There are a lot of songs that weren’t exactly original. Because the originals are so old they are part of the public domain and can be used without fearing a lawsuit. “There’s a place for Us” is from Beethoven’s Emperor Concerto second movement, “I don’t know how to love him” is from the second movement of Mendelssohn’s Violin Concerto, “Don’t sit under the apple tree” is from an old folk song, “I can’t help falling in love with you” is from a 1790’s French song called “Plaisir D’Amour”. There are many other examples as well.
(The Great Courses – Great American Music: Broadway Musicals)