to
believe from what is known of wild animals that _all_ would pair in the
spring. In the majority of cases it is most difficult to imagine where
the check falls, generally no doubt on the seeds, eggs, and young; but
when we remember how impossible even in mankind (so much better known
than any other animal) it is to infer from repeated casual observations
what the average of life is, or to discover how different the percentage
of deaths to the births in different countries, we ought to feel no
legitimate surprise at not seeing where the check falls in animals and
plants. It should always be remembered that in most cases the checks are
yearly recurrent in a small regular degree, and in an extreme degree
during occasionally unusually cold, hot, dry, or wet years, according to
the constitution of the being in question. Lighten any check in the
smallest degree, and the geometrical power of increase in every
organism will instantly increase the average numbers 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{230}. Fully to realise 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 S.
America, of the robins 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 every one of the inhabitants of a country will
_ordinarily_ remain 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{231}, where the cold checks life); and that ordinarily each
individual of each species holds its place, either by its own struggle
and capacity of acquiring nourishment in some period (from the egg
upwards) of its life, or by the struggle of its parents (in short lived
organisms, when the main check occurs at long intervals) against and
compared with other individuals of the _same_ or _different_ species.

{230} This