who received lands on condition of paying a tithe of their produce as rent, and of undertaking their own defence. This was a new piece of frontier policy which was often imitated in later times. [Illustration: Roman _Limes_] It seems to have been the Flavian emperors, Vespasian and Domitian, who advanced a step farther. On the other side of the Rhine and beyond these Agri Decumates the Romans began to construct a line of forts and wooden watch-towers linked by a rampart of earth, and known as the _Limes Trans-Rhenanus_. This frontier of Upper Germany left the Rhine between Linz and Andernach, crossed the Lahn at Ems, and then turned eastwards north of Wiesbaden (Aquæ Mattiacæ) and Frankfort. After Saalburg it runs on a north-easterly curve to Gruningen, whence it turns south, and continues for more than 100 miles through Aschaffenburg and Worth to join the Rhætian _limes_ at Lorch. From Lorch the Rhætian _limes_ goes eastwards to join the Danube a few miles above Regensburg. At first perhaps it was little more than a police and customs limit, but it gradually grew into a formidable barrier behind which the Roman Empire rested in a too profound security. Trajan continued it. Hadrian strengthened it with a wall and palisade. Commodus further fortified and extended it. A similar bulwark ran along the Danube. This policy of setting up immobile defences like the Great Wall of China is always a dangerous one. Useful at first and visibly strong, it tends to lull the defenders into a false security. The camps and forts grew into towns, the armies into peaceful citizens living with their wives and children and devoting themselves to trade and husbandry. Meanwhile the barbarians on the other side were growing stronger and learning the art of war as fast as the Romans were forgetting it.