During the more than three and a half centuries intervening between the battle of the Teutoberg and the crossing of the Danube by the Visigoths, the intermingling of the two peoples steadily increased. On the one hand were numerous Roman travelers and traders who visited the Germans living along the frontier and learned what sort of people they were. The soldiers of the legions stationed on the Rhine and Danube also added materially to Roman knowledge in this direction. But much more important was the influx of Germans into the Empire to serve as soldiers or to settle on lands allotted to them by the government. Owing to a general decline of population, and especially to the lack of a sturdy middle class, Rome found it necessary to fill up her army with foreigners and to reward them with lands lying mainly near the frontiers, but often in the very heart of the Empire. The over-population of Germany furnished a large class of excellent soldiers who were ready enough to accept the pay of the Roman emperor for service in the legions, even if rendered, as it often was, against their kinsmen who were menacing the weakened frontier. From this source the Empire had long been receiving a large infusion of German blood before any considerable tribe came within its bounds to settle in a body. Indeed, if there had occurred no sudden and startling overflows of population from the Germanic countries, such as the Visigothic invasion, it is quite possible that the Roman Empire might yet have fallen completely into the hands of the Germans by the quiet and gradual processes just indicated. As it was, the pressure from advancing Asiatic peoples on the east was too great to be withstood, and there resulted, between the fourth and sixth centuries, a series of notable invasions which left almost the entire Western Empire parceled out among new Germanic kingdoms established by force on the ruins of the once invincible Roman power. The breaking of the frontier by the West Goths (to whom the Emperor Aurelian, in 270, had abandoned the rich province of Dacia), during the reign of Gratian in the West and of Valens in the East, was the first conspicuous step in this great transforming movement.