All through 2009 public talks and activities are commemorating the work of the famous evolutionist Charles Darwin. First published 150 years ago in the book On the Origin of Species, Darwin’s ideas live on as the central organising concept of modern biology. The geneticist Theodosius Dobzhansky said “Nothing in biology makes sense except in the light of evolution."

Yet when it was first published Darwin’s book provoked a storm of controversy, some of which continues today. What turned an fairly ordinary young man into the great thinker who wrote the Origin of Species?  Darwin always said that it was the Beagle voyage that made him what he eventually became:  'The voyage of the Beagle has been by far the most important event in my life and has determined my whole career.' Five years at sea, travelling around the world on a British surveying ship, he certainly saw many sights. On that famous voyage he also began an intellectual journey. He returned home with a torrent of new ideas to contemplate.

Cape Town was one of the final ports of call. When Darwin arrived in Simon’s Bay in May 1836 many of the major excitements and achievements of the Beagle voyage lay behind him. He had already been at sea for more than four years, and the ship’s arrival at the Cape signalled the last leg of the voyage. Darwin was looking forward to being reunited with his family in England and to the thrill of unpacking his specimens. He hoped to join the world of London science as an equal.  Perhaps this accounts for his somewhat dismissive remark of the Cape area that ‘there is very little of interest.”

During his time in Cape Town Darwin nevertheless made a tour of 3 or 4 days into the interior, a 170 kilometre journey that took him to Paarl Rock, across the Franschhoek Pass, and back via the Cape Flats. More significantly for his later ideas, on 15 June 1836, three days before they sailed, he and Captain FitzRoy were invite to dine with the British astronomer John Herschel, then living in Feldhausen. Herschel lived in Cape Town for several years to observe the southern heavens and work with Thomas Maclear, the Astronomer Royal.  Several years beforehand he had written an influential book for scientists on the correct methods for stating a theory and what can legitimately be inferred about the ‘causes’ of things that happen in the world around us. Darwin had read this as an undergraduate and was eager to meet the great man.

This dinner party therefore has all the makings for a great detective story. There are no records of what might have been discussed. But we know that there were unusual scientific ideas bubbling in Darwin’s head, ready to take shape. And it is entirely possible that his meeting with Herschel helped Darwin to formulate his evolutionary theory.

By the time Darwin met Herschel, he had energetically collected natural history specimens widely and carefully. Several other European naturalists had covered some of the same territory beforehand, including the French naturalist Alcide d’Orbigny, collecting for the Paris Museum of Natural History. In South America, Darwin collected insects, small invertebrates, birds, spiders, corals, molluscs, mammals, and many large fossils that seemed to be related to the species that still lived in the area—giant species of sloth, llama, and armadillo. Had these become extinct from not having enough to eat? He hoped to ask Dr Andrew Smith, the expert on South African fauna, about this. Perhaps he could learn something from Smith about the foodstuffs of the rhino or elephant. He already had on board his collection of animals and plants from the Galapagos Islands.

Darwin also had learned a great deal about the geology of the regions he visited by taking with him a new analysis, written by the avant-garde geologist Charles Lyell, called the Principles of Geology, and published 1830-1833, while Darwin was aboard the Beagle.  Darwin took an important message from Lyell’s book --that many constantly repeated small changes can lead to big effects. Whereas Lyell applied this idea to geological history of the earth and the origin of landforms, Darwin applied it to the whole of the natural world, including humans. It became the single most important idea in Darwin’s intellectual life—the key idea that eventually underpinned the Origin of Species. 

Another theme occupying Darwin’s mind was the underlying unity behind the great diversity of human beings that he was encountering around the globe. Darwin was particularly interested in indigenous peoples and, like many of his family, also a fervent abolitionist. Captain FitzRoy had intended to set up a Christian mission station in Tierra del Fuego by repatriating three individuals whom he had previously taken to England to educate. These three, plus a missionary, were settled in Woollya Sound in 1835 while the Beagle carried out its geographical survey.  To the captain’s dismay the attempt was a dreadful failure. But for Darwin the lesson was very instructive. He saw that humans were all the same under the skin—it was only different levels of education and what he called ‘civilization’ that separated them.  For the rest of the voyage, Darwin and FitzRoy paid special attention to the work of protestant missionaries at other ports of call, especially Tahiti and New Zealand. When they arrived in South Africa, they noticed that residents were critical of the local missionaries’ wish to buy land. Together, they wrote a letter to The South African Christian Recorder, asking settlers to think more positively about the good work that missionaries were doing in spreading the values of civilization (the letter is headed, ‘At Sea, 28th June, 1836. A letter, containing remarks on the moral state of Tahiti, New Zealand, &c.’ Published in South African Christian Recorder 2 (issue 4) September 1836: 221-238).  It seems likely that this topic was the subject of animated conversation at Herschel’s dinner table.

The most interesting conversation, however, was probably about the implications of Lyell’s geological writings. Herschel knew Lyell personally from his time in London and had only just finished reading another of Lyell’s books, Elements of Geology. Given Darwin’s great interest in Lyell, he would have surely questioned Herschel about it. We also know that Herschel had written a letter to Lyell in February 1836 expressing admiration for Lyell’s courage in facing up to difficult issues about the origin of species. While Lyell never proposed a natural origin for species, like Darwin eventually did, he suggested in this little book that this might be a reasonable conclusion. Herschel also thought this would be so. He wrote to Lyell saying  (in ornate nineteenth century prose), “I allude to that mystery of mysteries, the replacement of extinct species by others. Many will doubtless think your speculations too bold . . . but we are led, by all analogy, to suppose that God operates through a series of intermediate causes, and that in consequence the origination of fresh species, could it ever come under our cognizance, would be found to be a natural in contradistinction to a miraculous process.” In short, Herschel was saying that God probably did not directly involve himself in the creation of new species. Herschel was a deeply religious man yet could nevertheless see that the origin of living beings could be explained scientifically—or as he puts it here, by ‘natural’ processes.

Whatever was discussed around Herschel’s table, Darwin was very impressed. In his diary he wrote, “I dined out several days, — with Mr Maclear (the astronomer), with Colonel Bell, and with Sir J. Herschel; this last was the most memorable event which, for a long period, I have had the good fortune to enjoy.” 

We know that as he traveled back across the Atlantic to Britain, his mind revolved ceaselessly around how to make sense of his collections. There are some hiunts in a notebook that he thought carefully about species definitions and wondered about their origins. Very soon after arriving home, he gave his Galapagos bird specimens to the London ornithologist John Gould, who identified them for him as distinct species of finch and mocking bird.  And at that moment in March 1837, less than a year after meeting Herschel in Cape Town, he became a committed evolutionist—although as yet without any idea as to a mechanism.      

Herschel’s words echoed in Darwin’s mind for years afterwards.  When in 1859 he came to compose the Origin of Species, he used Herschel’s phrase about species origins being the “mystery of mysteries.” The very first sentence of the Origin of Species shows the lasting impact of this one meeting in Feldhausen on a young man’s mind:

“WHEN on board H.M.S. 'Beagle,' as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers. On my return home, it occurred to me, in 1837, that something might perhaps be made out on this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it.”

Janet Browne is the Aramont professor of the History of Science, Harvard University