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In the first place, they indicate much more clearly the nature of the revolutions to which they have already been subjected. Shells certainly announce the fact, that the sea has once existed in the places where they have been formed; but the changes which have taken place in their species, when rigorously inquired into, may have arisen from slight changes in the nature of the fluid in which they lived, or merely in its temperature. They may even have been produced by causes still more accidental. We can never be perfectly assured that certain species, and even genera, inhabiting the bottom of the sea, and occupying certain fixed spaces, for a longer or shorter time, may not have been driven away and supplanted by other species or genera.

In regard to quadrupeds, on the contrary, every thing is precise. The appearance of their bones in strata, and still more of their entire carcases, announces, either that the stratum itself which contains them has, at a former period, been laid dry, or, at least, that dry land must have existed in its neighbourhood. Their disappearance renders it certain, that this stratum has been inundated, or that the dry land in question has ceased to exist. It is from them, therefore, that we learn with perfect certainty the important fact of repeated irruptions of the sea, which the shells and other marine productions could not of themselves have proved; and it is by a careful investigation of them, that we may hope to ascertain the number and the epochs of these irruptions.