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well determined, have served me as guides in this research. I have called that series of events the ancient history of the earth, in which we distinguish only a succession of periods, without any determination of time, because we can discover there general causes only, acting according to certain circumstances, indicated indeed by the phenomena, but which no longer exist, and of which, consequently, we cannot compare the effects with time. The scene is now about to change; and I shall therefore call that the modern history which I am now going to enter upon, in which we shall find every thing that a fixed chronology requires.

HISTORY OF THE EARTH SINCE THE BIRTH OF OUR

CONTINENTS.

12. The two first objects we have to consider in this new period of our globe, are, the change that took place in its external temperature at the time of the revolution I have been describing, and the origin of the population of the new continents. With respect to the first, we have to recollect, that the rays of the sun are not of themselves calorific; that they do not become so with regard to the earth, but by passing through its atmosphere, and falling on different bodies, and by their being there so modified as to produce the immediate cause of heat, which, with all naturalists, I have called fire. We also know, that the production of fire by the rays of the sun, is, cæteris paribus, more or less abundant, according to the state of the atmosphere, and of the several bodies; and that the permanence of the igneous fluid, whether free or combined with other substances, and consequently the preserva

tion of the heat produced by the rays of the sun, depend on the nature of the operations that take place in the atmosphere, and at the surface of the soil. Now, our atmosphere was formed by degrees, in proportion as the substances of our strata were precipitated in the liquid that produced them; and we see that it underwent successive changes, by what happened to the vegetables of the earth, at the same time that the races of marine animals also suffered changes by corresponding modifications of the liquid of the sea. Lastly, we have seen, that these two classes of changes had, for an immediate cause, certain successive revolutions of the bottom of that liquid. Here, then, is the point I shall set out from, to determine the first object of enquiry in this new state of the earth; namely, a great change of temperature in the regions without the tropics, in consequence of which, animals living there before that revolution, as is rendered evident by the quantity of their carcases found in our superficial strata, could no longer exist in them.

13. Since the great revolution which gave birth to the first lands on our globe, there has been none equally considerable, except that which destroyed those lands, and gave birth to our present continents. The atmosphere must have again undergone a great change in this latter revolution. There happened certainly at that time also a very great change in the dry surface of our globe; since the lands that were swallowed up consisted only of primordial strata : whereas the new continents have at their surface, and to a great depth, all the posterior strata; so that the primordial beds appear only here and there, owing to convulsions undergone by the whole mass of strata.

Lastly, after the precipitation of sand upon its ancient bed, the sea itself was no longer the same compound, and the nature of its exhalations was changed.

14. Here, then, are two very great changes which have taken place on our globe at the birth of our continents, and from these may have resulted very sensible modifications of the influence of the solar rays, not only with regard to heat, but with respect to all their operations. Without doubt we could not conclude from these, arguing à priori, that certain animals, which formerly lived without the tropics, could live there no longer in the new state of things; for we are still too ignorant of the composition of the atmosphere to trace its causes and effects to this depth: but, finding in our superficial loose strata carcases of elephants and rhinoceroses in such a state of preservation as proves they cannot have been deposited there a great number of ages, we are led to the general conclusion, that not very long ago a great change must have taken place in the physical causes operating at the earth's surface, subsequently to which those animals have ceased to exist in the regions which they previously could occupy. We know in general, in this respect, that differences in the state of the atmosphere, as well as in the nature of soils, sensibly modify the action of the rays of the sun, whether with regard to the production of heat, or the duration of the heat produced. Accordingly, permanent changes of this kind may have produced the great actual difference between the temperatures of winters and summers, and even between the days and nights in our regions'. Naturalists had

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Previously to the birth of our continents," says the author, "the same animals, marine as well as terrestrial, lived equally in all

contented themselves with assigning as causes of these differences those of the positions of the sun; because the intermediate causes by which its rays produce heat had not hitherto been considered; but although those causes are not as yet well defined, they are, notwithstanding, at this day, admitted by all observant philosophers. Now those intermediate causes may formerly have been such, as constantly and more completely to produce what we observe to a certain degree in the actual state of the globe; namely, that external heat results only as a secondary effect, greatly modified by intermediate causes, from the different positions of the sun; insomuch that in our regions, the nights are often as hot as the days which precede or follow them, -that mild winters precede or follow cool summers; and that these modifications differ in different coun

latitudes; for in every latitude we find in our superficial strata, particular kinds of marine and amphibious animals, such as the great pearl nautilus, and the hippopotamus, (which at present live only between the tropics), as in like manner we discover in them the carcases of the elephant and the rhinoceros. Doubtless at that period the atmosphere was such, that by a longer preservation of the calorific effects produced in it by the solar rays, the heat was much more equal at the earth's surface. Whoever has noticed the great differences which sometimes occur even in the present state of the globe, in the temperature of the winters in northern regions, without our being able to assign a cause for them, will not refuse to admit the inference which those facts necessarily seem to require, and respecting which geology supplies us with general views, namely, that before our continents were abandoned by the waters of the sea, the temperature was considerably higher at the surface of the earth than at present.' -See the Author's " Remarques sur l'Origine des Etres organisés," a dissertation forming the seventh Letter of the French Edition of this work. ED.

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tries at the same latitude. It is, therefore, very natural to conclude, that previously to that revolution of the globe which gave birth to our lands, the respective states of the atmosphere, of the sea, and of the soil was such, that the differences of the external heat were not so great between the different latitudes; and that in our regions its vicissitudes, from day to night, and from summer to winter, were less considerable than they are at present; which sufficiently accounts for some species of animals living in those regions formerly, which at this day can exist only in latitudes where the sun renews more constantly its productive effect of heat. Lastly, a circumstance immediately proving that at the time when our lands were abandoned by the waters of the sea, some change of this nature occurred in the physical causes which act upon the globe, is, that this revolution has affected not only some species of terrestrial, but different kinds of marine animals. For we likewise find, in our superficial loose strata, some shells of a race that was destroyed by the same revolution, and of others which since that time have been found to exist only between the tropics. Thus these phenomena of the organized beings on our globe, inexplicable by means of every slow cause, and which thus announce a great revolution at some determined epoch, are accounted for through the intervention of particular physical causes of a known kind, by the same revolution which we have already seen characterized by so many other phenomena.

15. The other object we have to settle, before we enter on the history of the new continents, is, the commencement of their population. In treating of

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