be extraordinarily rapid, if the power were exercised to the utmost. It never is exercised to the utmost, and yet, in the most favourable circumstances known to exist, which are those of a fertile region colonized from an industrious and civilized community, population has continued for several generations, independently of fresh immigration, to double itself in not much more than twenty years. … It is a very low estimate of the capacity of increase, if we only assume that in a good sanitary condition of the people, each generation may be double the number of the generation which preceded it." James Mill wrote: "That population therefore has such a tendency to increase as would enable it to double itself in a small number of years, is a proposition resting on the strongest evidence, which nothing that deserves the name of evidence has been brought on the other side to oppose."
Mr. McCulloch tells us that "it has been established beyond all question that the population of some of the states of North America, after making due allowance for immigration, has continued to double for a century past in so short a period as twenty, or at most five-and-twenty years." M. Moreau de Jonnès gives us the following table of the time in which the population of each of the under-mentioned countries would double itself:—
| Turkey | would take | … | … | 555 | years |
| Switzerland | would take„ | … | … | 227 | years„ |
| France | would take„ | … | … | 138 | years„ |
| Spain | would take„ | … | … | 106 | years„ |
| Holland | would take„ | … | … | 100 | years„ |
| Germany | would take„ | … | … | 76 | years„ |
| Russia | would take„ | … | … | 43 | years„ |
| England | would take„ | … | … | 43 | years„ |
| United States | would take„ | … | … | 25 | years„ |
| (Without reckoning immigrants.) | |||||
We shall take but a narrow view of the law of population if we confine ourselves exclusively to human beings. Man is but the highest in the animal kingdom, not a creature apart from it, and the law of population runs through the animal and the vegetable worlds. To take the commonest illustration: the horse is but a slowly breeding animal, producing but one at a birth, and that at considerable intervals of time; yet how small a proportion of the horses of a country are either stallions or brood mares; the reproductive organs of the colt are destroyed in the enormous majority of those born, and, nevertheless, our production of horses suffices for the vast needs of our commercial and luxurious classes. Darwin, in his "Origin of Species," writes:—"There is no exception to the rule that every organic being naturally increases at so high a rate that, if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in a few thousand years, there would literally not be standing room for his progeny. Linnæus has calculated that if an annual plant produced only two seeds—and there is no plant so unproductive as this—and their seedlings next year produced two and so on, then in twenty years there would be a million plants. The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be under the mark to assume that it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth three pair of young in this interval; if this be so, at the end of the fifth century there would be alive 15,000,000 elephants, descended 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 many 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, 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 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
