Notwithstanding this history, it was reserved for Charles Darwin, in 1859, seconded by A. R. Wallace, to convert the scientific world to evo lutional views. The new theory he specially advocated was that of natural selection. Darwin claimed that there was a universal tendency of fortuitous variation; its causes, he thought, were only in part known. He showed that favor able variations were preserved, and that natural selection has a directive force. He dwelt con vincingly on the facts of competition, of the struggle for existence, and on the biological en vironment. At the same time his Origin of Species was a massive and irresistible argu ment for the doctrine of descent; and as further expounded and upheld by Hooker. Huxley. Fritz Haeckel, and others, it became generally accepted. Darwin was the prince of observers and experimenters. He was also an expert systemat ist, a clear, persuasive writer; and into what ever field he entered his work was epoch-making.
Meanwhile in England, as early as 1852, Der bert Spencer advocated evolution from a philo sophical point of view. He proposed the term evo
lution both for the inorganic and organic world. His broad, synthetic mind grasped the full signifi cance of cosmic evolution, and he extended the doctrine of descent to human history, human society, morals, ethics, and religion. He is the philosopher of science, and of all that pertains to man. Ile has worked rather along Lamarck ian lines, holding that natural selection as such was of secondary importance, as compared with the primary factors of organic evolution.
Since Darwin's death, under the leadership of a Alfred Russel Wallace, and especially of Weis mann, the School of Neo-Darwinism (q.v.) has arisen. \Veismann, distinguished by his great work on the embryology and metamorphism of insects, and his investigations on temperature forms, has shown that heredity has a physical basis, the chromatin being the bearer of heredity. He asserts the 'all-sufficiency' of natural selec tion, and claims that variability is due to sexual reproduction.