The gradual expansion and increase of complexity in the movements of the suite, made not only possible but logically necessary by the structural potencies of these great principles of statement, contrast, and reassertion, and of antithesis of keys, led eventually to a new phase of musical structure, the third and last in the evolution we have been tracing. The suite, in the seventeenth century the most successfully cultivated of all the forms of pure music, gave place in the eighteenth century to a still higher form, the sonata, which has held the position of supremacy ever since. The sonata form is, not only by tradition but by natural right, the norm of modern musical structure. Almost all the chief works of all the great composers from Haydn and Mozart to Brahms and Tschaïkowsky are cast in this mould, as we easily realize if we remember that not only those pieces specifically named “sonatas,” but also trios, quartets, quintets, and the like, and overtures and concertos and symphonies, are but pieces in sonata-form intended for various groups of instruments. The string quartet is a sonata for two violins, a viola, and a 'cello; the concerto is a sonata for solo instrument with orchestral accompaniment; and the symphony is a sonata on a large scale, for orchestra. This remarkable prevalence of a single type of structure in modern music means far more than the accidental survival, by inertia, of an artificial convention; it means that this type of structure is on the whole the best possible embodiment of variety and unity in tonal effects; that it is the natural outgrowth of more primitive forms; and that it is elastic enough to admit into its uniform scheme of order the most diverse expressions of individual temperaments and ideals. Tschaïkowsky’s intuition of beauty in tones is different enough from Haydn’s; and the formal medium of which both can avail themselves without violence to their genius must obviously be founded deep in universal human psychology.