English naturalist, who is remembered mainly for his theory of evolution, which he based largely on observations made in 1832–6 during a voyage around the world on HMS Beagle, which was engaged on a mapping survey. After receiving a letter from Alfred Russel Wallace (1823–1913) outlining the similar conclusions he had reached, on 1 July 1858 Sir Charles Lyell and Sir Joseph Dalton Hooker (1817–1911) presented separate papers by Darwin and Wallace at a meeting of the Linnean Society (neither Darwin nor Wallace was present). In 1859 Darwin published a longer account in his book, On the Origin of Species by Means of Natural Selection. In this he presented powerful evidence suggesting that change (evolution) has occurred among species, and proposed natural selection as the mechanism by which it occurs. His theory may be summarized as follows: (a) the individuals of a species show variation; (b) on average, more offspring are produced than are needed to replace their parents; (c) populations cannot expand indefinitely and, on average, population sizes remain stable; (d) therefore there must be competition for survival; (e) it is the best-adapted variants (the fittest) that survive and reproduce. Since environmental conditions change over long periods of time, a process of natural selection occurs which favours the emergence of different variants and ultimately of new species (the ‘origin of species’). This theory is known as Darwinism. The subsequent discovery of chromosomes and genes, and the development of the science of genetics, have led to a better understanding of the ways in which variation may be caused. Modified by this modern knowledge, Darwin’s theory is called ‘neo-Darwinism’.