recurrent yearly in a small, regular
degree, and in an extreme degree during unusually cold, hot, dry, or wet
years, according to the constitution of the being in question. Lighten
any check in the least degree, and the geometrical powers of increase in
every organism will almost instantly increase the average number of the
favoured species. Nature may be compared to a surface on which rest ten
thousand sharp wedges touching each other and driven inwards by
incessant blows. Fully to realize these views much reflection is
requisite. Malthus on man should be studied; and all such cases as those
of the mice in La Plata, of the cattle and horses when first turned out
in South America, of the birds by our calculation, &c., should be well
considered. Reflect on the enormous multiplying power _inherent and
annually in action_ in all animals; reflect on the countless seeds
scattered by a hundred ingenious contrivances, year after year, over the
whole face of the land; and yet we have every reason to suppose that the
average percentage of each of the inhabitants of a country usually
remains constant. Finally, let it be borne in mind that this average
number of individuals (the external conditions remaining the same) in
each country is kept up by recurrent struggles against other species or
against external nature (as on the borders of the Arctic regions, where
the cold checks life), and that ordinarily each individual of every
species holds its place, either by its own struggle and capacity of
acquiring nourishment in some period of its life, from the egg upwards;
or by the struggle of its parents (in short-lived organisms, when the
main check occurs at longer intervals) with other individuals of the
_same_ or _different_ species.

But let the external conditions of a country alter. If in a small
degree, the relative proportions of the inhabitants will in most cases
simply be slightly changed; but let the number of inhabitants be small,
as on an island, and free access to it from other countries be
circumscribed, and let the change of conditions continue progressing
(forming new stations), in such a case the original inhabitants must
cease to be as perfectly adapted to the changed conditions as they were
originally. It has been shown in a former part of this work, that such
changes of external conditions would, from their acting on the
reproductive system, probably cause the organization of those beings
which were most affected