Darwin's theories
Darwin's theories included several related ideas. They were: (1) that evolution had occurred; (2) that most evolutionary change was gradual, requiring thousands or millions of years; (3) that the primary mechanism for evolution was a process called natural selection; and (4) that the millions of species present on earth today arose from a single original life form through a branching process called speciation, by which one species can give rise to two or more species. Darwin set forth his theories in his book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1859).
Excerpt adapted from the "Charles Darwin" article, The World Book Encyclopedia © 1999