In the meantime, as the musical world has moved forward in respect to structure from the symphony to the symphonic poem, followed by its logical sequence the tone-poem, in which the elements of various forms have been incorporated, so has there been progress and even revolution in the technical material of music itself. Dargomijsky was probably the pioneer in using the whole-tone scale, as may be seen in the third act of his opera 'The Stone Guest,' composed in 1869. Rimsky-Korsakoff elaborated on his foundation as early as 1880 in his opera _Sniégourutchka_. Moussorgsky showed unusually individual harmonic tendencies, as the first edition of _Boris Godounoff_ before the revisions and alterations by Rimsky-Korsakoff clearly demonstrate. After casual experiments by Chabrier, d'Indy, and Fauré, Debussy founded an original harmonic system, in which modified modal harmony, a remarkable extension of whole-tone scale chords, the free use of ninths, elevenths and thirteenths are the chief ingredients. Dukas has imitated Debussy to some extent, Ravel owes much to him; both have developed independently, Ravel in particular has approached if not crossed the boundaries of poly-harmony. Scriabine, following the natural harmonic heritage of the Russians, has evolved an idiom of his own possessing considerable novelty but disfigured by monotony, in that it consists chiefly of transpositions of the thirteenth-chord with the alteration of various constituent intervals. What he might not have accomplished can only be conjectured, since his career has been terminated by his sudden death. Although Richard Strauss has greatly enlarged modern harmonic resource, his results must be regarded on the whole as a by-product of his contrapuntal virtuosity. In his treatise on harmony Schönberg refers to his 'discovery' of the whole-tone scale long after both Russians and French had used it, but it is noteworthy that Schönberg arrived at the conception of this scale and its chords with an absolute and unplagiaristic independence. The most recent developments affecting the technical character of music are poly-harmony, or simultaneous use of chords in different keys, and free dissonant counterpoint. Striking instances of the former type of anarchic experiment may be found in the music of Igor Stravinsky, whose reputation has been made by the fantastic imagination and the dramatic sincerity of his ballets 'The Bird of Fire,' _Petrouchka_, 'The Ceremonial of Spring,' and 'The Nightingale.' In these he has mingled Russian and French elements, fusing them into a highly personal and extremely dissonant style, which in its pungent freedom and ingenious mosaic of tonalities is both highly diverting and poignantly expressive. Stravinsky is one of the most daring innovators of to-day, and both his dramatic vitality and the audacity of his musical conceptions mark him as a notable figure from whom much may be expected.