Puzzled by the geographical distribution of wildlife and fossils he collected on the voyage, Darwin began detailed investigations and, in 1838, devised his theory of natural selection.[15] Although he discussed his ideas with several naturalists, he needed time for extensive research and his geological work had priority.[16] He was writing up his theory in 1858 when Alfred Russel Wallace sent him an essay that described the same idea, prompting immediate joint submission of both their theories to the Linnean Society of London.[17] Darwin's work established evolutionary descent with modification as the dominant scientific explanation of diversification in nature.[18] 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.
Darwin published his theory of evolution with compelling evidence in his 1859 book On the Origin of Species.[19][20] 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.[18][21] Darwin's scientific discovery is the unifying theory of the life sciences, explaining the diversity of life.
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Both families were largely Unitarian, though the Wedgwoods were adopting Anglicanism. Robert Darwin, a freethinker, had baby Charles baptised in November 1809 in the Anglican St Chad's Church, Shrewsbury, but Charles and his siblings attended the local Unitarian Church with their mother. The eight-year-old Charles already had a taste for natural history and collecting when he joined the day school run by its preacher in 1817. That July, his mother died. From September 1818, he joined his older brother Erasmus in attending the nearby Anglican Shrewsbury School as a boarder.[25]
Darwin spent the summer of 1825 as an apprentice doctor, helping his father treat the poor of Shropshire, before going to the well regarded University of Edinburgh Medical School with his brother Erasmus in October 1825. Darwin found lectures dull and surgery distressing, so he neglected his studies.[26] He learned taxidermy in around 40 daily hour-long sessions from John Edmonstone, a freed black slave who had accompanied Charles Waterton in the South American rainforest.[27]
On their first stop ashore at St Jago in Cape Verde, Darwin found that a white band high in the volcanic rock cliffs included seashells. FitzRoy had given him the first volume of Charles Lyell's Principles of Geology, which set out uniformitarian concepts of land slowly rising or falling over immense periods,[II] and Darwin saw things Lyell's way, theorising and thinking of writing a book on geology.[50] When they reached Brazil, Darwin was delighted by the tropical forest,[51] but detested the sight of slavery, and disputed this issue with Fitzroy.[52]
Early in March, Darwin moved to London to be near this work, joining Lyell's social circle of scientists and experts such as Charles Babbage,[86] who described God as a programmer of laws. Darwin stayed with his freethinking brother Erasmus, part of this Whig circle and a close friend of the writer Harriet Martineau, who promoted the Malthusianism that underpinned the controversial Whig Poor Law reforms to stop welfare from causing overpopulation and more poverty. As a Unitarian, she welcomed the radical implications of transmutation of species, promoted by Grant and younger surgeons influenced by Geoffroy. Transmutation was anathema to Anglicans defending social order,[87] but reputable scientists openly discussed the subject and there was wide interest in John Herschel's letter praising Lyell's approach as a way to find a natural cause of the origin of new species.[72]
While developing this intensive study of transmutation, Darwin became mired in more work. Still rewriting his Journal, he took on editing and publishing the expert reports on his collections, and with Henslow's help obtained a Treasury grant of 1,000 to sponsor this multi-volume Zoology of the Voyage of H.M.S. Beagle, a sum equivalent to about 115,000 in 2021.[91] He stretched the funding to include his planned books on geology, and agreed to unrealistic dates with the publisher.[92] As the Victorian era began, Darwin pressed on with writing his Journal, and in August 1837 began correcting printer's proofs.[93]
Fully recuperated, he returned to Shrewsbury in July. Used to jotting down daily notes on animal breeding, he scrawled rambling thoughts about marriage, career and prospects on two scraps of paper, one with columns headed "Marry" and "Not Marry". Advantages under "Marry" included "constant companion and a friend in old age ... better than a dog anyhow", against points such as "less money for books" and "terrible loss of time".[101] Having decided in favour of marriage, he discussed it with his father, then went to visit his cousin Emma on 29 July. He did not get around to proposing, but against his father's advice he mentioned his ideas on transmutation.[102]
Darwin's book The Structure and Distribution of Coral Reefs on his theory of atoll formation was published in May 1842 after more than three years of work, and he then wrote his first "pencil sketch" of his theory of natural selection.[114] To escape the pressures of London, the family moved to rural Down House in Kent in September.[115] On 11 January 1844, Darwin mentioned his theorising to the botanist Joseph Dalton Hooker, writing with melodramatic humour "it is like confessing a murder".[116][117] Hooker replied "There may in my opinion have been a series of productions on different spots, & also a gradual change of species. I shall be delighted to hear how you think that this change may have taken place, as no presently conceived opinions satisfy me on the subject."[118]
Darwin completed his third geological book in 1846. He now renewed a fascination and expertise in marine invertebrates, dating back to his student days with Grant, by dissecting and classifying the barnacles he had collected on the voyage, enjoying observing beautiful structures and thinking about comparisons with allied structures.[123] In 1847, Hooker read the "Essay" and sent notes that provided Darwin with the calm critical feedback that he needed, but would not commit himself and questioned Darwin's opposition to continuing acts of creation.[124]
Though Darwin saw no threat, on 14 May 1856 he began writing a short paper. Finding answers to difficult questions held him up repeatedly, and he expanded his plans to a "big book on species" titled Natural Selection, which was to include his "note on Man". He continued his researches, obtaining information and specimens from naturalists worldwide including Wallace who was working in Borneo.[132]
In mid-1857 he added a section heading; "Theory applied to Races of Man", but did not add text on this topic. On 5 September 1857, Darwin sent the American botanist Asa Gray a detailed outline of his ideas, including an abstract of Natural Selection, which omitted human origins and sexual selection. In December, Darwin received a letter from Wallace asking if the book would examine human origins. He responded that he would avoid that subject, "so surrounded with prejudices", while encouraging Wallace's theorising and adding that "I go much further than you."[132]
Darwin's book was only partly written when, on 18 June 1858, he received a paper from Wallace describing natural selection. Shocked that he had been "forestalled", Darwin sent it on that day to Lyell, as requested by Wallace,[133][134] and although Wallace had not asked for publication, Darwin suggested he would send it to any journal that Wallace chose. His family was in crisis with children in the village dying of scarlet fever, and he put matters in the hands of his friends. After some discussion, with no reliable way of involving Wallace, Lyell and Hooker decided on a joint presentation at the Linnean Society on 1 July of On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection. On the evening of 28 June, Darwin's baby son died of scarlet fever after almost a week of severe illness, and he was too distraught to attend.[135]
There was little immediate attention to this announcement of the theory; the president of the Linnean Society remarked in May 1859 that the year had not been marked by any revolutionary discoveries.[136] Only one review rankled enough for Darwin to recall it later; Professor Samuel Haughton of Dublin claimed that "all that was new in them was false, and what was true was old".[137] Darwin struggled for thirteen months to produce an abstract of his "big book", suffering from ill health but getting constant encouragement from his scientific friends. Lyell arranged to have it published by John Murray.[138]
On the Origin of Species proved unexpectedly popular, with the entire stock of 1,250 copies oversubscribed when it went on sale to booksellers on 22 November 1859.[139] In the book, Darwin set out "one long argument" of detailed observations, inferences and consideration of anticipated objections.[140] In making the case for common descent, he included evidence of homologies between humans and other mammals.[141][III] Having outlined sexual selection, he hinted that it could explain differences between human races.[142][IV] He avoided explicit discussion of human origins, but implied the significance of his work with the sentence; "Light will be thrown on the origin of man and his history."[143][IV] His theory is simply stated in the introduction: 2ff7e9595c
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