Early folk song collectors also often collected (what are now known as) nursery rhymes, including in Scotland Sir Walter Scott and in Germany Clemens Brentano and Achim von Arnim in Des Knaben Wunderhorn (1806–1808). The first, and possibly the most important academic collection to focus in this area was James Orchard Halliwell's The Nursery Rhymes of England (1842) and Popular Rhymes and Tales in 1849, in which he divided rhymes into antiquities (historical), fireside stories, game-rhymes, alphabet-rhymes, riddles, nature-rhymes, places and families, proverbs, superstitions, customs, and nursery songs (lullabies). By the time of Sabine Baring-Gould's A Book of Nursery Songs (1895), folklore was an academic study, full of comments and footnotes. A professional anthropologist, Andrew Lang (1844–1912) produced The Nursery Rhyme Book in 1897. The early years of the 20th century are notable for the illustrations to children's books including Caldecott's Hey Diddle Diddle Picture Book (1909) and Arthur Rackham's Mother Goose (1913). The definitive study of English rhymes remains the work of Iona and Peter Opie.