ONE OF THE MAJOR DIFFERENCES between the music of the classical period (late eighteenth century) and that of the nineteenth century was the tendency of Romantic composers to resort explicitly to external, non-musical sources to form the basis for the creation of new instrumental compositions. Nature proved valuable in this regard; examples include such works as Liszt's piano piece Au lac de Wallenstadt (Années de Pèlerinage I) (At the Wallenstadt Lake, Years of Pilgrimage I, 1858), or Mendelssohn's Hebrides Overture (1832), which consciously try to reproduce in sonic terms a visual phenomenon — or, at least, an emotional recollection of the scene. However, with famous examples by Vivaldi (Le quattro stagione, The Four Seasons, 1725) and Beethoven (Symphony No. 6, “Pastoral,” 1808) providing precedents, nature depiction was not really a new departure in the nineteenth century, but rather the continuation of an existing tradition on a more widespread basis. For many composers, including Schumann and Liszt, the impetus that other art forms could give to the production of musical works was even more important than nature, and truly constituted one of the major innovations of the period. If few in the nineteenth century went as far as Wagner in advocating a Gesamtkunstwerk — a union of all arts to the service of music-based drama — virtually all musicians were conscious of being part of a broader cultural movement. The stimulus provided by literature proved hugely important for many composers, even in the field of purely instrumental music.