An enquiry into the facts of nature will show that all forms of vegetable and animal life are capable of reproducing themselves in almost boundless profusion. Darwin, in his work on The Origin of Species, points this out with the greatest clearness. He says: "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 with 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." After giving the example of the slow-breeding elephant, he continues: "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 less than ten years. Several of the plants, such as the cardoon and a tall thistle, now most numerous over the wild 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 favorable, and that there has consequently been less destruction of the old and young, and that nearly all the young have been able to breed. In such cases, the geometrical rate 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 among animals there are very few that 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."