Under these influences ecclesiastical architecture advanced with rapid strides. No longer hampered by monastic restrictions, it called into its service the laity, whose guilds of masons and builders carried from one diocese to another their constantly increasing stores of constructive knowledge. By a wise division of labor, each man wrought only such parts as he was specially trained to undertake. The master-builder--bishop, abbot, or mason--seems to have planned only the general arrangement and scheme of the building, leaving the precise form of each detail to be determined as the work advanced, according to the skill and fancy of the artisan to whom it was intrusted. Thus was produced that remarkable variety in unity of the Gothic cathedrals; thus, also, those singular irregularities and makeshifts, those discrepancies and alterations in the design, which are found in every great work of mediæval architecture. Gothic architecture was constantly changing, attacking new problems or devising new solutions of old ones. In this character of constant flux and development it contrasts strongly with the classic styles, in which the scheme and the principles were easily fixed and remained substantially unchanged for centuries. [Illustration: FIG. 106.--PLAN OF SAINTE CHAPELLE, PARIS, SHOWING SUPPRESSION OF SIDE-WALLS.] +STRUCTURAL PRINCIPLES.+ The pointed arch, so commonly regarded as the most characteristic feature of the Gothic styles, was merely an incidental feature of their development. What really distinguished them most strikingly was the systematic application of two principles which the Roman and Byzantine builders had recognized and applied, but which seem to have been afterward forgotten until they were revived by the later Romanesque architects. The first of these was the _concentration of strains_ upon isolated points of support, made possible by the substitution of groined for barrel vaults. This led to a corresponding concentration of the masses of masonry at these points; the building was constructed as if upon legs (Fig. 105). The wall became a mere filling-in between the piers or buttresses, and in time was, indeed, practically suppressed, immense windows filled with stained glass taking its place. This is well illustrated in the +Sainte Chapelle+ at Paris, built 1242-47 (Figs. 106, 122). In this remarkable edifice, a series of groined vaults spring from slender shafts built against deep buttresses which receive and resist all the thrusts. The wall-spaces between them are wholly occupied by superb windows filled with stone tracery and stained glass. It would be impossible to combine the materials used more scientifically or effectively. The cathedrals of Gerona (Spain) and of Alby (France; Fig. 123) illustrate the same principle, though in them the buttresses are internal and serve to separate the flanking chapels. [Illustration: FIG. 107.--EARLY GOTHIC FLYING BUTTRESS.]