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
Foreign Mercenaries - Adjutants of the Legion
It was an innovation--hazardous beyond doubt even in the view of its author--to which the untrustworthy character of the cavalry furnished by the subjects compelled him,(34) that Caesar for the first time deviated from the old Roman system of never fighting with mercenaries, and incorporated in the cavalry hired foreigners, especially Germans.
34. Cf. V. VII. The Gallic Plan of War
Another innovation was the appointment of adjutants of the legion (-legati legionis-). Hitherto the military tribunes, nominated partly by the burgesses, partly by the governor concerned, had led the legions in such a way that six of them were placed over each legion, and the command alternated among these; a single commandant of the legion was appointed by the general only as a temporary and extraordinary measure. In subsequent times on the other hand those colonels or adjutants of legions appear as a permanent and organic institution, and as nominated no longer by the governor whom they obey, but by the supreme command in Rome; both changes seem referable to Caesar's arrangements connected with the Gabinian law.(35)
35. Cf. V. III. Overthrow of the Senatorial Rule, and New Power of Pompeius
The reason for the introduction of this important intervening step in the military hierarchy must be sought partly in the necessity for a more energetic centralization of the command, partly in the felt want of capable superior officers, partly and chiefly in the design of providing a counterpoise to the governor by associating with him one or more colonels nominated by the Imperator.
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Reference address : http://www.ellopos.net/elpenor/rome/5-11-old-republic-new-monarchy.asp?pg=67