Why does Polybius believe that the Roman Constitution is a good example of the cycle?

Polybius believes the Roman Constitution exemplifies the cycle of political evolution because it effectively combines elements of monarchy, aristocracy, and democracy, thus balancing their strengths and mitigating their weaknesses. This mixed constitution allows for stability and adaptability, enabling Rome to withstand corruption and tyranny. By integrating these different forms of governance, Polybius argues that Rome is able to achieve a sustainable system that can endure the natural cycles of political decay and renewal.