Shorter, black-and-white daily strips began to appear early in the 20th century, and became established in newspapers after the success in 1907 of Bud Fisher's Mutt and Jeff. Humour strips predominated at first, and in the 1920s and 1930s strips with continuing stories in genres such as adventure and drama also became popular. Thin periodicals called comic books appeared in the 1930s, at first reprinting newspaper comic strips; by the end of the decade, original content began to dominate. The success in 1938 of Action Comics and its lead hero Superman marked the beginning of the Golden Age of Comic Books, in which the superhero genre was prominent.