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.
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