from Cape Comorin to the Himalayas, 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 naturalized 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 anyhow exist, and that the geometrical tendency to increase must be checked by destruction at some period of life."
Mr. John Stuart Mill also remarks: "The power of multiplication inherent in all organic life may be regarded as infinite. There is no species of vegetable or animal, which, if the earth were entirely abandoned to it, and to the things on which it feeds, would not in a small number of years overspread every region of the globe of which the climate was compatible with its existence."
The rapid multiplication of rabbits in Australia has lately given a startling instance of reproductive power: a number of rabbits were taken over and let loose; the district was thinly peopled, so they were not shot down to any great extent; their natural enemies, the hawks, weasels, &c., that prey on their young in England, were not taken over with them; food was abundant, and there was no check to keep them back: the consequence was that whole districts were overrun by them, and the farmers were at their wits' end to save their crops from the swarming rodents. In France, again, owing to the wholesale destruction of small birds, there was a perfect plague of insects, and the inhabitants of many districts have striven to import birds, so as to prevent the insects from practically destroying the vegetation.
While in the vegetable and animal kingdoms the rapidity of the increase is generally far greater than in the human race, we have yet seen how rapidly man has been found to
