ants which have become
common throughout whole islands in a period of less than ten years.
Several of the plants now most numerous over the wide plains of La
Plata, clothing square leagues of surface almost to the exclusion of
all other plants, 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 instances could be given, no one
supposes that the fertility of these animals or plants has been
suddenly and temporarily increased in any sensible degree. The obvious
explanation is that the conditions of life have been very 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. In such
cases the geometrical ratio of increase, the result of which never fails
to be surprising, simply explains the extraordinarily rapid increase and
wide diffusion of naturalised productions in their new homes.

In a state of nature almost every plant produces seed, and amongst
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 most rapidly stock every station
in which they could any how exist, and that the 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 forget that
thousands are annually slaughtered for food, and that in a state of
nature an equal number would have somehow to be disposed of.

The only difference between organisms which annually produce eggs or
seeds by the thousand, and those which produce extremely few, is,
that the slow-breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever so large. The
condor lays a couple of eggs and the ostrich a score, and yet in the
same country the condor may be the more numerous of the two: the Fulmar
petrel lays but one egg, yet it is believed to be the most numerous bird
in the world. One fly deposits hundreds of eggs, and another, like the
hippobosca, a single one; but this difference does not determine how
many individuals of the two species can be supported in a district.
A large nu