But after the institution of tribunes a change in the constitution gradually took place, so that it was neither aristocratic nor popular exclusively, but was composed of both elements, and was a system of balance of power between the various classes. The more complete the balance of power, the closer is the resemblance to a constitutional government. When one class acted as a check against another class, as gradually came to pass, until the subversion of liberties by successful generals, the senate, the magistrates, and the people in their assemblies shared between them the political power, but the senate had a preponderating influence. The judicial, the legislative, and the executive authority was as well defined in Roman legislation as it is in English or American. No person was above the authority of the laws; no one class could subvert the liberties and prerogatives of another class,--even the senate could not override the constitution. The consuls, elected by the centuries, presided over the senate and over the assemblies of the people. There was no absolute power exercised at Rome until the subversion of the constitution, except by dictators chosen by the senate in times of imminent danger. Nor could senators elect members of their own body; the censors alone had the right of electing from the ex-magistrates, and of excluding such as were unworthy. The consuls could remain in office but a year, and could be called to account when their terms of office had expired. The tribunes of the people ultimately could prevent a consul from convening the senate, could seize a consul and imprison him, and could veto an ordinance of the senate itself. The nobles had no exclusive privilege like the feudal aristocracy of mediaeval Europe, although it was their aim to secure the high magistracies to the members of their own body. The term _nobilitas_ implied that some one of a man's ancestors had filled a curule magistracy. A patrician, long before the reforms of the Gracchi, had become a man of secondary importance, but the nobles were aristocrats to the close of the republic, and continued to secure the highest offices; they prevented their own extinction by admitting into their ranks those who distinguished themselves,--that is, exercising their influence in the popular elections to secure the magistracies from among themselves.