Evolution is an all-pervading concept in modern science applied to physical as well as biological processes. The phrase theory of evolution, however, is most commonly associated with organic evolution, the origin and evolution of the enormous diversity of life on this planet.

The idea of organic evolution arose from attempts to explain the immense diversity of living organisms and dates from the dawn of culture. Attempts to formulate a naturalistic explanation of the phenomena of evolution in one form or another had been proposed, and by the end of the eighteenth century a number of naturalists from Carolus Linnaeus to Jean-Baptiste Lamarck had questioned the prevailing doctrine of fixity of species. Lamarck came the closest to proposing a complete theory of evolution, but for several reasons his theory fell short and was not widely accepted.

Two English naturalists conceived the first truly complete theory of evolution. In a...