But not merely was the valley of the lower Main in front of Mentz brought within the military frontier-line; in south-western Germany also the boundary was pushed forward in a still greater degree. The region of the Neckar, once possessed by the Celtic Helvetii, then for long a debateable borderland between these and the advancing Germans, and therefore named the Helvetian desert, subsequently perhaps occupied partially by the Marcomani, before these retreated to Bohemia (p. 29), came on the regulation of the Germanic boundaries after the battle of Varus into the same position as the greater portion of the right bank of the lower Rhine. Here, too, there must have been a frontier-line already at that time marked off, within which Germanic settlements were not tolerated. Thereupon individual, mostly Gallic, immigrants, who had not much to lose, settled down, as on an unenclosed moor, in these fertile but little protected regions, which went at that time by the name of _agri decumates_.[77] This private occupation, which was, it may be conjectured, merely tolerated by the government, was followed by the formal taking possession of it probably under Vespasian. As already, about the year 74, a highway was carried from Strassburg on the right bank of the Rhine as far as Offenburg,[78] there must have been instituted about this time in this region a more earnest protection of the frontier than the mere prohibition of Germanic settlement furnished. What the father had begun the sons carried out. Perhaps even through the construction--whether by Vespasian, by Titus, or Domitian--of the "Flavian altars"[79] at the source of the Neckar, near the modern Rottweil--a settlement of which indeed we know nothing but the name--there was procured for the new upper Germany on the right of the Rhine a centre similar to what the Ubian altar was formerly intended to become for Great Germany, and soon afterwards the altar of Sarmizegetusa became for the newly-conquered Dacia. The first institution of the frontier-defence, to be described further on, by which the Neckar valley was brought within the Roman line, is thus the work of the Flavii, chiefly, doubtless, of Domitian,[80] who thereby carried further the construction at the Taunus. The military road on the right of the Rhine from Mogontiacum by way of Heidelberg and Baden in the direction of Offenburg--the necessary consequence of this annexation of the Neckar region--was, as we now know,[81] constructed by Trajan in the year 100, and was a part of the more direct communication established by that emperor between Gaul and the line of the Danube. There was employment for the soldiers at these works, but hardly for their arms; there were no Germanic tribes dwelling in the region of the Neckar, and still less can the narrow strip on the left bank of the Danube, which was thereby brought within the frontier line, have cost serious struggles. The nearest Germanic people of note there,