Appropriation is the idea that ate the art world. Go to any Chelsea gallery or international biennial and you’ll find it. It’s there in paintings of photographs, photographs of advertising, sculpture with ready-made objects, videos using already-existing film. After its hothouse incubation in the ‘70s, appropriation breathed important new life into art. This life flowered spectacularly over the decades -- even if it’s now close to esthetic kudzu.