cended from the first pair.

But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three following
seasons. Still more striking is the evidence from our domestic animals
of many kinds which have run wild in several parts of the world; if the
statements of the rate of increase of slow-breeding cattle and horses
in South America, and latterly in Australia, had not been well
authenticated, they would have been incredible. So it is with plants;
cases could be given of introduced plants which have become common
throughout whole islands in a period of less than ten years. Several of
the plants, such as the cardoon and a tall thistle, which are now the
commonest over the wide plains of La Plata, clothing square leagues
of surface almost to the exclusion of every other plant, have been
introduced from Europe; and there are plants which now range in India,
as I hear from Dr. Falconer, from Cape Comorin to the Himalaya, which
have been imported from America since its discovery. In such cases, and
endless others could be given, no one supposes that the fertility of
the animals or plants has been suddenly and temporarily increased in any
sensible degree. The obvious explanation is that the conditions of life
have been highly favourable, and that there has consequently been less
destruction of the old and young and that nearly all the young have been
enabled to breed. Their geometrical ratio of increase, the result
of which never fails to be surprising, simply explains their
extraordinarily rapid increase and wide diffusion in their new homes.

In a state of nature almost every full-grown plant annually produces
seed, and among animals there are very few which do not annually pair.
Hence we may confidently assert that all plants and animals are tending
to increase at a geometrical ratio--that all would rapidly stock every
station in which they could any how exist, and that this geometrical
tendency to increase must be checked by destruction at some period of
life. Our familiarity with the larger domestic animals tends, I think,
to mislead us; we see no great destruction falling on them, and we do
not keep in mind that thousands are annually slaughtered for food,
and that in a state of nature an equal number would have somehow to be
di