The French organ composer, Charles Marie Widor, left one of the fundamental legacies of organ music. Unfortunately, too often, these works are played without understanding, for mere speed and loudness. The most egregious example of this is Widor's famous Toccata from the Fifth Symphony. Widor did everything to slow organists down, without avail, writing various articulation marks, etc. The reality was, and is, that any organist playing the Widor Toccata slowly, in a way that reveals the interest and novelty of the harmonic progressions, would be accused by his brother organists of "not knowing how to play."
Widor's Organ Symphonies, Two Sets of Four
The first four symphonies, Op 13 (1872) are different from the second set of four, in that the individual movements of the first opus are often extremely long, and could even stand on their own as individual works. There are various versions of these sonatas, and unfortunately the versions published by Dover are worse -- for structural reasons and sometimes because the movements are different, with more interesting ones in the Schirmer -- than the version in the Schirmer edition.
The Dover edition makes every repeat possible, but the Schirmer edition is much more subtle in its repetition and non repetition of sections. And there is the beautiful Salve Regina in the Second Symphony, one of the few early movements of Widor based on the plain chant which is replaced in the Dover. These textual variations, quite extensive, have apparently not been analyzed, even in the so-called Urtext edition of Widor. There is a remark in the English wikipedia article on Widor, linked to above, that says Widor continually revised his symphonies. This obviously happened, especially with the early symphonies, but it needs to be made more precise.
The second set of four, Op 42 (1879), are more like French suites, with five and even six character movements -- march, scherzo, minuetto, etc -- strung together. But again the elaboration of the musical ideas is quite extensive, and beautifully done, and this come out if these pieces are not played for mere virtuosity. Widor really understood the sonority of the French organ, and although he was capable of writing a fugue, when he does so it looks thin on the page -- because he was letting the power of the organ carry it. The French romantic organ didn't have the full plenum of a Bach organ, and this organ, with all its 8 foot and 16 foot stops sounds clotted if a fugue is written as Bach would write it.
The Last Two Symphonies -- Widor and Plain Song
For the final two symphonies, the Gothique (Op 70, 1895) and the Romane (Op 73, 1900), Widor turned to plainchant, the chants of Christmas and Easter respectively. But the difference in treatment is very interesting -- in the first symphony Widor in general treats the plain song as a cantilena to be embellished. In the second, on the other hand, the use of the plain song is more sophisticated, more organic to the structure of the work.
Widor and Mahler
Why can Widor be compared to Mahler? They both were virtuosos (Widor on the organ, Mahler as a conductor, a virtuoso of the orchestra) who knew the ultimate potential of their instruments, and went to the utmost limits of the possible, and also the utmost limits of good taste, reworking both Wagner and the whole canon of Western music into their work. Widor however was a more forward looking figure than Mahler, anticipating rock n' roll, even minimalism, in the cycling between two dominant seventh chords in the finale of Symphony 2.
Unfortunately Widor is not well played. It is usually played only for effict, to see how fast and how loud the organist can play. This trashes the musical ideas of the pieces. But when Widor is played as he should be played, the beauty and sublimity of his work stands out.