Once considered a relative rarity, endosymbiosis, the living together of one organism inside another, has increasingly become recognized as a major factor in the evolution of life forms. The word endosymbiosis comes from Greek words meaning "inside," "with," and "living." Endosymbiosis in biology is a subdivision of the more general concept, symbiosis, which refers to living beings of different species living together for most of the life history of a member of at least one of those species. (In the case of the bacteria it suffices to say "living together of different types" because bacteria often cannot clearly be assigned to species.) Ectosymbiosis is a more familiar notion, an association between organisms of different species where one is attached in some way to the outside of the other. Barnacles adhere to the hairy, wet surfaces of whales where the pattern of barnacle distribution is used by whales to...