Regarding form in music there is much prejudice one way or the other. The sonnet in poetry certainly is a rigid form; and yet those poets who have mastered it have produced extremely effective and highly artistic poems, and poems abounding in profound emotional expression. Walt Whitman, on the other hand, was quite formless, and yet he is sure to be ranked in time as one of the greatest poets of his age. Wagner's idea was that the symphonic form had reached its climax with Beethoven's Ninth Symphony; yet it is by no means incredible that if Wagner in his maturer years had undertaken to compose a symphony, the result would have disproved his own theory.
The symphonic form, however, or, to be more exact, the sonata form, seems to hamper every modern composer when he writes for the pianoforte, and the fact that most of Beethoven's pianoforte music was written in this form appears to be the reason for his works somewhat falling into disuse. On the other hand, the form is undoubtedly holding out better in the orchestral version of the sonata, the symphony, because the tone color of orchestral instruments gives it greater variety. Tschaikowsky, Dvorak and Brahms have worked successfully, and the two former even brilliantly, in this form; and if Brahms in his symphonies appears too continent, too classically reserved, it would seem to be not so much the form itself which is to blame, as his lack of skill in instrumentation.
My own personal preference is for the freer form developed by Liszt in the symphonic poem, in which a leading motive, or possibly several motives skillfully varied dominate the whole composition and give it esthetic and psychological unity; and for the still freer development of instrumental music in the tone poem of Richard Strauss. But neither the symphonic poems of Liszt nor the tone poems of Strauss are formless music. That should be well understood, although it should be borne in mind with equal distinctness that these manifestations of the genius of two great composers show a complete liberation from the shackles of the classical symphony. In the end the test is found in the music itself. If the music of a symphonic poem which sets out to express a given title or a given motto, if the music of a tone poem which starts out to interpret a programmatic story or device, is worthy to be ranked with the great productions of the art, it not only is profoundly interesting as music, but gains immensely in interest through its incidental secondary meaning. It is the old story of art for art's sake--art for the purpose of merely gratifying the eye or the ear--or art for the purpose of conveying something besides itself to the beholder or the listener; and it seems to me that, in the history of the art, art for art's sake has always been the more primitive expression and eventually has been obliged to give way.