Lynn Margulis is Distinguished Professor in the Department of Geosciences at the University of Massachusetts, Amherst, and the recipient of the 1999 National Medal of Science. She is the author of more than one hundred articles and ten books, including " Symbiosis and Cell Evolution " (second edition 1993). Dorion Sagan is the author of " Biospheres " (1990). As partners of Sciencewriters, Margulis and Sagan have also written " Microcosmos " (California, 1996), " What Is Sex ? " (1990), " Garden of Microbial Delights " (1995), " Mystery Dance " (1991), and several guides to videos of live organisms.