will have suffered to a certain extent. On the other hand, the temperate productions, after migrating nearer to the equator, though they will have been placed under somewhat new conditions, will have suffered less. And it is certain that many temperate plants, if protected from the inroads of competitors, can withstand a much warmer climate than →their own. Hence, it seems to me possible, bearing in mind that the tropical productions were in a suffering

state,

state

and could not have presented a firm front against intruders, that a certain number of the more vigorous and dominant temperate forms might have penetrated the native

ranks,

ranks

and have reached or even crossed the equator. The invasion would, of course, have been greatly favoured by high land, and perhaps