Since the motif is the animating force of the entire fugue, it is obvious that upon its pointedness, variety, and interest will depend the vitality of the composition as a whole. Bach accordingly spares no pains in the construction of his motifs. Much as they differ in length, expression, and style, all are brimful of interest. Each embodies some striking musical idea; some persuasive or emphatic rhythm, some definite tonal design which either by its oddness or by its utter naturalness and inevitability lays firm hold upon the attention at once, and coerces interest whenever it recurs. Here are a few motifs from the “Well-Tempered Clavichord”: [Music: score] [Illustration: FIGURE VII.] The variety is wonderful, even in these five subjects; and it will be seen at once how provocative of musical thought they are, like condensed aphorisms, packed with suggestions that send the mind questing through endless vistas of imagination. As for the further treatment of the fugal motif, the actual formal rules, despite the awe they have immemorially aroused in the popular mind, are few and simple. After the first announcement of the subject by a single voice, it is answered by a second voice, at an interval of a fifth above;[27] then again stated by a third voice, and answered by a fourth. This process goes on until each voice has had a chance to enunciate the motif, after which the conversation goes on more freely; the subject is announced in divers keys, by divers voices; episodes, in a congruous style, vary the monotony; at last the subject is emphatically asserted by the various voices in quick succession (“stretto”) and with some little display or grandiloquence the piece comes to an end. But simple as is this scheme, it gives the composer ample opportunity to develop his theme with the utmost ingenuity, to subject it to the most surprising metamorphoses, and to place it in ever new lights and postures. [Music: score] [Illustration: FIGURE VIII.] Practically all the possibilities of developing a motif were exploited by Bach in his marvelous fugues. The development of the motif means, in the most general terms, the repetition of it in forms sufficiently like the original one to be recognizable, yet sufficiently unlike it to be novel and interesting, to exhibit it, as has just been said, in “new lights and postures.” Now, since the identity of the motif depends on the fixed metrical and harmonic relations of its constituent tones, it is obvious that variation of it will have to consist in slight alterations of these metrical or of these harmonic relations, or of both, managed with such skill that they do in effect vary, without disintegrating, the motif. Our next task, then, will be to describe the chief means, both metrical and harmonic, by which the motif, in the hands of Bach and of all his successors, is modified without being destroyed.