For nearly a hundred years after Vespasian's accession to the throne, there was no serious interruption to the peaceful intercourse of the two races. During this time, we must take it for granted that a gradual change must have been growing up in the habits and ideas of the Germans. It is probable that they then began to collect in villages; to use stone as well as wood in building their houses and fortresses; to depend more on agriculture and less on hunting and fishing for their subsistence; and to desire the mechanical skill, the arts of civilization, which the Romans possessed. The extinction of many smaller tribes, also, taught them the necessity of learning to subdue their internal feuds, and assist instead of destroying each other. On the north of them was the sea; on the east the Sarmatians and other Slavonic tribes, much more savage than themselves: in every other direction they were confronted by Rome. The complete subjugation of their Celtic neighbors in Gaul was always before their eyes. In Hermann's day, they were still too ignorant to understand the necessity of his plan of union; but now that tens of thousands of their people had learned the extent and power of the Roman Empire, and the commercial intercourse of a hundred years had shown them their own deficiencies, they reached the point where a new development in their history became possible. [Sidenote: 166.] Such a development came to disturb the reign of the noble Emperor, Marcus Aurelius, in the latter half of the second century. About the year 166, all the German tribes, from the Danube to the Baltic, united in a grand movement against the Roman Empire. The Marcomanni, who still inhabited Bohemia, appear as their leaders, and the Roman writers attach their name to the long and desperate war which ensued. We have no knowledge of the cause of this struggle, the manner in which the union of the Germans was effected, or even the names of their leaders: we only know that their invasion of the Roman territory was several times driven back and several times recommenced; that Marcus Aurelius died in Vienna, in 181, without having seen the end; and that his son and successor, Commodus, bought a peace instead of winning it by the sword. At one time, during the war, the Chatti forced their way through the Tithe-Lands and Switzerland, and crossed the Alps: at another, the Marcomanni and Quadi besieged the city of Aquileia, on the northern shore of the Adriatic.