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Secondly, The nature of the revolutions which have altered the surface of the globe, must have exerted a more powerful action upon terrestrial quadrupeds, than upon marine animals. As these revolutions have consisted chiefly of changes in the bed of the sea, and as the waters must have destroyed all the quadrupeds which they reached, if their irruption was general, it would necessarily have destroyed the entire class; or if it only overwhelmed certain continents at one time, it would at least have destroyed the species peculiar to those continents, without having the same effect upon the marine animals. On the other hand, millions of aquatic animals would have been left dry, or buried under newly-formed strata, or thrown violently on the coasts; while their races would still have been preserved in some more peaceful parts of the sea, whence they might again be propagated after the agitation of the waters had ceased.

Thirdly, This more complete action is also more easily ascertained. It is more easy to demonstrate its effects, because, the number of quadrupeds being limited, and the greater part of their species, at least the large ones, being known, we have more means of determining whether fossil bones belong to them, or to a species that is now lost. As, on the other hand, we are very far from being acquainted with all the testaceous animals and fishes which inhabit the sea, and as we are still probably ignorant of the greater number of those which live in deep water, it is impossible to know with certainty, whether a species which occurs in a fossil state, may not still exist somewhere alive. And hence, we see naturalists persisting in giving the name of pelagic shells, that is to say, shells inhabiting the open sea, to the belemnites, cornua-ammonis, and other testaceous remains, which have hitherto been found only in the older strata; meaning by this, that if they have not yet been discovered in a living state, it is because they inhabit the depths of the sea, far beyond the reach of our nets.