the tendency of individuals that are better adapted to a particular environment to be more successful at surviving and producing offspring. This concept is inherent in the theory of evolution by natural selection, as proposed in the 1850s by British naturalists Charles Darwin and Alfred Russel Wallace (1823–1913). See also competition for resources; Darwinian fitness; social Darwinism. [coined by Herbert Spencer in Principles of Biology (1864) and adopted by Darwin as a synonym for natural selection in the fifth edition of his Origin of Species (1869, Chapter 4)]