A collection of revised, collected, and new essays written by Ernst Mayr in time for his 100th birthday. Mayr, the most eminent evolutionary biologist of the past century, explores biology as an autonomous science, offers insights on the history of evolutionary thought, critiques the contributions of philosophy to the science of biology, and comments on several of the major ongoing issues in evolutionary theory. Notably, he explains that Darwin's theory of evolution is actually five separate theories, each with its own history, trajectory and impact. He points out that a number of the perennial Darwinian controversies may well have been caused by the confounding of the five separate theories into a single composite. Those interested in evolutionary theory, or the philosophy and history of science will find useful ideas in this book, which should appeal to virtually anyone with a broad curiosity about biology.