Изображения страниц
PDF
EPUB

It should be observed that the northern forms found on the mountain-ranges of the intertropical regions and in the southern parts of the southern hemisphere are not arctic, but belong to the temperate zones: as Mr. H. C. Watson has recently remarked, "in receding from polar towards equatorial latitudes, the Alpine or mountain floras really become less and less arctic." Of these forms, some few are identical with northern temperate species, or are varieties of them, whilst others are ranked by all naturalists as closely allied to, but specifically distinct from, their northern representatives.

Now let us see what light can be thrown on the foregoing facts, on the belief, supported as it is by a large body of geological evidence, that the whole world, or a large part of it, was simultaneously colder during the Glacial period than at present. 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, the tropical plants and animals will have retreated from both sides towards the equator, followed in the rear by the temperate productions, and these by the arctic; but with the latter we are not now concerned. 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 overlooked. 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. Hence they will have suffered much extinction; how much no one can say; perhaps formerly the tropics supported as many species as we now see 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 lowest, most protected, and warmest districts. 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. 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 that proper to them. Hence, it seems to me possible, bearing in mind that the tropical productions were in a suffering 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, and have reached or 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 would have afforded an asylum for the 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, 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. We might of course speculate on the land having been formerly higher than at present in various parts of the tropics, where temperate forms apparently have crossed; but as the lines of migration have been so numerous, such speculations would be rash. Hence I am forced to believe that in certain regions, as in India, some temperate productions entered and crossed even the lowlands of the tropics at the period when the cold was most intense,-when arctic forms in Europe had migrated over at least twenty-five degrees of latitude, 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 from five to six thousand feet. During this the coldest period, large spaces of the tropical lowlands were probably clothed with a mingled tropical and temperate vegetation, like that now growing with strange luxuriance at the base of the Himalaya, at the height of four or five thousand feet, as so graphically described by Hooker.

So again, on the island of Fernando Po, in the Gulf of Guinea, Mr. Mann found temperate European forms first beginning to appear at the height of about five thousand feet. On the mountains of Panama, at the height of only two thousand feet, Dr. Seemann found the vegetation like that of Mexico, "with forms of the torrid zone harmoniously blended with those of the temperate." So that under certain conditions of climate it is certainly possible that strictly tropical forms might have co-existed for an indefinitely long period mingled with temperate forms.

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.

« ПредыдущаяПродолжить »