This is an intellectual biography of Benjamin Kidd, a leading Social Darwinist in the years before World War I, and a social prophet in the tradition of Comte and Spencer. His first book Social Evolution, published in 1894, was an immediate and enormous success around the world. In it, Kidd developed a collectivist form of Social Darwinism in tune with the values of Progressivism in America and the 'new liberalism' in Britain. By many it was regarded as the basis for a properly scientific sociology, and the combination of its claims to scientific methodology, with an emphasis on non-rational forces as the agents of progress accurately caught the temper of its times. Launched on his career as a writer, Kidd's subsequent books and journalism continued to exercise extraordinary influence. His 'social imperialism', linking a bio-political defence of empire with a programme of social reform, won currency in the Anglo-American world at a time of expansionary fervour.