When Arnold Schoenberg was a young man he was living in the late 19th century. If you look at the way composers wrote music at this time you will find that their compositions became more and more chromatic. This means that more and more sharps and flats were used during the piece, and the music went from one key to another (modulated) very quickly. Richard Wagner wrote very chromatic music, especially in his opera "Tristan and Isolde".