Theory of Evolution
Darwin's exposure to specimens all over the globe raised important questions. Other naturalists believed that all species either came into being at the start of the world, or were created over the course of natural history. In either case, the species were believed to remain much the same throughout time. Darwin, however, noticed similarities among species all over the globe, along with variations based on specific locations, leading him to believe that they had gradually evolved from common ancestors. He came to believe that species survived through a process called "natural selection," where species that successfully adapted to meet the changing requirements of their natural habitat thrived, while those that failed to evolve and reproduce died off.
In 1858, after years of further scientific investigation, Darwin publically introduced his revolutionary theory of evolution in a letter read at a meeting of the Linnean Society. On November 24, 1859, he published a detailed explanation of his theory in his best-known work, On the Origin of Species by Means of Natural Selection.