Yet another useful web link; this time about Polybius

Among those are the sections of Polybius's sixth book in which he analysed the basic structure of the Roman political system and so, he believed, the secrets of its success. Rome thrived, he boldly argued, because it was a perfect “mixed constitution”, that is, part monarchy (in its consuls), part aristocracy (in the Senate), part democracy (in the popular assemblies). 
http://www.the-tls.co.uk/tls/public/article1266167.ece