Lynn Margulis, one of the most provocative scientific thinkers of our time, and her son, the writer Dorion Sagan, here present a selection from their many essays published in the last decade and a half. Margulis's scientific contributions are legendary. Her proposal that eukaryotic cells (the cells of all multicellular animals and plants) are made up of symbiotic unions of more primitive cells was at first widely derided but is now mainstream science. She has described the previously unrecognized role microbial life plays in the maintenance of all life on earth. And she is, with James Lovelock, one of the founders of Gaia theory. In these essays, perhaps better than in any of her other books, one can see how these apparently unrelated interests combine into a single, coherent scientific world-view about the natural tendency of living systems to form complex interactive communities. This is Margulis and Sagan's fourth book.