The evolution of organisms was commonly accepted by naturalists in the middle decades of the 19th century. The distribution of exotic species in South America, in the Galápagos Islands, and elsewhere and the discovery of fossil remains of long-extinguished animals confirmed the reality of evolution in Darwin’s mind. The intellectual challenge was to explain the origin of distinct species of organisms, how new ones adapted to their environments, that “mystery of mysteries,” as it had been labeled by Darwin’s older contemporary, the prominent scientist and philosopher Sir John Herschel (1792–1871).
Early in the Notebooks of 1837 to 1839, Darwin registers his discovery of natural selection and repeatedly refers to it as “my theory.” From then until his death in 1882, Darwin’s life would be dedicated to substantiating natural selection and its companion postulates, mainly the pervasiveness of hereditary variation and the enormous fertility of organisms, which much surpassed the capacity of available resources. Natural selection became for Darwin “a theory by which to work.” He relentlessly pursued observations and performed experiments to test the theory and resolve presumptive objections.
Alfred Russel Wallace (1823–1913) is famously given credit for discovering, independently of Darwin, natural selection as the process accounting for the evolution of species. On June 18, 1858, Darwin wrote to Charles Lyell that he had received by mail a short essay from Wallace such that “if Wallace had my [manuscript] sketch written in  he could not have made a better abstract.” Darwin was thunderstruck.
Darwin and Wallace had started occasional correspondence in late 1855. At the time Wallace was in the Malay Archipelago collecting biological specimens. In his letters, Darwin would offer sympathy and encouragement to the occasionally dispirited Wallace for his “laborious undertaking.” In 1858, Wallace came upon the idea of natural selection as the explanation for evolutionary change and he wanted to know Darwin’s opinion about this hypothesis, because Wallace, as well as many others, knew that Darwin had been working on the subject for years, had shared his ideas with other scientists, and was considered by them as the eminent expert on issues concerning biological evolution.
Darwin was uncertain how to proceed about Wallace’s letter. He wanted to credit Wallace’s discovery of natural selection, but he did not want altogether to give up his own earlier independent discovery. Eventually, Sir Charles Lyell and Joseph Hooker proposed, with Darwin’s consent, that Wallace’s letter and two of Darwin’s earlier writings would be presented at a meeting of the Linnean Society of London. On July 1, 1858,