Charles Darwin spent five years, from 1831 to 1836, as the ship's naturalist aboard HMS Beagle on a survey voyage around South America and the Pacific, collecting specimens and taking notes that would occupy him for the rest of his career. The finches and tortoises of the Galapagos Islands, varying subtly from island to island, became a famous part of the story he later told, though the idea of natural selection itself crystallized gradually over the following two decades of reading, breeding experiments with pigeons, and correspondence rather than in a single moment of insight on the islands.
The theory Darwin eventually published rests on a small number of observations that, taken together, have a striking logical force. Organisms produce more offspring than their environment can support, individuals vary from one another in heritable ways, and some variations make their bearers more likely to survive and reproduce than others. Given enough time, Darwin argued, this simple process would accumulate small advantages generation after generation, gradually transforming populations and, eventually, producing new species, all without any guiding intelligence or purpose. He delayed publishing for years, aware how the idea would be received, until he learned that Alfred Russel Wallace, working independently in the Malay Archipelago, had arrived at essentially the same theory, the two men's work was presented jointly to the Linnean Society in 1858, and Darwin published On the Origin of Species the following year.
The book's central claim, that species are not fixed but change over time through an unguided natural process, remains the organizing theory of all of biology, though Darwin himself knew nothing of genes or DNA, the mechanism by which variation arises and is inherited was worked out only in the following century, by scientists building on the rediscovered work of Gregor Mendel.