Cadences serve the same purpose in music, then, as do the punctuation marks in rhetoric; and an idea of the senselessness and confusion of a musical composition, if left devoid of cadences in sufficient number and force, may be gleaned from an experimental test of the effect of a page of prose, read with persistent disregard of its commas, colons, and other marks of "cadence."
Another evidence of Form in music, that is at once subtle and powerful, rests upon what might be termed the linear quality of melody. The famous old definition of a line as a "succession of points," tallies so accurately with that of melody (as a "succession of single tones"), that it is not only proper, but peculiarly forceful, to speak of melodies as tone-lines. Our conception of a melody or tune, our ability to recognize and reproduce it, depends far more upon its undulations, its rising, falling, or resting level, than upon its rhythmic features (the varying lengths of its tones). These movements trace a resonant line before our mind's eye as surely, though perhaps not as distinctly, as the pencil of the artist traces the lines of an image upon the paper; and this process is going on constantly, from beginning to end, in every piece of music. In a portrait it describes the contours of face and figure,—in a word, the Form; in the musical composition it fulfils, to a great extent, the self-same mission, that of defining the Form. One clear, predominating tone-line traces the "air" or tune of the piece; and this is often the only line that arrests the hearer's attention; but there are other tone-lines, less prominent and less extended and coherent, gliding along harmoniously beside the Melody proper, which (something like the shading in a picture) contribute to the richness of the design, and perform their share in proving and illuminating the Form of the whole.
This is most salient in music for orchestra, where each player describes an individual tone-line, rendered all the more distinct and recognizable by the specific "color" of his instrument; and that is the chief, perhaps the sole, reason why the orchestra is esteemed the most complete and perfect medium of musical expression.
UNITY AND VARIETY.—As much as opinions and beliefs may differ, among music critics, as to the necessity of Form in music, and the conditions of its existence, no reasonable objection can be taken to the hypothesis that Clearness and Attractiveness are the two vital requisites upon which the enjoyment of any art depends. The artist's utterances or creations must be intelligible, and they must be interesting. The lack, partial or total, of either of these qualities neutralizes the force of the intended impression, in precise proportion to the default.
In musical composition these two requisites are embodied in the principles of Unity and Variety.
Unity—in its various technical phases of Uniformity, Regularity, Similarity, Equality, Agreement, or whatever other synonym we may find it convenient to use—is the condition out of which the composer must secure intelligibility, clearness, definiteness of expression. Glance at Ex. 2, and note the evidences of unity (similarity) in the rhythmic and melodic formation of the first four measures.
Variety—in its most comprehensive application—is the medium he must employ to arouse and sustain the hearer's interest. Glance again at Ex. 2, and note the contrast between the two halves of the first four measures, and between these and the following two measures.
These conditions are, of course, squarely opposed to each other, though their interaction is reciprocal rather than antagonistic; and, from what has been said, it is obvious that they are of equal importance. Hence, as was declared on the second page, the great problem of the art-creator consists in so balancing their operations that neither may encroach upon the domain of the other. For too constant and palpable Unity will inevitably paralyze interest; while too much Variety will as surely tend to obscure the distinctness of the design.