Thus, deposits of various kinds are now in process of formation in our existing seas, as, for example, in the Arctic Ocean, the Atlantic, and the Pacific, and many of these deposits are known to us by actual examination and observation with the sounding-lead and dredge.

But it is hardly necessary to add that the animal remains contained in these deposits--the fossils of some future period--instead of being identical, are widely different from one another in their characters.
We have seen, then, that the entire stratified series is capable of subdivision into a number of definite rock-groups or "formations," each possessing a peculiar and characteristic assemblage of fossils, representing the "life" of the "period" in which the formation was deposited.

We have still to inquire shortly how it came to pass that two successive formations _should_ thus be broadly distinguished by their life-forms, and why they should not rather possess at any rate a majority of identical fossils.

It was originally supposed that this could be explained by the hypothesis that the close of each formation was accompanied by a general destruction of all the living beings of the period, and that the commencement of each new formation was signalised by the creation of a number of brand-new organisms, destined to figure as the characteristic fossils of the same.

This theory, however, ignores the fact that each formation--as to which we have any sufficient evidence--contains a few, at least, of the life-forms which existed in the preceding period; and it invokes forces and processes of which we know nothing, and for the supposed action of which we cannot account.