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THE CONDITION OF SOUTH AMERICA DURING THE TERTIARY PERIOD.
I HAVE thought it well to bring to a conclusion the argument derivable from the geology of the old continent and its adjacent islands, because, when we consider the case of South America, to which the present chapter will be exclusively devoted, we find ourselves most emphatically in a new world, and surrounded by forms anomalous at first sight, although strictly analogous to the existing fauna of that country, and clearly adapted to its conditions.
The tertiary geology of South America is on the grandest scale, and of the most instructive nature. Flanked by the great mountain chain of the Andes which runs parallel to the western coast, this country is still the seat of disturbances which ought to be studied as exhibiting the true elements of geological causation and illustrating almost every great geological principle. That part of the continent extending from the mountains eastward towards the sea is divided into vast plains drained by the river Amazon and the Rio de la Plata, and separated by a succession of transverse mountain ridges, comparatively unimportant with reference to the subject we have now to consider.
Almost the whole tract of plain country has been affected by strictly tertiary changes, and elevation has
taken place at such long intervals and by such slow degrees as hardly to interfere with the condition of things obtaining at the time.* A long succession of animals nearly allied to, but in many cases quite distinct from, its present inhabitants, dwelt on this rising continent; and corresponding groups seem to have existed ever since the first elevation of the country, fragments of them being embedded in the gravel and other deposits at the mouths of the great rivers.
Throughout the whole of Brazil, and in the provinces of La Plata and Buenos Ayres, remains of the extinct quadrupeds formerly tenanting these districts are occasionally met with, and are sometimes not only abundant, but preserved in the most wonderful state of perfection. Some of these skeletons exhibit nearly every bone of the animal; the strong cuirasses of others have scarcely a fragment removed from its true position; and these are found on the banks of the rivers, and in the adjacent mud, while numerous detached bones occur in the caverns in Brazil, and are distributed as widely and buried as safely as the bones of elephants or hyænas in the corresponding places of deposit in England and Europe. We have but to examine the fragments, and re-construct the animals, to learn the zoological condition of the great South American continent during the tertiary period, which indeed may there be regarded as rather passing away, than actually past.
But, first of all, let us consider the nature of the country itself in which these remains are found; and,
*An account of these will be found in Mr. Darwin's valuable work on the "Geology of South America," published while these sheets were passing through the press.
since there has probably been but little difference in this respect, we shall thus learn at the same time the conditions under which the ancient inhabitants may have lived.
The almost boundless plains, to which in South America the name "Pampas" is given, are localities equally remarkable and interesting to the zoologist, the botanist, and the geologist. They are not actually level, but rather gently undulating; yet, at the same time, the change of level is so gradual and small, that the undulations more resemble the swell of a great ocean in a calm, than any smaller or more visible hills. Over these tracts the traveller may pass for a hundred miles, without seeing any change either in the nature or the products of the soil, and without meeting with a single pebble. They exhibit the appearance of a sea-bottom which has remained for a long period undisturbed; and it is impossible to conceive anything more monotonous, or in that respect more dreary, than a journey over a desert so boundless. A succession of broad flat terraces, of different elevation, but in all respects similar, characterises also the whole district of Patagonia from the sea to the mountain chain on the western coast.
But it must not be imagined that the vegetation in those tracts partakes of the dreary and monotonous aspect of the country. It is, on the contrary, rich to a degree scarcely imaginable in a country and climate like ours. It exhibits occasionally clumps of well-grown trees, but more commonly the rapid and rank luxuriance of tropical districts. The whole of that part of South America, which is spread out in flat valleys between the branches and trunks of the
noblest rivers in the world, is provided throughout with an unfailing supply of moisture, and, consequently, enjoys perpetual fertility; and, as the rivers are frequently changing their course, they thus deposit the rich alluvial soil in various parts, and with it also bury the trunks of trees and the carcases of animals washed away in the occasional floods, or lying dead on the river banks. There is an abundant and never-failing supply for the most voracious of vegetable-feeding animals; and no amount of destruction seems to check, even for a short time, the rapid increase of the grasses and other plants that are indigenous.
At the time when America was first discovered, this vast district was chiefly tenanted by a small number of species of animals of very strange habits and structure, and of which it may, perhaps, be sufficient to say in general language, that they are represented by the sloth, the armadillo, and the ant-eater: we shall presently see what kind of sloths and armadillos were its inhabitants at a yet earlier period. Besides these, there also existed, among the animals indigenous in this continent, a kind of camel called the llama, several moderate-sized carnivora, a remarkable group of monkeys, and some interesting forms of rodent or gnawing animals. The group first mentioned (called by naturalists Edentata, or toothless, from the absence of cutting teeth) includes the most interesting both of recent and fossil species; but, before describing these, it will be better first of all to consider the structure and habits of those which do not belong to this group, but exhibit analogies with the more common types of animal structure in other parts of the world. Amongst these we find a pa
chydermatous species, called the Toxodon,* showing many curious points of resemblance to the dinothere, but more nearly approaching the rodents (e. g. beaver, &c.) in some important respects. There are also the remains of another interesting and very large species, called the Macrauchenia, which was a sort of camel, connecting the pachyderms with the ruminants. These have been found to possess considerable interest, and assist in bringing the whole group of fossils more immediately into comparison with those of other parts of the world.
The Toxodon, like the Dinotherium, is chiefly known by portions of the skull, and is almost as remarkable for the position and arrangement of its gnawing teeth, as the giant of the middle tertiary period in Enrope seems to have been for its singular tusks, and their position in the lower jaw. The dimensions of the skull show that the Toxodon must have rivalled the largest quadrupeds in this respect; and its general proportions, its peculiarities of form, and its structure, prove clearly that this extinct genus differed essentially from any other animal hitherto described.
The general form of the skull of the Toxodon seems to present no analogies with that of the elephant, or indeed with any of the larger quadrupeds. The teeth, of which there are seven grinders on each side of the upper jaw, and two incisors, one of them extremely large, and almost like those of a beaver, sufficiently indicate the peculiarities in this respect; and from these the name of the genus has been derived. One peculiarity in the skull worthy of notice is
* Tožov (toxon), a bow; odovç (odous), a tooth.