What I believe to be the true and sufficient answer has been given by no historian: but the key to it is only fifty years old. The reason is that no race or species, vegetable or animal or human, can maintain--much less raise--its organic level unless its best be selected for parenthood. It is true of a race as of an individual that it must work for its living--so to speak--if it is not to degenerate. When the terms are too easy, down you go. The tape-worm has given up even digesting for its living, and we know its degeneracy--all hooks and mouth. Society works and hands over its predigested food to such social parasites amongst ourselves. You must struggle or you will degenerate--even if only with rhyme or counterpoint, not necessarily for bread. “Effort is the law,” as Ruskin said: whether for a livelihood or for enjoyment. Living things are the product of the struggle for existence: we are thus evolved strugglers by constitution: and directly we cease to struggle we forfeit the possibilities of our birthright. “Thou, O God,” said Leonardo, “hast given all good things to man at the price of labour.” The case is the same with races. Directly the conditions become too easy, selection ceases, and it is as successful to be incompetent or lazy or vicious as to be worthy. The hard conditions that kept weeding out the unworthy are now relaxed and the fine race they made goes back again. Finally there occurs the phenomenon of _reversed selection_, when it is fitter to be bad than good, cowardly than brave--as when religious persecution murders all who are true to themselves and spares hypocrites and apostates: or when healthy children are killed in factories whilst feeble-minded children or deaf-mutes are carefully tended until maturity and then sent into the world to reproduce their maladies. Under reversed selection such results are obtained as a breeder of race-horses or plants would obtain if he went to work on similar lines: the race degenerates rapidly: and if it be an Imperial race its Empire comes crashing down about its ears. All Empires and civilisations hitherto have involved the partial or complete arrest or reversal of the process of natural selection: and the racial degeneration which necessarily ensued has been the cause of their invariable doom. When a primitive race is making its way by force, selection is stringent. The weak, cowardly, diseased, stupid are expunged from generation to generation. As civilisation advances, a higher ethical level is reached: all true civilisation tending to abrogate and ameliorate the struggle for existence. The diseased and weakly and feeble-minded are no longer left to pay the penalty sternly exacted by Nature for unfitness: they are allowed to survive and multiply. A successful race can apparently afford to permit this, as a race that is fighting for its existence cannot. But in reality no race can afford this absolutely fatal process.