Journey:
After a week with student friends at Barmouth, Darwin returned home on 29 August to find a letter from Henslow proposing him as a suitable (if unfinished) gentleman naturalist for a self-funded supernumerary place on HMS Beagle with captain Robert FitzRoy, more as a companion than a mere collector. The ship was to leave in four weeks on an expedition to chart the coastline of South America. Robert Darwin objected to his son's planned two-year voyage, regarding it as a waste of time.
After delays, the voyage began on 27 December 1831; it lasted almost five years. As FitzRoy had intended, Darwin spent most of that time on land investigating geology and making natural history collections, while the Beagle surveyed and charted coasts. He kept careful notes of his observations and theoretical speculations, and at intervals during the voyage his specimens were sent to Cambridge together with letters including a copy of his journal for his family. He had some expertise in geology, beetle collecting and dissecting marine invertebrates, but in all other areas was a novice and ably collected specimens for expert appraisal. Despite suffering badly from seasickness, Darwin wrote copious notes while on board the ship. Most of his zoology notes are about marine invertebrates, starting with plankton collected in a calm spell.
On their first stop ashore at St. Jago, 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, and Darwin saw things Lyell's way, theorising and thinking of writing a book on geology.
When they reached Brazil Darwin was delighted by the tropical forest, but detested the sight of slavery. The survey continued to the south in Patagonia. They stopped at Bahía Blanca, and in cliffs near Punta Alta Darwin made a major find of fossil bones of huge extinct mammals beside modern seashells, indicating recent extinction with no signs of change in climate or catastrophe. He identified the little-known Megatherium by a tooth and its association with bony armour which had at first seemed to him like a giant version of the armour on local armadillos. The finds brought great interest when they reached England.
On rides with gauchos into the interior to explore geology and collect more fossils, Darwin gained social, political and anthropological insights into both native and colonial people at a time of revolution, and learnt that two types of rhea had separate but overlapping territories. Further south he saw stepped plains of shingle and seashells as raised beaches showing a series of elevations. He read Lyell's second volume and accepted its view of "centres of creation" of species, but his discoveries and theorising challenged Lyell's ideas of smooth continuity and of extinction of species.
Three Fuegians on board, who had been seized during the first Beagle voyage and had spent a year in England, were taken back to Tierra del Fuego as missionaries. Darwin found them friendly and civilised, yet their relatives seemed "miserable, degraded savages", as different as wild from domesticated animals. To Darwin the difference showed cultural advances, not racial inferiority. Unlike his scientist friends, he now thought there was no unbridgeable gap between humans and animals. A year on, the mission had been abandoned. The Fuegian they had named Jemmy Button lived like the other natives, had a wife, and had no wish to return to England.
Darwin experienced an earthquake in Chile and saw signs that the land had just been raised, including mussel-beds stranded above high tide. High in the Andes he saw seashells, and several fossil trees that had grown on a sand beach. He theorised that as the land rose, oceanic islands sank, and coral reefs round them grew to form atolls.
On the geologically new Galápagos Islands Darwin looked for evidence attaching wildlife to an older "centre of creation", and found mockingbirds allied to those in Chile but differing from island to island. He heard that slight variations in the shape of tortoise shells showed which island they came from, but failed to collect them, even after eating tortoises taken on board as food. In Australia the marsupial rat-kangaroo and the platypus seemed so unusual that Darwin thought it was almost as though two distinct Creators had been at work. He found the Aborigines "good-humoured & pleasant", and noted their depletion by European settlement.
The Beagle investigated how the atolls of the Cocos (Keeling) Islands had formed, and the survey supported Darwin's theorising. FitzRoy began writing the official Narrative of the Beagle voyages, and after reading Darwin's diary he proposed incorporating it into the account. Darwin's Journal was eventually rewritten as a separate third volume, on natural history.
In Cape Town Darwin and FitzRoy met John Herschel, who had recently written to Lyell praising his uniformitarianism as opening bold speculation on "that mystery of mysteries, the replacement of extinct species by others" as "a natural in contradistinction to a miraculous process". When organising his notes as the ship sailed home, Darwin wrote that if his growing suspicions about the mockingbirds, the tortoises and the Falkland Islands Fox were correct, "such facts undermine the stability of Species", then cautiously added "would" before "undermine". He later wrote that such facts "seemed to me to throw some light on the origin of species".
When the Beagle reached Falmouth, Cornwall, on 2 October 1836, Darwin was already a celebrity in scientific circles as in December 1835 Henslow had fostered his former pupil's reputation by giving selected naturalists a pamphlet of Darwin's geological letters. Darwin visited his home in Shrewsbury and saw relatives, then hurried to Cambridge to see Henslow, who advised on finding naturalists available to catalogue the collections and agreed to take on the botanical specimens. Darwin's father organised investments, enabling his son to be a self-funded gentleman scientist, and an excited Darwin went round the London institutions being fêted and seeking experts to describe the collections. Zoologists had a huge backlog of work, and there was a danger of specimens just being left in storage.
Charles Lyell eagerly met Darwin for the first time on 29 October and soon introduced him to the up-and-coming anatomist Richard Owen, who had the facilities of the Royal College of Surgeons to work on the fossil bones collected by Darwin. Owen's surprising results included other gigantic extinct ground sloths as well as the Megatherium, a near complete skeleton of the unknown Scelidotherium and a hippopotamus-sized rodent-like skull named Toxodon resembling a giant capybara. The armour fragments were actually from Glyptodon, a huge armadillo-like creature as Darwin had initially thought. These extinct creatures were related to living species in South America.
In mid-December Darwin took lodgings in Cambridge to organise work on his collections and rewrite his Journal. He wrote his first paper, showing that the South American landmass was slowly rising, and with Lyell's enthusiastic backing read it to the Geological Society of London on 4 January 1837. On the same day, he presented his mammal and bird specimens to the Zoological Society. The ornithologist John Gould soon announced that the Galapagos birds that Darwin had thought a mixture of blackbirds, "gros-beaks" and finches, were, in fact, twelve separate species of finches. On 17 February Darwin was elected to the Council of the Geological Society, and Lyell's presidential address presented Owen's findings on Darwin's fossils, stressing geographical continuity of species as supporting his uniformitarian ideas.
By mid-March, Darwin was speculating in his Red Notebook on the possibility that "one species does change into another" to explain the geographical distribution of living species such as the rheas, and extinct ones such as the strange Macrauchenia which resembled a giant guanaco. His thoughts on lifespan, asexual reproduction and sexual reproduction developed in his "B" notebook around mid-July on to variation in offspring "to adapt & alter the race to changing world" explaining the Galápagos tortoises, mockingbirds and rheas.