At once a spirited defense of Darwinian explanations of biology and an elegant primer on evolution for the general reader, What Evolution Is has several audiences in mind: those scientists and nonscientists who accept evolutionary thinking but do not know exactly how it works, and those who accept evolution but are not sure the Darwinian explanation is correct. With rare clarity, Mayr poses the questions at the heart of evolution - What is the evidence for evolution on earth? What is the origin and role of organic diversity? - and describes in refreshingly nontechnical language how the search for answers has over the years revealed solutions to the most challenging problems posed by evolutionary theory. In a provocative final section, Mayr considers how our improved understanding of evolution has affected the viewpoints and values of modern man.