It is generally acknowledged that throughout the history of mediaeval art the Church was not only the most generous protectress of the artist, but also the most fruitful source of his inspiration. This is true in a special sense of the art of music. The Church, of course, has always had her own music, namely, the plainsong, which, as Dom Shebbeare wrote in a recent number of Blackfriars, she evolved in the process of evolving a liturgy. Plainsong had been brought to its highest stage of perfection by the beginning of the seventh century, under Saint Gregory I, the great classifier and arranger of the music of the liturgy, but it remained a living musical language for many centuries after the time of the great Pope. As new Feasts were instituted, new music was composed for them, and fresh melodies for the Ordinary Chants of the Mass were written throughout the Middle Ages. All this new liturgical music was written according to the rules of a long-established tradition, but side by side with the unchanging music of the Church, there has existed in Europe for a thousand years another branch of the art, derived from plainsong, yet differing profoundly from it, chiefly by reason of the fact that it has undergone an unending series of changes and developments.