any similar stocks, or each from one only, the descendants of
which have spread themselves gradually from a particular point over the
habitable lands and waters? Thirdly, how far the duration of each
species of animal and plant is limited by its dependence on certain
fluctuating and temporary conditions in the state of the animate and
inanimate world? Fourthly, whether there be proofs of the successive
extermination of species in the ordinary course of nature, and whether
there be any reason for conjecturing that new animals and plants are
created from time to time, to supply their place?

_Whether species have a real existence in nature._--Before we can
advance a step in our proposed inquiry, we must be able to define
precisely the meaning which we attach to the term species. This is even
more necessary in geology than in the ordinary studies of the
naturalist; for they who deny that such a thing as a species exists,
concede nevertheless that a botanist or zoologist may reason as if the
specific character were constant, because they confine their
observations to a brief period of time. Just as the geographer, in
constructing his maps from century to century, may proceed as if the
apparent places of the fixed stars remained absolutely the same, and as
if no alteration were brought about by the precession of the equinoxes;
so, it is said, in the organic world, the stability of a species may be
taken as absolute, if we do not extend our views beyond the narrow
period of human history; but let a sufficient number of centuries
elapse, to allow of important revolutions in climate, physical
geography, and other circumstances, and the characters, say they, of the
descendants of common parents may deviate indefinitely from their
original type.

Now, if these doctrines be tenable, we are at once presented with a
principle of incessant change in the organic world; and no degree of
dissimilarity in the plants and animals which may formerly have existed,
and are found fossil, would entitle us to conclude that they may not
have been the prototypes and progenitors of the species now living.
Accordingly M. Geoffroy St. Hilaire has declared his opinion, that there
has been an uninterrupted succession in the animal kingdom, effected by
means of generation, from the earliest ages of the world up to the
present day, and that the ancient animals whose remains have been
preserved in the strata, however different, may nevertheless have been