Editing On the Origin of Species (section)

===Inception of Darwin's theory===
{{See also|Charles Darwin's education|Inception of Darwin's theory}}
[[File:Darwin tree.png|upright|thumb|In mid-July 1837 Darwin started his "B" notebook on ''Transmutation of Species'', and on page 36 wrote "I think" above his first [[tree of life (biology)|evolutionary tree]].]]

Darwin went to [[University of Edinburgh|Edinburgh University]] in 1825 to study medicine. In his second year he neglected his medical studies for [[natural history]] and spent four months helping [[Robert Edmund Grant|Robert Grant]]'s research into [[marine invertebrate]]s. Grant revealed his enthusiasm for the transmutation of species, but Darwin rejected it.<ref>{{Harvnb|Browne|1995|pp=80–88}}</ref> Starting in 1827, at [[University of Cambridge|Cambridge University]], Darwin learnt science as [[natural theology]] from botanist [[John Stevens Henslow]], and read [[William Paley|Paley]], [[John Herschel]] and [[Alexander von Humboldt]]. Filled with zeal for science, he studied [[catastrophism|catastrophist]] [[geology]] with [[Adam Sedgwick]].<ref>{{Harvnb|Bowler|2003|pp=148–149}}</ref><ref>{{harvnb|Browne|1995|pp=133–140}}</ref>

In December 1831, he joined the [[Second voyage of HMS Beagle|''Beagle'' expedition]] as a gentleman naturalist and geologist. He read [[Charles Lyell]]'s ''[[Principles of Geology]]'' and from the first stop ashore, at [[Santiago, Cape Verde|St. Jago]], found [[Uniformitarianism (science)|Lyell's uniformitarianism]] a key to the geological history of landscapes. Darwin discovered fossils resembling [[Glyptodon|huge armadillo]]s, and noted the geographical distribution of modern species in hope of finding their "centre of creation".<ref>{{Harvnb|Larson|2004|pp=56–62}}</ref> The three [[Fuegians|Fuegian]] missionaries the expedition returned to [[Tierra del Fuego]] were friendly and civilised, yet to Darwin their relatives on the island seemed "miserable, degraded savages",<ref>{{Harvnb|Darwin|1845|pp=[http://darwin-online.org.uk/content/frameset?itemID=F14&viewtype=text&pageseq=218 205–208]}}</ref> and he no longer saw an unbridgeable gap between humans and animals.<ref>{{Harvnb|Browne|1995|pp=244–250}}</ref> As the ''Beagle'' neared England in 1836, he noted that species might not be fixed.<ref name=xix>{{Harvnb|Keynes|2000|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F1840&pageseq=22 xix–xx]}}</ref><ref>{{Harvnb|Eldredge|2006}}</ref>

[[Richard Owen]] showed that fossils of extinct species Darwin found in South America were allied to living species on the same continent. In March 1837, ornithologist [[John Gould]] announced that [[Darwin's rhea]] was a separate species from the previously described [[Rhea (bird)|rhea]] (though their territories overlapped), that [[mockingbird]]s collected on the [[Galápagos Islands]] represented three separate species each [[Endemism|unique to]] a particular island, and that several distinct birds from those islands were all classified as [[Darwin's finch|finches]].<ref>{{Harvnb|Quammen|2006|pp=24–25}}</ref> Darwin began speculating, in a series of notebooks, on the possibility that "one species does change into another" to explain these findings, and around July sketched a [[genealogy|genealogical]] branching of a single [[tree of life (biology)|evolutionary tree]], discarding Lamarck's independent [[Lineage (evolution)|lineages]] progressing to higher forms.<ref>{{harvnb|Herbert|1980|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F1583e&pageseq=9 7–10]}}</ref><ref>{{Harvnb|van Wyhe|2008|p=44}}</ref><ref>{{Citation | url=http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=CUL-DAR121.-&pageseq=1|title=Darwin's Notebook B: Transmutation of species. pp. 1–13, 26, 36, 74|access-date=16 March 2009}}</ref> Unconventionally, Darwin asked questions of [[fancy pigeon]] and [[animal husbandry|animal breeders]] as well as established scientists. At the zoo he had his first sight of an ape, and was profoundly impressed by how human the [[orangutan]] seemed.<ref>{{Harvnb|Desmond|Moore|1991|pp=240–244}}</ref>

In late September 1838, he started reading [[Thomas Malthus]]'s ''[[An Essay on the Principle of Population]]'' with [[Malthusian trap|its statistical argument]] that human populations, if unrestrained, breed beyond their means and [[struggle for existence|struggle to survive]]. Darwin related this to the struggle for existence among wildlife and botanist [[Augustin Pyramus de Candolle|de Candolle's]] "warring of the species" in plants; he immediately envisioned "a force like a hundred thousand wedges" pushing well-adapted variations into "gaps in the economy of nature", so that the survivors would pass on their form and abilities, and unfavourable variations would be destroyed.<ref name=wyhe/><ref>{{Harvnb|Larson|2004|pp=66–70}}</ref><ref>{{Citation|url=http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=CUL-DAR123.-&pageseq=112|title=Darwin's Notebook D: Transmutation of species. pp. 134–135|access-date=8 April 2009}}</ref> By December 1838, he had noted a similarity between the act of breeders selecting traits and a Malthusian Nature selecting among variants thrown up by "chance" so that "every part of newly acquired structure is fully practical and perfected".<ref>{{Citation |url=http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=CUL-DAR124.-&pageseq=63|title=Darwin's Notebook E: Transmutation of species. p. 75|access-date=14 March 2009}}</ref>

Darwin now had the basic framework of his theory of natural selection, but he was fully occupied with his career as a geologist and held back from compiling it until his book on ''[[The Structure and Distribution of Coral Reefs]]'' was completed.<ref name=vw186>{{Harvnb|van Wyhe|2007|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=A544&pageseq=10 186–187]}}</ref><ref>{{harvnb|Browne|1995|p=436}}</ref> As he recalled in his autobiography, he had "at last got a theory by which to work", but it was only in June 1842 that he allowed himself "the satisfaction of writing a very brief abstract of my theory in pencil".<ref name="AB 120">{{Harvnb|Darwin|1958|p=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F1497&pageseq=124 120]}}</ref>