Origin of Species Variorum

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

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

→ 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

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

↑ 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.

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

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.

↑ 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.

↑ 1 blocks not present in 1866 1869 1872; present in 1859 1860 1861

But the great fact to bear in mind is, that all tropical productions will have suffered to a certain extent.

→ in its turn be 1872

be 1869

→ OMIT 1872

in their turn 1869

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 →became more and more intense, we know that Arctic forms invaded the temperate

productions,

regions;

and

and,

→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. ↑ The inhabitants of these

lowlands

hot lowlands

would at the same time

migrate

have migrated

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

have been

driven to their former homes or

have been

destroyed, being replaced by the equatorial forms returning from the south. Some, however, of the northern temperate forms would

almostc

almost

ertainly

certainly

ascend

have ascended

any adjoining high land, where, if sufficiently lofty, they would

long

have long

survive,

survived

like the Arctic forms on the mountains of Europe. They might

survive,

have survived,

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. ↑ ↑ ↑

In the regular course of events the southern hemisphere would →in its turn be subjected to a severe Glacial period, with the northern hemisphere rendered warmer; and then the southern temperate forms would →OMIT invade the equatorial lowlands. The northern forms which had before been left on the mountains would now descend and mingle with the southern forms. These latter, when the warmth returned, would return to their former homes, leaving some few species on the mountains, and carrying southward with them some of the northern temperate forms which had descended from their mountain fastnesses. Thus, we should have some few species identically the same in the northern and southern temperate zones and on the mountains of the intermediate tropical regions. But the species left during a long time on these

mountains

mountains,

or in opposite hemispheres, would have to compete with many new forms and would be exposed to somewhat different physical conditions; hence they would be eminently liable to modification, and would generally now exist as varieties or as representative species; and this is the