say, after the
greater part of the existing fauna and flora were established on the
earth, there has still been a new species superadded, may point to man
himself as furnishing the required illustration--for man must be
regarded by the geologist as a creature of yesterday, not merely in
reference to the past history of the organic world, but also in relation
to that particular state of the animate creation of which he forms a
part. The comparatively modern introduction of the human race is proved
by the absence of the remains of man and his works, not only from all
strata containing a certain proportion of fossil shells of extinct
species, but even from a large part of the newest strata, in which all
the fossil individuals are referable to species still living.

To enable the reader to appreciate the full force of this evidence, I
shall give a slight sketch of the information obtained from the newer
strata, respecting fluctuations in the animate world, in times
immediately antecedent to the appearance of man.

In tracing the series of fossiliferous formations from the more ancient
to the more modern, the first deposits in which we meet with assemblages
of organic remains, having a near analogy to the fauna of certain parts
of the globe in our own time, are those commonly called tertiary. Even
in the Eocene, or oldest subdivision of these tertiary formations, some
few of the testacea belong to existing species, although almost all of
them, and apparently all the associated vertebrata, are now extinct.
These Eocene strata are succeeded by a great number of more modern
deposits, which depart gradually in the character of their fossils from
the Eocene type, and approach more and more to that of the living
creation. In the present state of science, it is chiefly by the aid of
shells that we are enabled to arrive at these results, for of all
classes the testacea are the most generally diffused in a fossil state,
and may be called the medals principally employed by nature, in
recording the chronology of past events. In the Miocene deposits, which
are next in succession to the Eocene, we begin to find a considerable
number, although still a minority, of recent species, intermixed with
some fossils common to the preceding epoch. We then arrive at the
Pliocene strata, in which species now contemporary with man begin to
preponderate, and in the newest of which nine-tenths of the fossils
agree with species still inhabiting the nei