_In patris sempiturnus es filius._ The practice of Organum, crude as it may seem to modern ears, was or immense historical importance, as the first embodiment of that principle of combining various parts simultaneously which in due time produced all the resources of polyphony and of harmony. It is not necessary to examine here, in detail, all the stages of that long and weary journey which the mediæval composers made from this starting-point of Organum to the highly developed contrapuntal music of the sixteenth century. In all its aspects it was essentially a growth in definiteness, coherence, and heterogeneity. The parts were combined with more and more freedom, both as to their comparative rate of movement and as to the purity of the chords they made at prominent points (less harmonious intervals being gradually tolerated); the number of parts was increased, in spite of the great difficulties that each additional part must have meant to writers with inadequate experience and models; experiments were tried in combining together tunes already composed, popular songs and the like, trimming and twisting and compressing or expanding them to make them fit; the device of imitation, of which more will be said presently, was introduced in the interests of sense and coherence;[12] one experiment after another was tried, one resource after another was utilized, until eventually, in the sixteenth century, the art of ecclesiastical counterpoint[13] was fully established. To this sixteenth-century music it is difficult for modern ears to listen appreciatively. The exact value and significance of chords, cadences, and melodic phrases, like the exact significance of words in language, depends so largely upon current usage and the mental habits it reposes upon, that it is as much an effort for modern listeners to comprehend mediæval music as it is for the modern reader to understand the vocabulary of Chaucer or Shakespeare. Just as words, in the course of long service, gradually take on new associations, new shades of suggestion, and even, in extreme cases, a significance quite opposite to their original one, so the material of music, as used to-day, has hundreds of associations and subtle shades of value, developed only during the last three hundred years, but nevertheless permeating our minds so thoroughly that it is almost impossible for us to think them away.