For the constitution of the Roman army we must consult the estimable Polybius,[855] its early historian, Livy, and the latest of the great authorities, Vegetius, in the days of Valentinian II. (A.D. 375–92); not forgetting Varro,[856] who treats of weapon changings. Whilst the militia consisted of three bodies, the citizens, the allies, who were sworn, and the auxiliaries or mercenaries; the characteristic of Roman organisation was the Legion—that is, _legere_ (they chose). Emerging by slow degrees from the Phalanx or close column,[857] it learnt to prefer for battle the _acies instructa_, haye or line, and the _acies sinuata_, with wings; and it reserved for especial purposes the _agmen pilatum_ or close array, and the _agmen quadratum_ or hollow square. The reason of the change is manifest. The Phalanx or oblong herse was irresistible during the compact advance. The wise Egyptian inventors made it perfect for the Nile Valley. But it lost virtue in woodlands and highlands; it was liable to be broken when changing front, and the long unwieldy spears which it required caused confusion on broken ground. The Legion consisted, strictly speaking, of heavy-armed infantry—of Milites, from _Mil-es_, because reckoned by their thousands. They were preceded by the Velites, Ferentarii, or Rorarii, ‘light infantry,’ _éclaireurs_, who cleared the way for action; in the first century they were reinforced by the Accensi Velati.[858] Whilst the Auxiliaries fought with bows and arrows, and some, like the Etruscans, with the ‘funda’ or sling, the Veles carried two to seven light throw-spears (_hastæ velitariæ_) about three feet long in the shaft, with a nine-inch lozenge-shaped head of iron.[859] For close quarters he wore on his right side a Parazonium-dagger, and on the right a broad cut-and-thrust blade of moderate size. His defences were an apron of leather strips, studded with metal; and a Parma,[860] the small round shield, like the Cetra, some three feet in diameter.[861] [Illustration: FIG. 270.] [Illustration: FIG. 271.—1, 2. HELMETS OF HASTARII (FROM TRAJAN’S COLUMN); 3, 4. HELMETS OF HASTARII; 5. BRONZE HELMET (FROM CANNÆ).]