Page - 80 - in The Origin of Species

80 ORIGIN' OF SPECIES be so, after a period of from 740 to 750 years there would be nearly nineteen million elephants alive, 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 several 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 in- credible. So it is with plants: cases could be given of intro- duced 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, which are now the commonest over the wide plains of La Plata, cloth- ing square leagues of surface almost to the exclusion of everv 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 discover)-. In such cases, and endless others could be given, no one sup- poses, that the fertility of the animals or plants has been suddenly and tem.porarily increased in any sensible degree. The obvious explanation is that the conditions of life have been higlilv favourable, and that there has constantlv 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, simplv explains their extraordinarily rapid increase and wide diffusion in their new homes. In a state of nature almost ever}' full-gro\sTi plant annually produces seed, and amongst animals there are very few which do not annually pair. Hence we may confidently as- sert, that all plants and animals are tending to increase at a geometrical ratio,—that all would rapidly stock ever}- station in which they could anyhow exist,—and that this geomet- rical tendencv to increase must be checked bv destruction at