Plagiarism is basic to all culture." Pete Seeger claims that his father, a Harvard musicologist, told him that. To which I could only reply (plagiarizing Jerry Lee Lewis), "You're so right you don't know what you're saying."
Seeger was, of course, talking mostly about the folk cultural process by which the same stories and tunes get passed down and reinterpreted from generation to generation. Today that process continues in popular culture. If you don't believe me, read Dave Marsh's landmark work of cultural criticism, Louie Louie.