ample, 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. The problem is an undeniably
difficult one, and it will not be possible here to give more than
a mere outline of the modern views upon the subject. Without
entering into the at present inscrutable question as to the manner
in which new life-forms are introduced upon the earth, it may be
stated that almost all modern geologists hold that the living
beings of any given formation are in the main modified forms of
others which have preceded them. It is not believed that any
general or universal destruction of life took place at the
termination of each geological period, or that a general introduction
of new forms took place at the commencement of a new period.
It is, on the contrary, believed that the animals and plants
of any given period are for the most part (or exclusively) the
lineal but modified descendants of the animals and plants of
the immediately preceding period, and that s