The Biology of the Second Reich: Social Darwinism and the Origins of World War I reveals the ideological and scientific roots of the war that gave rise to Nazi Germany and the horrors of the Holocaust. To German intellectuals in the late 19th and early 20th centuries, wars of annihilation were the natural outgrowth of Darwinian evolution’s survival of the fittest mantra, a “natural law, without which the organic world...could not continue to exist at all,” observed German zoologist Gustav Jaeger in 1870. Darwin clearly saw the implications of natural selection for human society. In his The Descent of Man,...