Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
As, moreover, both the extraordinarily admitted senators and the quaestors were nominated by the -comitia tributa-, the senate, hitherto resting indirectly on the election of the people,(18) was now based throughout on direct popular election; and thus made as close an approach to a representative government as was compatible with the nature of the oligarchy and the notions of antiquity generally.
18. Cf. II. III. The Senate. Its Composition
The senate had in course of time been converted from a corporation intended merely to advise the magistrates into a board commanding the magistrates and self-governing; it was only a consistent advance in the same direction, when the right of nominating and cancelling senators originally belonging to the magistrates was withdrawn from them, and the senate was placed on the same legal basis on which the magistrates' power itself rested.
The extravagant prerogative of the censors to revise the list of the senate and to erase or add names at pleasure was in reality incompatible with an organized oligarchic constitution. As provision was now made for a sufficient regular recruiting of its ranks by the election of the quaestors, the censorial revisions became superfluous; and by their abeyance the essential principle at the bottom of every oligarchy, the irremoveable character and life-tenure of the members of the ruling order who obtained seat and vote, was definitively consolidated.
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Reference address : https://www.ellopos.net/elpenor/rome/4-10-sullan-constitution.asp?pg=25