Book excerpt: Biological evolution is a fact--but the many conflicting theories of evolution remain controversial even today. In 1966, simple Darwinism, which holds that evolution functions primarily at the level of the individual organism, was threatened by opposing concepts such as group selection, a popular idea stating that evolution acts to select entire species rather than individuals. George Williams's famous argument in favor of the Darwinists struck a powerful blow to those in opposing camps. His Adaptation and Natural Selection, now a classic of science literature, is a thorough and convincing essay in defense of Darwinism; its suggestions for developing effective principles for dealing with the evolution debate and its relevance to many fields outside biology ensure the timelessness of this critical work.