Polybius's political cycle

"Now the first of these [political forms] to come into being is monarchy, it's growth being natural and unaided; and next arises kingship derived from monarchy by the aid of art and by the correction of defects. Monarchy first changes into its vicious allied form, tyranny; and next, the abolishment of both gives birth to aristocracy. Aristocracy by its very nature degenerates into oligarchy; and when the commons inflamed by anger take vengeance on this government for its unjust rule, democracy comes into being; and in due course the licence and lawlessness of this form of government produces mob-rule to complete the series. The truth of what I have just said will be quite clear to anyone who pays due attention to such beginnings, origins, and changes as are in each case natural. For he alone who has seen how each form naturally arises and develops, will be able to see when, how, and where the growth, perfection, change, and end of each are likely to occur again. And it is to the Roman constitution above all that this method, I think, may be successfully applied, since from the outset its formation and growth have been due to natural causes." -- Histories, Book VI