After arriving on Aeternum, the 19th Legion shows clear evidence of modifying the organization of its centuries over time. Initial attempts at reorganization were a direct response to the peculiar phenomena of resurrection.
If the Legion here evolved as a response to changing material conditions, it is only logical to conclude that the legions of the Roman Empire did the same, though perhaps not in as dramatic of a fashion.
(If I ever get off this wretched island to publish my findings, my theories will be vindicated. I can’t wait to see the looks on those Florentine imbeciles’ faces!)