Besides the development of the sectional form, another technical change acted to break down the old obstacles to characteristic expression. An essential feature of the mediaeval music, consequent upon the very nature of the Gregorian modes, was the very slight employment of chromatic alteration of notes, and the absence of free dissonances. Modulation in the modern sense cannot exist in a purely diatonic scheme. The breaking up of the modal system was foreshadowed when composers became impatient with the placidity and colorlessness of the modal harmonies and began to introduce unexpected dissonances for the sake of variety. The chromatic changes that occasionally appear in the old music are scattered about in a hap-hazard fashion; they give an impression of helplessness to the modern ear when the composer seems about to make a modulation and at once falls back again into the former tonality. It was a necessity, therefore, as well as a virtue, that the church music of the old régime should maintain the calm, equable flow that seems to us so pertinent to its liturgic intention. For these reasons it may perhaps be replied to what has been said concerning the devotional ideal embodied in the calm, severe strains of the old masters, that they had no choice in the matter. Does it follow, it may be asked, that these men would not have written in the modern style if they had had the means? Some of them probably would have done so, others almost certainly would not. Many writers who carried the old form into the seventeenth century did have the choice and resisted it; they stanchly defended the traditional principles and condemned the new methods as destructive of pure church music. The laws that work in the development of ecclesiastical art also seem to require that music should pass through the same stages as those that sculpture and painting traversed,—first, the stage of symbolism, restraint within certain conventions in accordance with ecclesiastical prescription; afterwards, the deliverance from the trammels of school formulas, emancipation from all laws but those of the free determination of individual genius. At this point authority ceases, dictation gives way to persuasion, and art still ministers to the higher ends of the Church, not through fear, but through reverence for the teachings and appeals which the Church sends forth as her contribution to the nobler influences of the age.