What Darwin did for life through time, Vernadsky did for life through space on a geological scale. --Lynn Margulis This classic 1926 book is the first sustained expression of the idea, now widely accepted, that life is a geological force. It founded the field of biogeochemistry and is one of the seminal works in what is now known as Gaia theory. This edition, the first unexpurgated English translation, is fully annotated for the modern reader.