es at an enormous ratio. Suppose in a certain spot
there are eight pairs of 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 bird) there will
be 2048 birds, instead of the original sixteen. As this increase is
quite impossible, we must conclude either that birds do not rear nearly
half their young, or that the average life of a bird is, from accident,
not nearly seven years. Both checks probably concur. The same kind of
calculation applied to all plants and animals affords results more or
less striking, but in very few instances more striking than in man.

Many practical illustrations of this rapid tendency to increase are on
record, among which, during peculiar seasons, are the extraordinary
numbers 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 actually _swarmed_ with 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 number than usual 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 and favourable country, 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 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 checks fall--though 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 duration of life is, or to discover the
different percentage of deaths to births in different countries, we
ought to feel no surprise at our being unable to discover where the
check falls in any animal or plant. It should always be remembered, that
in most cases the checks are