While much has been written upon Social Darwinism, the historical impact of Darwinism upon theories of war and human aggression has been sadly neglected. This book is the first to study this discourse in depth. It challenges the received view that Darwinism generated essentially aggressive and warlike social values and pugnacious images of humankind. Paul Crook reconstructs the influential discourse of 'peace biology', whose liberal vision was of a basically free humanity, not fettered by iron laws of biological necessity or governed by violent genes. By exploring a gamut of Darwinian readings of history and war, mainly in the English-speaking world to 1919, this study throws new light upon militarism, peace movements, the origins of World War I and British social thought.