Herbert Spencer (1820–1903) was a British philosopher famous for applying the theory of natural selection to society. He was in many ways the first true sociological functionalist. In fact, while Durkheim is widely considered the most important functionalist among positivist theorists, it is known that much of his analysis was culled from reading Spencer's work, especially his "Principles of Sociology" (1874–96). In describing society, Spencer alludes to the analogy of a human body. Just as the structural parts of the human body—the skeleton, muscles, and various internal organs—function independently to help the entire organism survive, social structures work together to preserve society.