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
Nevertheless the original character of the council of elders bequeathed many and important legal consequences to the Roman senate. To express the matter briefly, the position of the senate as something other and more than a mere state-council--than an assemblage of a number of trusty men whose advice the king found it fitting to obtain--hinged entirely on the fact that it was once an assembly, like that described by Homer, of the princes and rulers of the people sitting for deliberation in a circle round the king.
So long as the senate was formed by the aggregate of the heads of clans, the number of the members cannot have been a fixed one, since that of the clans was not so; but in the earliest, perhaps even in pre-Roman, times the number of the members of the council of elders for the community had been fixed without respect to the number of the then existing clans at a hundred, so that the amalgamation of the three primitive communities had in state-law the necessary consequence of an increase of the seats in the senate to what was thenceforth the fixed normal number of three hundred.
Moreover the senators were at all times called to sit for life; and if at a later period the lifelong tenure subsisted more -de facto- than -de jure-, and the revisions of the senatorial list that took place from time to time afforded an opportunity to remove the unworthy or the unacceptable senator, it can be shown that this arrangement only arose in the course of time. The selection of the senators certainly, after there were no longer heads of clans, lay with the king; but in this selection during the earlier epoch, so long as the people retained a vivid sense of the individuality of the clans, it was probably the rule that, when a senator died, the king should call another experienced and aged man of the same clanship to fill his place.
It was only, we may surmise, when the community became more thoroughly amalgamated and inwardly united, that this usage was departed from and the selection of the senators was left entirely to the free judgment of the king, so that he was only regarded as failing in his duty when he omitted to fill up vacancies.
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Reference address : https://www.ellopos.net/elpenor/rome/1-05-original-constitution-rome.asp?pg=28