ed on the share which a change of climate
may have had in rendering the Mediterranean unfit for the habitation of
certain Testacea, which still continued to thrive in the Indian Ocean,
and of others which were now only represented by analogous forms within
the tropics. He must also have been aware that other extrinsic causes,
such as the progress of human population, or the increase of some one of
the inferior animals, might gradually lead to the extirpation of a
particular species, although its fecundity might remain to the last
unimpaired. If, therefore, amid the vicissitudes of the animate and
inanimate world, there are known causes capable of bringing about the
decline and extirpation of species, it became him thoroughly to
investigate the full extent to which these might operate, before he
speculated on any cause of so purely hypothetical a kind as "the
diminution of the prolific virtue."

If it could have been shown that some wild plant had insensibly dwindled
away and died out, as sometimes happens to cultivated varieties
propagated by cuttings, even though climate, soil, and every other
circumstance, should continue identically the same--if any animal had
perished while the physical condition of the earth, and the number and
force of its foes, with every other extrinsic cause, remain unaltered,
then might we have some ground for suspecting that the infirmities of
age creep on as naturally on species as upon individuals. But, in the
absence of such observations, let us turn to another class of facts, and
examine attentively the circumstances which determine the _stations_ of
particular animals and plants, and perhaps we shall discover, in the
vicissitudes to which these stations are exposed, a cause fully adequate
to explain the phenomena under consideration.

_Stations of plants and animals._--Stations comprehend all the
circumstances, whether relating to the animate or inanimate world, which
determine whether a given plant or animal can exist in a given place; so
that if it be shown that stations can become essentially modified by the
influence of known causes, it will follow that species, as well as
individuals, are mortal.

Every naturalist is familiar with the fact, that although in a
particular country, such as Great Britain, there may be more than three
thousand species of plants, ten thousand insects, and a great variety in
each of the other classes; yet there will not be more than a hundred,
perhaps n