In reading the words of the Catholic liturgy from the Missal we must remember that they were written to be sung, and in a certain limited degree acted, and that we cannot receive their real force except when musically rendered and in connection with the ceremonies appropriated to them. For the Catholic liturgy is in conception and history a musical liturgy; word and tone are inseparably bound together. The immediate action of music upon the emotion supplements and reinforces the action of the text and the dogmatic teaching upon the understanding, and the ceremony at the altar makes the impression still more direct by means of visible representation. All the faculties are therefore held in the grasp of this composite agency of language, music, and bodily motion; neither is at any point independent of the others, for they are all alike constituent parts of the poetic whole, in which action becomes prayer and prayer becomes action. The music of the Catholic Church as it exists to-day is the result of a long process of evolution. Although this process has been continuous, it has three times culminated in special forms, all of them coincident with three comprehensive ideas of musical expression which have succeeded each other chronologically, and which divide the whole history of modern music into clearly marked epochs. These epochs are those (1) of the unison chant, (2) of unaccompanied chorus music, and (3) of mixed solo and chorus with instrumental accompaniment. (1) The period in which the unison chant was the only form of church music extends from the founding of the congregation of Rome to about the year 1100, and coincides with the centuries of missionary labor among the Northern and Western nations, when the Roman liturgy was triumphantly asserting its authority over the various local uses. (2) The period of the unaccompanied contrapuntal chorus, based on the mediaeval key and melodic systems, covers the era of the European sovereignty of the Catholic Church, including also the period of the Counter-Reformation of the sixteenth century. This phase of art, culminating in the works of Palestrina in Rome, Orlandus Lassus in Munich, and the Gabrielis in Venice, suffered no decline, and gave way at last to a style in sharp contrast with it only when it had gained an impregnable historic position. (3) The style now dominant in the choir music of the Catholic Church, _viz._, mixed solo and chorus music with free instrumental accompaniment, based on the modern transposing scales, arose in the seventeenth century as an outcome of the Renaissance secularization of art. It was taken up by the Catholic, Lutheran, and Anglican Churches, and was moulded into its present types under the influence of new demands upon musical expression which had already brought forth the dramatic and concert styles.