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
No doubt a certain hereditary character was inherent not merely in the nature of the senate as an institution, in so far as it rested from the outset on a representation of the clans,(18) but in the nature of aristocracy generally, in so far as statesmanly wisdom and statesmanly experience are bequeathed from the able father to the able son, and the inspiring spirit of an illustrious ancestry fans every noble spark within the human breast into speedier and more brilliant flame.
18. Cf. I. V. The Senate
In this sense the Roman aristocracy had been at all times hereditary; in fact, it had displayed its hereditary character with great naivete in the old custom of the senator taking his sons with him to the senate, and of the public magistrate decorating his sons, as it were by anticipation, with the insignia of the highest official honour--the purple border of the consular, and the golden amulet-case of the triumphator.
But, while in the earlier period the hereditariness of the outward dignity had been to a certain extent conditioned by the inheritance of intrinsic worth, and the senatorial aristocracy had guided the state not primarily by virtue of hereditary right, but by virtue of the highest of all rights of representation--the right of the excellent, as contrasted with the ordinary, man--it sank in this epoch (and with specially great rapidity after the end of the Hannibalic war) from its original high position, as the aggregate of those in the community who were most experienced in counsel and action, down to an order of lords filling up its ranks by hereditary succession, and exercising collegiate misrule.
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Reference address : https://www.ellopos.net/elpenor/rome/3-11-government-governed.asp?pg=19