By the early 1840's, Darwin had written up a detailed treatment of his theory of evolution. In the manuscript, Wallace proposed a theory of evolution virtually identical to the one Darwin had been sitting on for nearly 20 years. Darwin and Wallace delivered a joint paper on their theory in 1858 at the meeting of the Linnean Society. In the next 15 months, Darwin completed a 450 page essay that explained and elaborated on his idea. Organisms display variation among themselves.
Natural selection is the mechanism by which the process of evolution occurs.
As we shall see, there are other mechanisms in addition to natural selection by which evolution occurs.