WHATEVER ELSE art is good for, its chief effectiveness lies in propagating more art," says art historian Leo Steinberg. "All art is infected by other art."

There is ample evidence from every age to prove his point. The architecture of Greece subsequently changed the face of Rome and Paris, not to mention 20th-century Washington. A theme by Haydn inspired Brahms to challenge his master; two characters from Shakespeare blossomed into Tom Stoppard's "Rosenkrantz and Guildenstern"; the words of early American poets became the source of Leonard Bernstein's "Songfest." Even comedian Steve Martin's hilarious variation on King Tut and his treasure was, in its way, inspired by prior art or at least by an art exhibition.

Painting, too, has always leaned heavily upon its predecessors, no matter how different the results may seem, which is not too surprising in view of the fact that for centuries, artists learned chiefly by way of copying old masters. Even the mature Rubens continued to study color by copying the Venetian master Titian. His lively copy of Titian's "Adam and Eve" in fact, hangs in the Prado not far from the original.

There were, however, (and still are) more furtive borrowings that had less to do with inspiration, learning and "homage" than with finding prefabricated solutions to formal problems. The American painter John Trumbull, in a standing portrait of George Washington, for example, swiped the general's white steed straight out from behind Van Dyck's portrait of Charles I of Spain, then widely known through contemporary engravings. Van Dyck, in turn, had rustled that very same horse straight out of Titian's barn, specifically from his "Adoration of the Magi," a painting he had seen and sketched while in Italy.

In literature, the footnote is used to avoid plagarism. In painting, there are no footnotes, except for those provided later by historians of art, who spend some of their most exciting hours tracking down such clues.