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
System of Government-- Division and Classification of the Subjects
The system, on which this fabric was constructed and kept together, can no longer be ascertained in detail from the few notices that have reached us. Even the numerical proportions of the three classes of subjects relatively to each other and to the full burgesses, can no longer be determined even approximately;(39) and in like manner the geographical distribution of the several categories over Italy is but imperfectly known.
39. It is to be regretted that we are unable to give satisfactory information as to the proportional numbers. We may estimate the number of Roman burgesses capable of bearing arms in the later regal period as about 20,000. (Cf. I. VI. Time And Occasion of the Reform) Now from the fall of Alba to the conquest of Veii the immediate territory of Rome received no material extension; in perfect accordance with which we find that from the first institution of the twenty-one tribes about 259, (Cf. II. II. Coriolanus) which involved no, or at any rate no considerable, extension of the Roman bounds, no new tribes were instituted till 367.
However abundant allowance we make for increase by the excess of births over deaths, by immigration, and by manumissions, it is absolutely impossible to reconcile with the narrow limits of a territory of hardly 650 square miles the traditional numbers of the census, according to which the number of Roman burgesses capable of bearing arms in the second half of the third century varied between 104,000 and 150,000, and in 362, regarding which a special statement is extant, amounted to 152,573. These numbers must rather stand on a parallel with the 84,700 burgesses of the Servian census; and in general the whole earlier census-lists, carried back to the four lustres of Servius Tullius and furnished with copious numbers, must belong to the class of those apparently documentary traditions which delight in, and betray themselves by the very fact of, such numerical details.
It was only with the second half of the fourth century that the large extensions of territory, which must have suddenly and considerably augmented the burgess roll, began. It is reported on trustworthy authority and is intrinsically credible, that about 416 the Roman burgesses numbered 165,000; which very well agrees with the statement that ten years previously, when the whole militia was called out against Latium and the Gauls, the first levy amounted to ten legions, that is, to 50,000 men.
Subsequently to the great extensions of territory in Etruria, Latium, and Campania, in the fifth century the effective burgesses numbered, on an average, 250,000; immediately before the first Punic war, 280,000 to 290,000. These numbers are certain enough, but they are not quite available historically for another reason, namely, that in them probably the Roman full burgesses and the "burgesses without vote" not serving, like the Campanians, in legions of their own, --such, e. g., as the Caerites, --are included together in the reckoning, while the latter must at any rate -de facto- be counted among the subjects (Rom. Forsch. ii. 396).
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