On Darwin’s Trouble with the Finches

Scientific discovery, as any PhD student halfway through their project will tell you, is hard work: progress is step-wise, and the steps are small. Not surprisingly, however, the popular view of science overlooks the daily grind and focuses instead on the occasional flashes of inspiration that have punctuated its history. In this view, science progresses in a series of great leaps forward, and the scientists involved are mythic heroes grappling singlehandedly with the great problems of their time. The mythology also requires that these visionary moments acquire their own icons – Newton had his apple, Watt his kettle – and intellectual history is thus handily reduced to a series of mnemonics.

Evolutionary biology’s ‘Eureka!’ moment supposedly took place some time in September or October 1835, during the Beagle’s five-week visit to the Galapagos Islands. The Beagle had been at sea for nearly four years, and, as he wrote to his Cambridge mentor, John Henslow, Charles Darwin was increasingly anxious to get home: ‘I look forward with joy and interest to [visiting the Galapagos], both as being somewhat nearer to England, & for the sake of having a good look at an active Volcano.’ He had cause to expect some pyrotechnics as the island chain had been produced by continental drift of the earth’s crust over a sporadically active volcanic ‘hot spot’. The islands’ blasted aspect, however, made them unpopular with visitors. Herman Melville, who visited in 1841, had the usual reaction: ‘Take five-and-twenty heaps of cinders dumped here and there in an outsize city lot, imagine some of them magnified into mountains, and the vacant lot the sea; and you will have a fit idea of the general aspect of the [islands].’

Melville at least got to see what Darwin had come to see: the volcanoes where ‘toil the demons of fire, who at intervals irradiate the nights with strange spectral illumination’. Darwin was not so lucky: ‘The craters are all entirely inert; consisting indeed of nothing more than a ring of cinders.’ Even a recently active crater was producing only ‘a small jet of steam’. The earthquake he had experienced earlier in Chile would have to remain Darwin’s sole direct experience of major geological forces at work.

Happily, there was plenty of biology to distract him from his geological disappointment. Because the islands had been formed de novo through volcanic action, the only animals and plants present were either migrants or modified descendants of earlier arrivals. As visitors never failed to notice, this made for a bizarre flora and fauna: giant tortoises (galápagos in Spanish), cactuses, penguins, flightless cormorants, marine iguanas. What made the islands’ natural history especially remarkable was the lack of human contact. Fray Tomás de Berlanga, the Spanish cleric who stumbled on the Galapagos in 1535 en route to Peru from Panama, found no human presence, and there is no archaeological record of pre-European colonisation. This may be one of the very few cases in the New World in which the word ‘discovery’ is not a misnomer.

In Evolution’s Workshop, his chronological account of the biological exploration of the islands, Edward Larson tells the colourful story of the visitors who followed de Berlanga. Buccaneers eventually gave way to whalers, but the allure of the Galapagos – short on good harbours and fresh water – was never strong. The contrast with other comparably isolated island groups is striking. Hawaii, also volcanic in origin, and inhabited, too, by its own biological peculiarities, was colonised around the year 400 by Polynesians; by the time Captain Cook arrived in 1778, it was home to some 300,000 people.

The archipelago that Darwin visited was by no means virginal, nonetheless. Two hundred of Ecuador’s criminals were held in a penal settlement on Charles Island (modern Floreana); mariners had introduced domestic animals, notably goats, to several islands; and the practice of stocking up with giant tortoises as a source of fresh meat for the long voyage across the Pacific had long been established. The crew of the Beagle found tortoises already scarce in accessible coastal areas and managed to collect only four dozen – Darwin reported wistfully that ‘formerly single vessels have taken away as many as seven hundred of the animals.’ But in general the Galapagos offered him a biological world unsullied by human contact. The animals were so tame that he was able to push ‘a large Hawk’ off a branch with the end of his gun.

Darwin later explained that he endeavoured to make ‘as nearly a perfect collection’ in every branch of natural history ‘as time permitted’. Considering that the Beagle was in the Galapagos for only five weeks, that most of that time was spent at sea carrying out its primary mission of charting, and that Darwin visited only four of the 18 larger islands, he did a thorough job. At the time, however, he was still essentially a creationist for whom the adaptations of animals and plants were evidence of divine design. The different islands all appeared superficially similar – equatorial, volcanic – so surely the Great Designer would have created forms that would thrive equally well on all the islands. It didn’t occur to Darwin, therefore, to differentiate his collections by island, and he duly broke the cardinal rule of biological collecting by failing to record the precise locations from which his specimens were taken. In addition, he failed to respond to some obvious pointers to evolutionary patterns among the islands. An Englishman in charge of the convicts claimed that the tortoises varied from island to island to such an extent that he could tell which island a tortoise had come from simply by looking at the carapace. Today, the different island forms are recognised as distinct subspecies. Darwin, however, ignored this information. He failed to collect the carapaces that were lying around the Charles Island settlement, where they were used as flower pots and the tortoises harvested by the crew served as dinner, not data; maybe Darwin watched as the cook heaved the empty carapaces into the Beagle’s wake. Four small tortoises did make it back to England but were too young to have developed island-specific characteristics.

The finches – emblematic of Darwin’s greatest Galapagos triumph – were in fact his nadir. Not only did he fail to label which islands his specimens came from, but he completely overlooked the evolutionary significance of variation in shape and form of the finches’ beaks, and focused instead on their coloration, confessing that ‘there reigns to me an inexplicable confusion’ regarding the birds. For biology students today, the finches exemplify the process of ‘adaptive radiation’, whereby an ancestor – an early arrival on the islands – has given rise to a number of descendant species specialised for different ecological roles, or ‘niches’. On oceanic islands like the Galapagos, therefore, many niches that in non-island ecosystems are distributed among different groups of birds are monopolised by a single group of close relatives. Darwin missed the boat. He decided that the finches were not closely related to each other, but belonged instead to four different groups of birds: blackbirds, grossbeaks, warblers and true finches.

One group did prompt a modicum of evolutionary speculation, however. Darwin noted that the Galapagos mockingbirds varied from island to island: ‘The specimens from Chatham and Albemarle Isd. appear to be the same, but the other two are different. In each Isd. each kind is exclusively found; habits of all are indistinguishable.’ But even in this case, he failed to recognise the evolutionary implications, lamely concluding that the mockingbirds from different islands ‘are only varieties’ of a single species – analogous to a gardener’s differing cultivars of roses.

Frank Sulloway has shown convincingly not only that Darwin’s Galapagos visit was singularly short on ‘Eureka!’ moments, but that the rest of the voyage – across the Pacific to Polynesia, on to Australia, and back to England via the Cape of Good Hope – also lacked them. It was only when Darwin delivered his bird collections to John Gould, the Zoological Society’s eminent ornithologist, that things began to happen. In early March 1837, five months after returning to England and nearly eighteen months after leaving the Galapagos, Darwin met with Gould to discuss his collections. Of the 26 species of land birds that he had collected, 25 were new – unique to the Galapagos. More important, Gould had determined that the mockingbird ‘varieties’ were in fact species, and that Darwin’s assortment of blackbirds, grossbeaks, warblers and finches were closely related members of a single group. Furthermore, the fact that the finches were massively over-represented in the Galapagos fauna – Gould recognised 13 species among the 26 Darwin had collected – cast the evolution/ creation divide into stark relief. If evolution were at work, then these species were the modified descendants of an early finch immigrant; if creation, then the Almighty had for some inexplicable reason developed an inordinate fondness for finches in the Galapagos, and nowhere else. Within months, Darwin had opened his notebook on ‘Transmutation of Species’ and the protracted intellectual process that culminated in 1859 with the publication of the Origin of Species was under way. If we must pick out a single episode in the history of evolutionary biology as its defining moment, then it surely has to be Darwin’s meeting with Gould, though the image of the younger man being set straight on matters of bird classification by the older expert is unpromising material from which to fashion a legend.

Although he tried to make amends, Darwin’s hitherto unpropitious relationship with the finches did not improve after Gould’s revelation. He realised that, like the mockingbirds, they too might be differentiated by island, but any attempt to investigate this was stymied by his failure to label his specimens accordingly. However, others on the Beagle had also made collections in the Galapagos and been more conscientious about labelling them; Darwin tried to paper over the gaps in his data with information from their specimens. A major source was the Beagle’s captain, Robert Fitzroy, whose published view on the variation among the finches was that it was ‘one of those admirable provisions of Infinite Wisdom by which each created thing is adapted to the place for which it was intended’. It is ironic that, to buttress his evolutionary interpretation, Darwin had to borrow from a lifelong creationist, whose response to the publication of the Origin would be to appear, Bible in hand, at the debate held at the British Association for the Advancement of Science meeting the following year.

Using data from Fitzroy and others, Darwin fudged the result he wanted: the finches, he decided, were neatly divided up among the islands. In fact, they aren’t. Multiple species of finches often occur together on the same islands; theirs is a much more complicated evolutionary story than the mockingbirds’. Darwin’s claims about the distribution of the finches among the islands in his Voyage of the Beagle – claims that even under the best of circumstances would have been dubious, since he collected from only four of the many islands – had one more unfortunate impact on attempts to understand their evolution: curators at the British Natural History Museum used the information in the Voyage as the basis for relabelling the Galapagos finches in their care, thus ensuring that Darwin’s mistaken retrospective speculation became enshrined in the museum collections themselves.

Darwin must have known that his ideas about the finches were shaky. Too careful a scientist to incorporate questionable material into his great work, he made no mention of them in the Origin of Species. Such is the power of the myth, however, that it is widely assumed that Origin relies heavily on evidence from the birds. Despite the shortcomings of his analysis, the finches have subsequently become a paradigmatic case of evolution in action, now serving as the centrepiece in textbooks and as the basis for popular accounts of evolution like Jonathan Weiner’s The Beak of the Finch. But these developments are relatively recent: the riddle of the finches was only solved more than a century after Darwin’s visit to the Galapagos. David Lack, then a schoolteacher, studied the birds in 1938-39, and in 1947 published his influential Darwin’s Finches, in which he laid out the details of the finches’ adaptive radiation, and emphasised the importance of competition between similar species in determining which ones are present on a particular island. The idea is simply that two species with similar requirements cannot coexist: one will eliminate the other or, alternatively, they will diverge evolutionarily in order to minimise competition. Lack’s interpretation – which remains in some aspects controversial – provides one of the best and most thoroughly worked out examples of the process that Darwin outlined in the Origin. It is his work that has been incorporated into the Darwin myth.

Perhaps Lack’s most important contribution to the myth-making process was nomenclatural. The name ‘Darwin’s Finches’ had been suggested earlier for the group, but it was its use by Lack that established it. Interestingly, the choice was made partly for technical reasons. The obvious alternative, ‘Galapagos Finches’, is inaccurate because one species inhabits Cocos Island, lying between the Galapagos and Central America. Dolph Schluter, a biologist who has worked on the finches, told Larson that he thought a more appropriate name would be ‘Lack’s Finches’. Indeed, one of the virtues of Larson’s thorough history of the biological exploration of the Galapagos is its emphasis on Lack’s contributions, which, belonging neither to the classical era (of Darwin) nor to the modern era (of contemporary research), have not always received the attention they deserve.

The history of how the finches were interpreted by Darwin and others is the history of the intellectual revolution they are associated with. Darwin’s initial ideas, despite the evidence in front of him, were very much of his time, and they changed not because the Darwin of myth had a moment of visionary insight but, prosaically, because the Darwin of fact was part of a community of scientists. This is not, in any way, to diminish his status as a scientist. Indeed, Frank Sulloway has concluded that the chief offence of the Galapagos legend is that it ‘masks the complex nature of scientific discovery, and, thereby, the real nature of Darwin’s genius’.