Aristotle

When a change of constitution takes place, Kingship passes into Tyranny, because Tyranny is the bad form of monarchy, so that a bad king becomes a tyrant. Aristocracy passes into Oligarchy owing to badness in the rulers, who do not distribute what the State has to offer according to desert, but give all or most of its benefits to themselves, and always assign the offices to the same persons, because they set supreme value upon riches; thus power is in the hands of a few bad men, instead of being in the hands of the best men. Timocracy passes into Democracy, there being an affinity between them, inasmuch as the ideal of Timocracy also is government by the mass of the citizens, and within the qualification all are equal. Democracy is the least bad of the perversions, for it is only a very small deviation from the constitutional form of government. These are the commonest ways in which revolutions occur in states, since they involve the smallest change, and come about most easily.