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Marsupials of America and Australia, were all represented in Europe (and probably also in North America) during the earlier part of the Tertiary epoch. These facts, taken in their entifety, lead us to conclude that, during the whole of the Tertiary and perhaps during much of the Secondary periods, the great land masses of the earth were, as now, situated in the Northern Hemisphere; and that here alone were developed the successive types of vertebrata from the lowest to the highest. In the Southern Hemisphere there appear to have been three considerable and very ancient land masses, varying in extent from time to time, but always keeping distinct from each other, and represented, more or less completely, by Australia, South Africa, and South America of our time. Into these flowed successive waves of life, as they each in turn became temporarily united with some part of the northern land. Australia appears to have had but one such union, perhaps during the middle or latter part of the Secondary epoch, when it received the ancestors of its Monotremata and Marsupials, which it has since developed into a great variety of forms. The South African and South American lands, on the other hand, appear each to have had several successive unions and separations, allowing first of the influx of low forms only (Edentata, Insectivora and Lemurs); subsequently of Rodents and small Carnivora, and, latest of all, of the higher types of Primates, Carnivora and Ungulata.
During the whole of the Tertiary period, at least, the Northern Hemisphere appears to have been divided, as now, into an Eastern and a Western continent; always approximating and sometimes united towards the north, and then admitting of much interchange of their respective faunas; but on the whole keeping distinct, and each developing its own special family and generic types, of equally high grade, and generally belonging to the same Orders. During the Eocene and Miocene periods, the distinction of the Palaearctic and Nearctic regions was better marked than it is now; as is shown by the floras no less than by the faunas of those epochs. Dr. Newberry, in his Report on the Cretaceous and Tertiary floras of the Yellowstone and Missouri Rivers, states, that although the Miocene flora of Central North America corresponds generally with that of the European Miocene, yet many of the tropical, and especially the Australian types, such as Hakea and Hryandra, are absent. Owing to the recent discovery of a rich Cretaceous flora in North America, probably of the same age as that of Aix-la-Chapelle in Europe, we are able to continue the comparison; and it appears, that at this early period the difference was still more marked. The predominant feature of the European Cretaceous flora seems to have been the abundance of Proteacese, of which seven genera now living in Australia or the Cape of Good Hope have been recognised, besides others which are extinct. There are also several species of Pandanvs, or screw-pine, now confined to the tropics of the Eastern Hemisphere, and along with these, oaks, pines, and other more temperate forms. The North American Cretaceous flora, although far richer than that of Europe, contains no Proteacese or Pandani, but immense numbers of forest trees of living and extinct genera. Among the former we have oaks, beeches, willows, planes, alders, dog-wood, and cypress; together with such American forms as magnolias, sassafras, and liriodendrons. There are also a few not now found in America, as Araucaria and Cinnamomum, the latter still living in Japan. This remarkable flora has been found over a wide extent of countty—New Jersey, Alabama, Kansas, and near the sources of the Missouri in the latitude of Quebec—so that we can hardly impute its peculiarly temperate character to the great elevation of so large an area. The intervening Eocene flora approximates closely, in North America, to that of the Miocene period; while in Europe it seems to have been fully as tropical in character as that of the preceding Cretaceous period; fruits of Nipa, Pandanus, Anona, Acacia, and many Proteacese, occurring in the London clay at the mouth of the Thames.
These facts appear, at first sight, to be inconsistent, unless we suppose the climates of Europe and North America to have been widely different in these early times; but they may perhaps be harmonised, on the supposition of a more uniform and a somewhat milder climate then prevailing over the whole Northern Hemisphere; the contrast in the vegetation of these countries being due to a radical difference of type, and therefore not indicative of climate. The early European flora seems to have been a portion of that which now exists only in the tropical and sub-tropical lands of the Eastern Hemisphere; and, as much of this flora still survives in Australia, Tasmania, Japan, and the Cape of Good Hope, it does not necessarily imply more than a warm and equable temperate climate. The early North American flora, on the other hand, seems to have been essentially the 3ame in type as that which now exists there, and which, in the Miocene period, was well represented in Europe; and it is such as now flourishes best in the warmer parts of the United States. But whatever conclusion we may arrive at on the question of climate, there can be no doubt as to the distinctness of the florasof the ancient Nearctic and Palaearctic regions; and the view derived from our study of their existing and extinct faunas— that these two regions have, in past times, been more clearly separated than they are now—receives strong support from the unexpected evidence now obtained as to the character and mutations of their vegetable forms, during so vast an epoch as is comprised in the whole duration of the Tertiary period.
The general phenomena of the distribution of living animals, combined with the evidence of extinct forms, lead us to conclude that the Palaearctic region of early Tertiary times was, for the most part, situated beyond the tropics, although it probably had a greater southward extension than at the present time. It certainly included much of North Africa, and perhaps reached far into what is now the Sahara; while a southward extension of its central mass may have included the Abyssinian highlands, where some truly Palaearctic forms are still found. This is rendered probable by the fossils of Perim Island a little further east, which show that the characteristic Miocene fauna of South Europe and North India prevailed so far within the tropica. There existed, however, at the extreme eastern and western limits of the region, two extensive equatorial land-areas, our Indo-Malayan and West African sub-regions—both of which must have been united for more or less considerable periods with the northern continent They would then have received from it such of the higher vertebrates as were best adapted for the peculiar climatal and organic conditions which everywhere prevail near the equator; and these would be preserved, under variously modified forms, when they had ceased to exist in the less favourable and constantly deteriorating climate of the north. At later epochs, both these equatorial lands became united to some part of the great South African continent (then including Madagascar), and we thus have explained many of the similarities presented by the faunas of these distant, and generally very different countries.
During the Miocene period, when a subtropical climate pre vailed over much of Europe and Central Asia, there would be no such marked contrast as now prevails between temperate and tropical zones; and at this time much of our Oriental region, perhaps, formed a hardly separable portion of the great Palsearctic land. But when, from unknown causes, the climate of Europe became less genial, and when the elevation of the Himalayan chain and the Mongolian plateau caused an abrupt difference of climate on the northern and southern sides of that great mountain barrier, a tropical and a temperate region were necessarily formed; and many of the animals which once roamed over the greater part of the older and more extensive region, now became restricted to its southern or northern divisions respectively. Then came the great change we have already described (vol. i. p. 288), opening the newly-formed plains of Central Africa to the incursions of the higher forms of Europe; and following on this, a still further deterioration of climate, resulting in that marked contrast between temperate and tropical faunas, which is now one of the most prominent features in the distribution of animal as well as of vegetable forms.
It is not necessary to go into any further details here, as we have already, in our discussion of the origin of the fauna of the several regions, pointed out what changes most probably occurred in each case. These details are, however, to a great extent speculative; and they must remain so till we obtain as much knowledge of the extinct faunas and past geological history of the southern lands, as we have of those of Europe and North America. But the broad conclusions at which we have now arrived seem to rest on a sufficiently extensive basis of facts; and they lead us to a clearer conception of the mutual relations and comparative importance of the several regions than could be obtained at an earlier stage of our inquiries.
If our views of the origin of the several regions are correct, it is clear that no mere binary division—into north and south, or into east and west—can be altogether satisfactory, since at the dawn of the Tertiary period we still find our six regions, or what may be termed the rudiments of them, already established. The north and south division truly represents the fact, that the great northern continents are the seat and birth-place of all the higher forms of life, while the southern continents have derived the greater part, if not the whole, of their vertebrate fauna from the north; but it implies the erroneous conclusion, that the chief southern lands—Australia and South America—are more closely related to each other than to the northern continent. The fact, however, is that the fauna of each has been derived, independently, and perhaps at very different times, from the north, with which, they therefore have a true genetic relation; while any intercommunion between themselves has been comparatively recent and superficial, and has in no way modified the great features of animal life in each. The east and west division, represents—according to our views—a more fundamental diversity; since we find the northern continent itself so divided in the earliest Eocene, and even in Cretaceous times; while we have the strongest proof that South America was peopled from the Nearctic, and Australia and Africa from the Palaearctic region: hence, the Eastern and Western Hemispheres are the two great branches of the tree of life of our globe. But this division, taken by itself, would obscure the facts—firstly, of the close relation and parallelism of the Nearctic and Palaearctic regions, not only now but as far back as we can clearly trace them in the past; and, secondly, of the existing radical diversity of the Australian region from the rest of the Eastern Hemisphere.
Owing to the much greater extent of the old Palaarctic region (including our Oriental), and the greater diversity of