These elements of new blood were chiefly supplied by the Franks (both of the East and the West), the Burgundians, and the Normans, the latter being descendants of the Vikings from the Baltic. The Belgæ were a subdivision of the Franks, and made up of several tribes, Trevii, Eburones, Nervii, etc. Generally speaking, they were Germanic, with a considerable Celtic admixture. The Cluniac and Cistercian reforms came from Burgundy, which is partially within the limits of our study, though later they received great accessions of strength from natives of Flanders, Brabant, the Rhineland, and Champagne. During the eleventh century Normandy was the spiritual centre, the dynamic force, of Europe, while in the twelfth century the leadership was assumed by the Île de France, as wholly under the inspiration of the Cistercians as Normandy had been under that of the Cluniacs. It was during these two centuries that the great burst of Norman and of Gothic architecture occurred in the Île de France, in Normandy, and in Champagne. The contributions of the land we now know as Belgium were quite different; they were at the same time a product of mediæval culture and one of its causes, for they grew out of the deep and vital impulses beneath the whole epoch, while they seemed to determine many of its manifestations. The first of these, the Crusades, has already been referred to; the second, the great guild system, with its concomitant, the commune, and its result, a desire for personal, civic, and national liberty that became a passion, needs some consideration, since it is from this that came so much of the later mediæval art of Flanders and Brabant that is so priceless and so appallingly in danger of destruction. Just how and why the Flemings should have become a nation of weavers, merchants, and traders is hard to say, but even in the tenth century, weaving had become so important an industry a charter was granted the guild of weavers by Count Baldwin. The supply of wool came overseas from England, where an important market for the finished wares was also found, and as a result a close community of interests sprang up between Flanders and East Anglia. Without natural protection of any kind, the land lying open to any invasion, walled cities became imperative, as well as unions for self-defence, and so came the great and rich and defiant cities such as Ghent and Bruges, Ypres and Courtrai. When the nobles and knights flocked off on crusade, the citizens remained at home, and they were not slow to seize the opportunity offered them of acquiring, almost without protest, the civil power that, elsewhere, under a dominant and universal feudalism, remained in the hands of the barons.