In this groundbreaking book, Lynn Margulis and Dorion Sagan present an answer to one of the enduring mysteries of evolution--the source of inherited variation that gives rise to new species. Random genetic mutation, long believed to be the main source of variation, is only a marginal factor. As the authors demonstrate in this book, the more important source of speciation, by far, is the acquisition of new genomes by symbiotic merger. The result of thirty years of delving into a vast, mostly arcane literature, this is the first book to go beyond--and reveal the severe limitations of--the "Modern Synthesis" that has dominated evolutionary biology for almost three generations. Lynn Margulis, whom E. O. Wilson called "one of the most successful synthetic thinkers in modern biology," and her co-author Dorion Sagan have written a comprehensive and scientifically supported presentation of a theory that directly challenges the assumptions we hold about the variety of the living world. A challenger of the orthodox "neo-Darwinist" interpretation of evolution, microbiologist Margulis has made her professional mark touting an alternative: symbiogenesis. She and coauthor (and son) Sagan have presented their ideas in earlier popular works ( What Is Life? , 1995), but never as vigorously as in this volume. Essentially, the debate between neo-Darwinists and Margulis hinges on the definition of a species, and the manner in which a new one appears. To Margulis and Sagan, the neo-Darwinist model, which asserts random gene mutation as the source of inherited variations, is "wildly overemphasized," and to support their view, they delve deeply into the world of microbes. They detail the anatomy of cells with and without nuclei, positing a process of genome ingestion that creates a new species. Surprisingly, the upshot of Margulis' theories is the rehabilitation of Jean Baptiste de Lamarck, whose theory that supposedly acquired traits are hereditary has been ridiculed for 150 years. Polemical and provocative, Margulis and Sagan's work should set many to thinking that evolution has not yet been completely figured out. Gilbert Taylor Copyright © American Library Association. All rights reserved Lynn Margulis, Distinguished University Professor in the Department of Geosciences, University of Massachusetts, Amherst, is a member of the National Academy of Sciences and a recipient of the 1999 Presidential Medal of Science. She lives in Amherst, Massachusetts. Dorion Sagan is the author of Biospheres and, with Dr. Eric Schneider, Into the Cool: The New Thermodynamics of Life . He lives in New York City.