So-called Gothic architecture was not the product of a single generation; it was the continuous logical development of the Romanesque movement, just as the latter in its time had been the outcome of a gradual adaptation of old traditions to new-born exigencies. Thus our Aquitainian forbears, by their successful translation into stone of the eastern cupola, prepared the way for the groined vault, the embryo of which is clearly traceable in the pendentives of the dome at St. Front. The great churches which, towards the middle of the twelfth century, rose throughout the rich Western provinces that cluster about Aquitaine, were all constructed with groined vaults. In these examples we can discern no halting, tentative application of newly adopted principles. The work is that of consummate architects, who brought to their labours the assurance born of experienced skill, and in the later part of the twelfth century, the new system had replaced all others for the construction of vaults throughout Western Europe. The architects of the royal domain, and notably those of the Ile-de-France, had been the first to adopt the groined vault. Towards the close of the twelfth century their assimilation of the new principles, their native ingenuity, and professional hardihood alike urged them to its further development. They became the inventors of the flying buttress. The substitution of the groined vault for its parent, the cupola, was the direct consequence of the old tradition. The development was merely a stage in the march of ideas, a consummation logically arrived at in the track which the Romans, constructors not less bold though more prudent than their artistic progeny, had marked out for them. The groined vault, in short, is simply the growth of Roman principles perfected by continuous experiment. But the flying buttress, or rather the system of construction based on its use, caused a radical change in the art of building of the twelfth century. Stability, which in the ancient buildings was ensured by solid masses at the impost of vaults and arches, was replaced by the balance of parts. From this daring system some of the most marvellous of architectural effects have been won; but the innovation had a dangerous inherent weakness, inasmuch as it involved the exterior position of those essential vital organs for whose preservation the ancients had wisely provided, by keeping them within the building.