of creatures belonging to all climates,
and which have lived on land as well as in fresh and salt water. Let
us now suppose that by a series of changes, sudden or gradual, all the
present organized beings were swept away, and that, when the earth was
renewed by the power of the Creator, a new race of intelligent beings
could explore those parts of the former sea basins that had been
elevated into land. They would find the remains of multitudes of
creatures not existing in their time; and by the presence of these
they could distinguish the deposits of the former period from those
that belonged to their own. They could also compare these remains with
the corresponding parts of creatures which were their own
contemporaries, and could thus infer the circumstances in which they
had lived, the modes of subsistence for which they had been adapted,
and the changes in the distribution of land and water and other
physical conditions which had occurred. This, then, is precisely the
place which fossil organic remains occupy in modern geology, except
that our present system of nature rests on the ruins, not of one
previous system, but of several.

4. By the aid of the superposition of deposits and their organic
remains, geology can divide the history of the earth into distinct
periods. These periods are not separated by merely arbitrary
boundaries, but to some extent mark important eras in the progress of
our earth; though they usually pass into each other at their confines,
and the nature of the evidence prevents us from ascertaining the
precise length of the periods themselves, or the intervals in time
which may separate the several monuments by which they are
distinguished. The following table will serve to give an idea of the
arrangement at present generally received, with some of the more
important facts in the succession of animal and vegetable life, as
connected with our present subject. It commences with the oldest
periods known to geology, and gives in the animal and vegetable
kingdoms the _first appearance_ of each class, with a few notes of the
subsequent history of the principal forms. It must, however, be borne
in mind that farther discoveries may extend some classes farther back
than we at present know them, and that a more detailed table,
descending to orders and families, would give a more precise view of
the succession of life. Farther, the several geological formations
would admit of much subdivision, and are repr