Messiaen continually evolved new composition techniques, always integrating them into his existing musical style; his final works still retain the use of modes of limited transposition. For many commentators this continual development made every "major" work from the "Quatuor" onwards a conscious summation of all that Messiaen had composed up to that time. However, very few of these major works lack new technical ideas—simple examples being the introduction of communicable language in "Meditations", the invention of a new percussion instrument (the geophone) for "Des canyons aux etoiles...", and the freedom from any synchronisation with the main pulse of individual parts in certain birdsong episodes of "St. François d'Assise".