the doctrine of
their unlimited durability. But natural history has been successfully
cultivated for so short a period, that a few examples only of local, and
perhaps but one or two of absolute, extirpation can as yet be proved,
and these only where the interference of man has been conspicuous. It
will nevertheless appear evident, from the facts and arguments detailed
in the third book (from the thirty-seventh to the forty-second chapters,
inclusive) that man is not the only exterminating agent; and that,
independently of his intervention, the annihilation of species is
promoted by the multiplication and gradual diffusion of every animal or
plant. It will also appear, that every alteration in the physical
geography and climate of the globe cannot fail to have the same
tendency. If we proceed still farther, and inquire whether new species
are substituted from time to time for those which die out, and whether
there are certain laws appointed by the Author of Nature to regulate
such new creations, we find that the period of human observation is as
yet too short to afford data for determining so weighty a question. All
that can be done is to show that the successive introduction of new
species may be a constant part of the economy of the terrestrial system,
without our having any right to expect that we should be in possession
of direct proof of the fact. The appearance again and again of new
species may easily have escaped detection, since the numbers of known
animals and plants have augmented so rapidly within the memory of
persons now living, as to have doubled in some classes, and quadrupled
in others. It will also be remarked in the sequel (book iii. chap. 43),
that it must always be more easy if species proceeded originally from
single stocks, to prove that one which formerly abounded in a given
district has ceased to be, than that another has been called into being
for the first time. If, therefore, there be as yet only one or two
unequivocal instances of extinction, namely, those of the dodo and
solitaire (see ch. 41), it is scarcely reasonable as yet to hope that we
should be cognizant of a single instance of the first appearance of a
new species.

_Recent origin of man, and gradual approach in the tertiary fossils of
successive periods from an extinct to the recent fauna._--The geologist,
however, if required to advance some fact which may lend countenance to
the opinion that in the most modern times, that is to