But let the destruction of the adults be ever so heavy, if the number which can exist in any district be not wholly kept down by such causes⁠—or again let the destruction of eggs or seeds be so great that only a hundredth or a thousandth part are developed⁠—yet of those which do survive, the best adapted individuals, supposing that there is any variability in a favourable direction, will tend to propagate their kind in larger numbers than the less well adapted. If the numbers be wholly kept down by the causes just indicated, as will often have been the case, natural selection will be powerless in certain beneficial directions; but this is no valid objection to its efficiency at other times and in other ways; for we are far from having any reason to suppose that many species ever undergo modification and improvement at the same time in the same area.