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
Under such circumstances the only wonder is that the Roman patriciate did not disappear much more rapidly than it actually did. The fact of its still continuing for a prolonged period a numerous community can scarcely be accounted for by the bestowal of Roman burgess-rights on several distinguished foreign clans, which after emigrating from their homes or after the conquest of their cities received the Roman franchise--for such grants appear to have occurred but sparingly from the first, and to have become always the more rare as the franchise increased in value.
A cause of greater influence, in all likelihood, was the introduction of the civil marriage, by which a child begotten of patrician parents living together as married persons, although without -confarreatio-, acquired full burgess-rights equally with the child of a -confarreatio- marriage. It is at least probable that the civil marriage, which already existed in Rome before the Twelve Tables but was certainly not an original institution, was introduced for the purpose of preventing the disappearance of the patriciate.(4)
4. The enactments of the Twelve Tables respecting -usus- show clearly that they found the civil marriage already in existence. In like manner the high antiquity of the civil marriage is clearly evident from the fact that it, equally with the religious marriage, necessarily involved the marital power (v. The House-father and His Household), and only differed from the religious marriage as respected the manner in which that power was acquired. The religious marriage itself was held as the proprietary and legally necessary form of acquiring a wife; whereas, in the case of civil marriage, one of the general forms of acquiring property used on other occasions--delivery on the part of a person entitled to give away, or prescription--was requisite in order to lay the foundation of a valid marital power.
To this connection belong also the measures which were already in the earliest times adopted with a view to maintain a numerous posterity in the several households.(5)
5. Cf. I. V. The House-father and His Household
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Reference address : https://www.ellopos.net/elpenor/rome/1-06-burgesses-reformed-constitution.asp?pg=9