The Book of Lost Tales
In his "The Book of Lost Tales", Tolkien develops a theme that the diminutive fairy-like race of Elves had once been a great and mighty people, and that as Men took over the world, these Elves had "diminished" themselves. This theme was influenced especially by the god-like and human-sized "Ljósálfar" of Norse mythology, and medieval works such as "Sir Orfeo", the Welsh "Mabinogion", Arthurian romances and the legends of the "Tuatha Dé Danann".