"In Isaac Newton's lifetime, no more than a few thousand people had any idea what he looked like, though he was one of England's most famous men, yet now millions of people have quite a clear idea--based on replicas of copies of rather poorly painted portraits. Even more pervasive and indelible are the smile of Mona Lisa, The Scream of Edvard Munch, and the silhouettes of various fictional extraterrestrials. These are memes, living a life of their own, independent of any physical reality. "This may not be what George Washington looked like then," a tour guide was overheard saying of the Gilbert Stuart painting at the Metropolitan Museum of Art, "but this is what he looks like now." Exactly."