Natural selection, as introduced by Charles Darwin in the Origin of Species (1859), has always been a topic of great conceptual and empirical interest. This book puts Darwin's theory of evolution in historical context showing that, in important respects, his central mechanism of natural selection gives the clue to understanding the nature of organisms. Natural selection has important implications, not just for the understanding of life's history - single-celled organism to man - but also for our understanding of contemporary social norms, as well as the nature of religious belief. The book is written in clear, non-technical language, appealing not just to philosophers, historians, and biologists, but also to general readers who find thinking about important issues both challenging and exciting.