How do new species evolve? Although Darwin identified inherited variation as the creative force in evolution, he never formally speculated where it comes from. His successors thought that new species arise from the gradual accumulation of random mutations of DNA. But despite its acceptance in every major textbook, there is no documented instance of it. Lynn Margulis and Dorion Sagan take a radically new approach to this question. They show that speciation events are not, in fact, rare or hard to observe. Genomes are acquired by infection, by feeding, and by other ecological associations, and then inherited. "Acquiring Genomes" is the first work to integrate and analyze the overwhelming mass of evidence for the role of bacterial and other symbioses in the creation of plant and animal diversity. It provides the most powerful explanation of speciation yet given.