Читать книгу Essay on the Theory of the Earth - Georges baron Cuvier - Страница 37

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The most important consideration, that which, in fact, is the chief object of all my researches, and which establishes their legitimate connection with the Theory of the Earth, is to ascertain in what strata each species is found, and whether there may be some general laws, relative either to the zoological subdivisions, or to the greater or less resemblance of the species to those of the present day.

The laws which have been recognised with respect to these relations are very distinct and satisfactory.

In the first place, it is clearly ascertained that the oviparous quadrupeds appear much more early than the viviparous; that they are even more abundant, larger, and more varied, in the ancient strata than at the surface of the globe, as it exists at present.

The Ichthyosauri, the Plesiosauri, several species of Tortoise, and several species of Crocodile, are found beneath the chalk, in the deposits commonly called Jura formations. The Monitors of Thuringia would be still older, if, according to the Wernerian School, the copper-slate in which they are contained, along with a great variety of fishes supposed to have belonged to fresh-water, is to be placed among the oldest beds of the secondary formations. The enormous crocodiles and the great tortoises of Maestricht, are found in the chalk formation itself; but these are marine animals.

This earliest appearance of fossil bones seems, therefore, already to indicate, that dry lands and fresh waters had existed before the formation of the chalk deposits. But neither at this period, nor while the chalk was forming, nor even long after, have any bones of land-mammifera been encrusted; or, at least, the small number of these, which are alleged to have been found in strata of these dates, forms but a trifling exception.

We begin to find bones of marine mammifera, namely, of lamantins and seals, in the coarse shelly limestone which covers the chalk in the neighbourhood of Paris; but there are still no bones of terrestrial mammifera.

Notwithstanding the most assiduous investigation, I have not been able to discover any distinct trace of this class in any of the deposits preceding those which rest upon the coarse limestone. Certain lignites and molasses do in fact contain them; but I am very doubtful whether these deposits are all, as is commonly supposed, anterior to that limestone. The places where these bones have been found are so limited, both in extent and in number, as to induce us to suppose some irregularity, or some repetition of the formation containing them. On the contrary, the moment we arrive at the deposits which rest upon the coarse limestone, the bones of land-animals present themselves in great abundance.

As it is reasonable to believe that shells and fishes did not exist at the period of the formation of primitive rocks, we are also led to conclude that the oviparous quadrupeds began to exist along with the fishes, and at the commencement of the period during which the secondary rocks were formed; but that the land-quadrupeds did not appear upon the earth, at least in any considerable number, till long after, and until the coarse limestone strata, which contain the greater number of our genera of shells, although of species different from ours, had been deposited.

It is remarkable that those coarse limestone strata which are used at Paris for building, are the last formed strata which indicate a long and quiet continuance of the sea upon our continents. Above them, indeed, there are found formations containing shells and other marine productions; but these consist of collections of transported matters, sand, marls, sandstones, and clays, which rather indicate transportations that have taken place with more or less violence, than strata formed by tranquil deposition; and, if there be some rocky and regular strata of pretty considerable magnitude, beneath or above these transported matters, they generally exhibit indications of having been deposited from fresh water.

Almost all the known bones of viviparous quadrupeds, therefore, have been found either in those fresh-water formations, or in the alluvial formations; and consequently there is every reason to conclude that these quadrupeds have only begun to exist, or, at least, to leave their remains in the strata of our earth, after the last retreat of the sea but one, and during the state of things that preceded its last irruption.

But there is also an order in the disposition of these bones with regard to each other; and this order further announces a very remarkable succession in the appearance of the different species. All the genera which are now unknown, the Palæotheria, Anaplotheria, &c., with the position of which we are thoroughly acquainted, belong to the oldest of the formations of which we are now speaking, those which rest immediately upon the coarse limestone. It is chiefly these genera which occupy the regular beds that have been deposited from fresh-water, or certain alluvial beds of very ancient formation, generally composed of sand and rolled pebbles, and which were perhaps the earliest alluvium of that ancient world. Along with these there are also found some lost species of known genera, but in small numbers, and some oviparous quadrupeds and fishes, which appear to have been all inhabitants of fresh-water. The beds which contain them are always more or less covered by alluvial beds, containing shells, and other marine productions.

The most celebrated of the unknown species, which belong to known genera, or to genera closely allied to those which are known, such as the fossil elephants, rhinoceroses, hippopotami, and mastodons, do not occur along with those more ancient genera. It is in the alluvial formations alone that they are discovered, sometimes accompanied with marine shells, and sometimes with fresh-water shells, but never in regular stony beds. Every thing that is found along with these species is either unknown like themselves, or at least doubtful.

Lastly, the bones of species which are apparently the same as those that are still found alive, are never discovered, except in the last alluvial deposits formed on the sides of rivers, or on the bottoms of ancient pools or marshes now dried up, or in the substance of beds of peat, or in the fissures and caverns of some rocks; or, lastly, at small depths below the surface, in places where they may have been buried by the falling down of debris, or even by the hand of man; and their superficial position renders these bones, although the most recent of all, almost always the worst preserved.

It must not, however, be thought that this classification of the various geological positions of fossil remains, is as certain as that of the species, or that it is equally capable of demonstration. There are numerous reasons which prevent this from being the case.

In the first place, all my determinations of species have been made upon the bones themselves, or by means of good figures; whereas it has been impossible for me personally to examine all the places in which these bones have been discovered. I have very frequently been obliged to content myself with vague and ambiguous accounts, given by people who were not themselves well aware of what it was necessary to observe; and, more frequently still, I have been unable to procure any information whatever on the subject.

Secondly, these repositories of organic remains are subject to infinitely greater doubts, than the bones themselves. The same formation may appear recent in places where it shews itself at the surface, and ancient in those where it is covered by the beds which have succeeded it. Ancient formations may have been transported by partial inundations, and thus have covered recent bones; they may have fallen upon them by crumbling, and thus have enveloped and mingled them with the productions of the ancient sea, which they previously contained. Bones of ancient periods may have been washed out by the waters, and afterwards enveloped in recent alluvial formations. Lastly, recent bones may have fallen into the fissures or caverns of ancient rocks, and been enveloped by stalactites or other incrustations. In every individual instance, therefore, it becomes necessary to analyze and appreciate all those circumstances which might disguise the real origin of fossil remains; and it rarely happens that people who have collected bones have been themselves aware of this necessity, the consequence of which has been, that the true characters of their geological position have been almost always neglected or misunderstood.

Thirdly, there are some doubtful species, which must occasion more or less uncertainty in the results of our researches, until they have been clearly ascertained. Thus the horses and buffaloes that occur along with the elephants, have not yet received appropriate specific characters; and such geologists as are disinclined to adopt the different epochs which I have endeavoured to establish with regard to fossil bones, may, for many years to come, draw from thence an argument against my system, so much the more convenient as it is contained in my own work.

But allowing that these epochs are liable to some objections, from such as may only consider some particular case, I am not the less satisfied, that those who shall take a comprehensive view of the phenomena, will not be checked by such inconsiderable and partial difficulties, and will be led to conclude, as I have done, that there has been at least one, and very probably two, successions in the class of quadrupeds, previous to that which at the present day peoples the surface of the earth.