The second voyage of HMS Beagle, from 27 December 1831 to 2 October 1836, was the second survey expedition of HMS Beagle, under captain Robert FitzRoy who had taken over command of the ship on its first voyage after the previous captain, Pringle Stokes, committed suicide. FitzRoy had thought of the advantages of having someone onboard who could investigate geology, and sought a naturalist to accompany them as a supernumerary. At the age of 22, the graduate Charles Darwin hoped to see the tropics before becoming a parson and accepted the opportunity. He was greatly influenced by reading Charles Lyell's Principles of Geology during the voyage. By the end of the expedition, Darwin had made his name as a geologist and fossil collector and the publication of his journal (later known as The Voyage of the Beagle) gave him wide renown as a writer.

Beagle sailed across the Atlantic Ocean, and then carried out detailed hydrographic surveys around the coasts of southern South America, returning via Tahiti and Australia after having circumnavigated the Earth. The initial offer to Darwin told him the voyage would last two years; it lasted almost five.

Darwin spent most of this time exploring on land: three years and three months land, 18 months at sea. Early in the voyage, Darwin decided that he could write a geology book, and he showed a gift for theorising. At Punta Alta in Argentina, he made a major find of gigantic fossils of extinct mammals, then known from very few specimens. He collected and made detailed observations of plants and animals. His findings undermined his belief in the doctrine that species are fixed, and provided the basis for ideas which came to him when back in England, leading to his theory of evolution by natural selection.

Charles Robert Darwin (; DAHR-win; 12 February 1809 – 19 April 1882) was an English naturalist, geologist and biologist, best known for his contributions to evolutionary biology. His proposition that all species of life have descended from common ancestors is now widely accepted and considered a fundamental concept in science. In a joint publication with Alfred Russel Wallace, he introduced his scientific theory that this branching pattern of evolution resulted from a process that he called natural selection, in which the struggle for existence has a similar effect to the artificial selection involved in selective breeding. Darwin has been described as one of the most influential figures in human history, and he was honoured by burial in Westminster Abbey.Darwin published his theory of evolution with compelling evidence in his 1859 book On the Origin of Species. By the 1870s, the scientific community and a majority of the educated public had accepted evolution as a fact. However, many favoured competing explanations that gave only a minor role to natural selection, and it was not until the emergence of the modern evolutionary synthesis from the 1930s to the 1950s that a broad consensus developed in which natural selection was the basic mechanism of evolution. Darwin's scientific discovery is the unifying theory of the life sciences, explaining the diversity of life.Darwin's early interest in nature led him to neglect his medical education at the University of Edinburgh; instead, he helped to investigate marine invertebrates. Studies at the University of Cambridge (Christ's College) encouraged his passion for natural science. His five-year voyage on HMS Beagle established him as an eminent geologist whose observations and theories supported Charles Lyell's conception of gradual geological change, and publication of his journal of the voyage made him famous as a popular author.Puzzled by the geographical distribution of wildlife and fossils he collected on the voyage, Darwin began detailed investigations and in 1838 conceived his theory of natural selection. Although he discussed his ideas with several naturalists, he needed time for extensive research, and his geological work had priority. He was writing up his theory in 1858 when Alfred Russel Wallace sent him an essay that described the same idea, prompting immediate joint publication of both their theories. Darwin's work established evolutionary descent with modification as the dominant scientific explanation of diversification in nature. In 1871 he examined human evolution and sexual selection in The Descent of Man, and Selection in Relation to Sex, followed by The Expression of the Emotions in Man and Animals (1872). His research on plants was published in a series of books, and in his final book, The Formation of Vegetable Mould, through the Actions of Worms (1881), he examined earthworms and their effect on soil.