Lycurgus had perfectly well understood that all the above changes take place necessarily and naturally, and had taken into consideration that every variety of constitution which is simple and formed on one principle is precarious, as it is soon perverted into the corrupt form which is proper to it and naturally follows on it.
For just as rust in the case of iron and wood-worms and ship-worms in the case of timber are inbred pests, and these substances, even though they escape all external injury, fall a prey to the evils engendered in them, so each constitution has a vice engendered in it and inseparable from it. In kingship it is despotism, in aristocracy oligarchy, and in democracy the savage rule of violence; and it is impossible, as I said above, that each of these should not in course of time change into this vicious form.
Lycurgus, then, foreseeing this, did not make his constitution simple and uniform, but united in it all the good and distinctive features of the best governments, so that none of the principles should grow unduly and be perverted into its allied evil, but that, the force of each being neutralized by that of the others, neither of them should prevail and outbalance another, but that the constitution should remain for long in a state of equilibrium like a well-trimmed boat, kingship being guarded from arrogance by the fear of the commons, who were given a sufficient share in the government, and the commons on the other hand not venturing to treat the kings with contempt from fear of the elders, who being selected from the best citizens would be sure all of them to be always on the side of justice; so that that part of the state which was weakest owing to its subservience to traditional custom, acquired power and weight by the support and influence of the elders. The consequence was that by drawing up his constitution thus he preserved liberty at Sparta for a longer period than is recorded elsewhere.
Lycurgus then, foreseeing, by a process of reasoning, whence and how events naturally happen, constructed his constitution untaught by adversity, but the Romans while they have arrived at the same final result as regards their form of government, have not reached it by any process of reasoning, but by the discipline of many struggles and troubles, and always choosing the best by the light of the experience gained in disaster have thus reached the same result as Lycurgus, that is to say, the best of all existing constitutions.
(Source: Polybius, The Histories, Vol.IIΙ, Book VI, Loeb Classical Library)
Research-Selection for NovoScriptorium: Anastasius Philoponus