all annually
breed; and the moral restraint, which in some small degree checks the
increase of mankind, is entirely lost. Even slow-breeding mankind has
doubled in 25 years{228}, and if he could increase his food with greater
ease, he would double in less time. But for animals, without artificial
means, _on an average_ the amount of food for each species must be
constant; whereas the increase of all organisms tends to be geometrical,
and in a vast majority of cases at an enormous ratio. Suppose in a
certain spot there are eight pairs of [robins] birds, and that _only_
four pairs of them annually (including double hatches) rear only four
young; and that these go on rearing their young at the same rate: then
at the end of seven years (a short life, excluding violent deaths, for
any birds) there will be 2048 robins, instead of the original sixteen;
as this increase is quite impossible, so we must conclude either that
robins do not rear nearly half their young or that the average life of a
robin when reared is from accident not nearly seven years. Both checks
probably concur. The same kind of calculation applied to all vegetables
and animals produces results either more or less striking, but in
scarcely a single instance less striking than in man{229}.

{228} Occurs in _Origin_, Ed. i. p. 64, vi. p. 79.

{229} Corresponds approximately with _Origin_, Ed. i. pp. 64-65,
vi. p. 80.

Many practical illustrations of this rapid tendency to increase are on
record, namely during peculiar seasons, in the extraordinary increase of
certain animals, for instance during the years 1826 to 1828, in La
Plata, when from drought, some millions of cattle perished, the whole
country _swarmed_ with innumerable mice: now I think it cannot be
doubted that during the breeding season all the mice (with the exception
of a few males or females in excess) ordinarily pair; and therefore that
this astounding increase during three years must be attributed to a
greater than usual number surviving the first year, and then breeding,
and so on, till the third year, when their numbers were brought down to
their usual limits on the return of wet weather. Where man has
introduced plants and animals into a new country favourable to them,
there are many accounts in how surprisingly few years the whole country
has become stocked with them. This increase would necessarily stop as
soon as the country was fully stocked; and yet we have every reason