The authors of Mystery Dance: The Evolution of Human Sexuality (1991) return to the fundamental biological questions, this time taking on the slipperiest of all issues. Wisely, they avoid any temptation to present a single, simplistic answer to the question posed in the title. Instead, Margulis (Biology/Univ. of Mass., Amherst) and her collaborator/son end each chapter with an answer from a different perspective: astronomical, physical, bacteriological, evolutionary, and so forth. While the range they cover is thus greatly extended, the reader will quickly begin to note certain limitations. For example, the book's initial chapter excludes viruses from the definition of life, on the ground that they do not metabolize; but very few biologists would be comfortable with such a clear-cut demarcation. At the other end of the scale, the illustrations quite deliberately scant what most of us would think of as ""higher"" organisms; the only vertebrates shown in 80 full-color photographs are a pair of human skeletons and a fish. Even granted that vertebrates comprise a small fraction of all species, the decision still seems eccentric. Equally eccentric is the pervasive niceness of the authors' viewpoint: Any tough-minded biologist would laugh at the quaint exegesis of Darwinian competition as the idea that organisms ""knock up against each other and work things out."" Likewise, the text is skewed in favor of such fashionable but still controversial notions as the Gala theory. And while the book is full of interesting insights, many of them will be obscured by a prose style that rarely finds a middle ground between the muddiest kind of technical language and self-consciously ""poetic"" overwriting. Visually very attractive, this book will probably find a place on many coffee tables; but it would be surprising if any but the most dedicated readers persevered through the entire text.