Lest the reader get lost in the maze of technical details, however, it will be well now to revert to the general principles underlying all these musical phenomena, and to sum up, before passing on, the essential points we have been trying to come at. Those arts which, like poetry and music, present their matter to us in a temporal series, depend for that organization of variety into unity which is beauty (and the _sine qua non_ of all art) on the arousal in us of expectations, which are presently fulfilled. By first leading us to expect something, and then presenting it, they enable us to group our impressions, to feel that they are interrelated and mutually dependent, to get, in short, the sense of design or order. Music effects this by means of metrical and harmonic form, which act is the same way so far as they present unrestful, followed by restful, impressions, though in different ways so far as the technical basis of these impressions is concerned. Psychologically speaking, metrical and harmonic form cooperate to give music definite structure in our minds; to reclaim it from the condition of a mere sensuous or emotional stimulus, and engraft upon it the final and supreme beauty of order. All absolute or pure music depends for its structure on these two great formative agents of metrical and harmonic design; but the mode of their application progressed from simplicity to comparative complexity as music evolved from the choral song of the sixteenth century, out of which it grew, to the modern sonata and symphony. It would be quite impossible to examine in detail, here, all the stages of that progress. Our effort must be rather to define three well-marked phases of the many-sided growth in general and summary terms, taking for granted, meanwhile, the minor variations and modifications which elude our somewhat rough analysis. These three phases have in common certain essential traits. In each we see music making up its elementary units of effect, out of unorganized tones, by the aid of metrical and harmonic form; in each we see it combining these units into complex designs by means of the principles of variated repetition of them. The difference between the phases is that in the later ones the units are larger and more definite, and are combined into broader, more complex organisms.