Darwin had struck up a correspondence with botanist Joseph Dalton Hooker (1817–1911) soon after Hooker’s return in the fall of 1843 from a 4-year southern voyage under James Clark Ross, on which he served as both assistant surgeon and naturalist. (That was the acclaimed voyage that confirmed the existence of a vast new continent—Antarctica.) Knowing that Hooker was going to work on a flora of the southern lands, Darwin asked Henslow to forward his Galápagos plant collection to him in case they proved useful. Hooker thanked him warmly, and they started corresponding on points of common interest as scientists and travelers. Their friendship grew to the point where, in January of 1844, the year he penned the Essay, Darwin ventured to take Hooker into his confidence. “I have been now ever since my return engaged in a very presumptuous work & which I know no one individual who would not say a very foolish one,” Darwin ventured. He explained how he had been struck by the Galápagos species he observed, and the fossil mammals of South America, and how he resolved to collect any and all facts that might bear on the question of the nature of species. “I have read heaps of agricultural & horticultural books, & have never ceased collecting facts,” he continued. “At last gleams of light have come, & I am almost convinced (quite contrary to opinion I started with) that species are not (it is like confessing a murder) immutable.” Darwin hastily added a reassuring line distancing himself from the unpalatable ideas of Lamarck: “Heaven forfend me from Lamarck nonsense of a ‘tendency to progression’ ‘adaptations from the slow willing of animals’ &c.”—yet, he had to acknowledge, the conclusion he had come to was not so very different from the Frenchman’s, though the mechanism of change is very different. “I think I have found out (here’s presumption!) the simple way by which species become exquisitely adapted to various ends.” How would Hooker take this? Darwin was a bit worried: “You will now groan,” he wrote, “& think to yourself ‘on what a man have I been wasting my time in writing to.’ ”27 Far from contemptuous or dismissive, Hooker was intrigued and invited his new friend to tell him more.
The Barnacle Returneth
While Hooker was not dismissive of Darwin’s species theory, he wasn’t an instant convert either. His skepticism proved invaluable to Darwin: the botanist was a font of information and a fierce (but friendly) critic, constantly challenging Darwin to sharpen his arguments and defend his evidence. He helped Darwin recognize where the weaknesses of his theory lay. They exchanged long letters in which Darwin peppered Hooker with questions seeking data, observations, and Hooker’s judgment on matters relating to geographical distribution, variation, classification, structure, and species relationships. Darwin pored over botanical manuals and turned to Hooker for assessment of this or that observation, ever on the lookout for facts that would bear on his theory. It was in this vein that Darwin wrote Hooker expressing interest in an article on species and their variation by Frédéric Gérard, in the newly published Dictionnaire universel d’histoire naturelle. Darwin picked up on intriguing language used by Gérard, such as how species can develop certain traits (sounding like thinly veiled transmutationism), and how the Frenchman questioned whether “species” per se even existed on the basis of their often unclear boundaries. Darwin’s innocent query provoked a contemptuous reply from Hooker: Gérard was “evidently no Botanist,” he sniffed, but one of those “narrow-minded studiers of overwrought local floras.” He had little patience for armchair naturalists who make a big deal out of variable characters and questionable delineations of species that were, after all, issues discussed in cautious terms by careful and methodical botanists, yet “taken up by such men as Gerard, who have no idea what thousands of good species there are in the world.” Hooker declared that he was “not inclined to take much for granted from any one [who] treats the subject in his way & who does not know what it is to be a specific Naturalist himself.”28
Reading between the lines, Darwin realized that Hooker was indirectly referring to him and his grandiose theorizing, going on about species and varieties but really having little idea what these are like in nature. “How painfully . . . true is your remark that no one has hardly a right to examine the question of species who has not minutely described many,” he acknowledged. But, he continued, defensively: “I was, however, pleased to hear from Owen (who is vehemently opposed to any mutability in species) that he thought it was a very fair subject & that there was a mass of facts to be brought to bear on the question, not hitherto collected.” Darwin would take comfort in the fact that he had “dabbled in several branches of natural history and geology,” he said, and although he would get “more kicks than half-pennies,” he would pursue his species theory. At least he would improve upon Lamarck, he ventured: “Lamarck is the only exception, that I can think of, of an accurate describer of species at least in the Invertebrate kingdom, who has disbelieved in permanent species, but he in his absurd though clever work has done the subject harm, as has Mr Vestiges, and, as (some future loose naturalist attempting the same speculations will perhaps say) has Mr D.”29
Some of the acorn barnacles studied by Darwin. Courtesy of Richard Milner.
Hooker was quick to say that he wasn’t insinuating that Darwin was one of those armchair naturalists. Rather, he wrote, smoothing things over, “what I meant I still maintain, that to be able to handle the subject at all, one must have handled hundreds of species with a view to distinguishing them & that over a great part—or brought from a great many parts—of the globe.” But Darwin felt the truth of Hooker’s criticism. “All which you so kindly say about my species work,” he replied, “does not alter one iota my long self-acknowledged presumption in accumulating facts & speculating on the subject of variation, without having worked out my due share of species.” This exchange of letters took place in September 1845.30 Within a year Darwin was deep into a systematic study of barnacles, an undertaking that he first thought might take up to a year but in fact ballooned to 8 years. By the end of it he had “minutely described” many species, earning “the right to examine the question of species.”
Darwin’s barnacle work snowballed in part because other naturalists cheered him on: Hooker, certainly, but also Richard Owen at the Royal College of Surgeons, and the famed zoologist Louis Agassiz at Harvard, who called the world of barnacles a “great desideratum” desperately in need of study. The real coup came when the zoological keeper at the British Museum, Edward Gray, agreed to lend Darwin the Museum’s entire cirripede collection, much of it uncataloged. Imagine that: the whole national collection at his fingertips! But the other reason the project exploded was the unexpected discoveries he made—discoveries with profound implications for his species theory. As Janet Browne aptly put in her monumental Darwin biography, the barnacle studies would prove to be another of Darwin’s “explorations into the reproductive unknown.”31
Darwin’s barnacle odyssey began with the curious specimen he found on the windswept beach in the Chonos Archipelago. He showed it to Hooker, who had come to Down House in the fall of 1846 to meet Darwin’s old Beagle shipmate Bartholomew Sulivan. Just back from South America, Sulivan had completed a detailed survey of the Falkland Islands, among other accomplishments, as commander of his own ship. Back in 1835 when Darwin first found his burrowing barnacle, he commented in his notes that “it is manifest this curious little animal forms new genus.” Now he asked Hooker to help him name it, and they came up with Arthrobalanus—or “Mr. Arthrobalanus” in their frequent discussions of the odd crustacean. In fact this creature seemed doubly odd: besides its burrowing habit, Darwin thought he could discern two penises. It took a while, but he eventually realized that the Mr. was in fact a Ms. The “penises” were the cirri, filament-like feeding appendages, of female specimens. He wasn’t to discover the male of this species for several years yet—it turned out to be the smallest barnacle male known. In the meantime his skills at dissection got steadily better, helped by Hooker’s recommendations for improving the optics of his microscope. In one letter he thanked Hooker thrice over: for
helping with the microscope, for suggesting he use porcupine quills instead of glass tubes in dissection, and for the gift of “capital” chutney. Between the three, “I have many daily memorials of you,” he joked, and proudly reported that he devised a set of wood blocks to support his wrists while dissecting, “a splendid invention.” He was excited with his new cirripede hobbyhorse. “As you say,” he wrote to Hooker, “there is an extraordinary pleasure in pure observation . . . After having been so many years employed in writing my old geological observations it is delightful to use one’s eyes & fingers again.”32
This was November of 1846, and the Darwin household was growing in more ways than one: he and Emma enlarged the house over the past year to include a schoolroom and two bedrooms, as their family had grown to four children (1-year-old Georgy, 3-year-old Etty, 5-year-old Annie, and 7-year-old Willy)—and counting, with baby Bessy joining the family in July of 1847. By this time Darwin was deep into his barnacle work, and although he took some time to attend scientific meetings in London and Oxford now and then what he loved best was the “absolute rurality” of Down. His friends loved it too, and he regularly invited them for scientific respites from London. It was an idyll compared to the noise, smoke, and grit of the city, though at times it must have seemed almost as crowded: besides growing family and domestic staff there was a near-constant procession of visiting scientific friends (often with their wives and children) and relatives such as Charles’s brother ’Ras (despite professing to hate country life) and other siblings and their families. Their Aunt Sarah Wedgwood also made frequent appearances, having moved to the village of Downe to be close to Charles and Emma. Darwin loved the conviviality of family and friends, but it was also understood that he had to often retreat to his study, where he would “do his barnacles” in peace.
The following year, 1848, brought joy, sadness, and excitement: Franky, the latest addition to the family, came along in August, but just a few months later Darwin’s father, Dr. Robert Darwin, died in Shrewsbury. The excitement came from his most startling barnacle discoveries yet: while most barnacle species were hermaphrodites bearing both male and female sexual organs, he came across one (Ibla cumingii) that was not only single-sex, but had males reduced to tiny nubs adhering to the carapace of the comparatively gigantic females. The males were mere rudiments, eyeless, limbless, near-microscopic bags of sperm. This was astonishing: having separate sexes was strange enough, but there was practically nothing to compare with the radical difference in size and development of the two sexes in this species—rather as if human males were reduced to tiny testes adhering to women like ticks. He wrote excitedly to Henslow in April 1848 about the “microscopically minute” males:
But here comes the odd fact . . . [the males] become parasitic within the sack of the female, & thus fixed & half embedded in the flesh of their wives they pass their whole lives & can never move again.
“Is it not strange,” he mused, “that nature should have made this one genus unisexual [sexes separate], & yet have fixed the males on the outside of the females—the male organs in fact being thus external instead of internal.”33 It was strange, and things got even stranger when he realized that another barnacle, Scalpellum, was both hermaphroditic and had tiny “parasitic” males. Now why have these extra, or complemental males, as Darwin called them, when male equipment was already built-in, as it were? He wrote excitedly to Hooker: “I had observed some minute parasites adhering to [Scalpellum], & these parasites, I now can show, are supplemental males . . . so we have almost a polygamous animal, simple females alone being wanting.”34 He went back and scrutinized earlier specimens, rightly thinking he may have mistaken diminutive males for the odd parasite, and indeed found several new cases of complemental males. Recall how he had been mistaken about the double penis of what turned out to be Arthrobalanus females. Now he actually found Arthrobalanus males, which turn out to be the minutest of minute complementals but equipped with a penis nearly nine times the length of the male’s body.
It dawned on him that the sexual strategies of these barnacles were of tremendous significance: no less than evidence of an evolutionary differentiation from hermaphrodites into males and females. Animals and plants were initially hermaphroditic, he began to think, then slowly differentiated over eons into the two-sex arrangement so common today. Why was another matter, but this explained anomalous features of anatomy like the nonfunctional nipples of male mammals or rudimentary stamens of female flowers, and it dovetailed with a growing insight from his studies of reproduction and pollination (see Chapters 6 and 7); namely, that nature abhors perpetual self-fertilization. How and why the sexes formed was yet another exciting line of investigation. Something like a grand unified theory of life’s evolution driven by natural selection was taking shape in his mind. “I never [should] have made this out,” he exclaimed to Hooker, “had not my species theory convinced me, that an hermaphrodite species must pass into a bisexual species by insensibly small stages, & here we have it, for the male organs in the hermaphrodite are beginning to fail, & independent males ready formed.” This was the kind of evidence he needed. The odd sex lives of his barnacles may not be a smoking gun for transmutation, but they were not far from it: how else to explain such strange arrangements? It certainly wasn’t consistent with the conventional view of perfect design in nature. He half-joked in the same letter that Hooker would “perhaps wish my Barnacles & Species theory al Diabolo [to the devil] together. But I don’t care what you say, my species theory is all gospel.”35
And so it went as he was drawn in deeper and deeper, studying barnacles from every corner of the world. He traced homologies and reconstructed what he thought was the “archetypal” barnacle, showing how its 17 body parts derived from the 21-body-part archetype that the French zoologist Henri Milne-Edwards had identified for Crustacea as a whole. He was astounded by the variation he found, at multiple levels: abundant individual variation, but also unexpected structural and life history variations on a theme, from those puzzling complemental males to groups with elaborate cirri to yet others lacking appendages altogether. Those he classified in their own suborder: “Apoda,” the footless barnacles. An “apodal cirripede” may be a contradiction in terms, but as he marveled to Lyell, when it came to barnacles “truly the schemes & wonders of nature are illimitable.”36
By the time he was through, Darwin had produced the most comprehensive treatment of living and fossil cirripedes yet, culminating in a four-volume set of monographs, two published in 1851 and two in 1854.37 It was a huge accomplishment, earning him the Royal Society’s highest honor, the Royal Medal, in 1853, before the study was even fully published. Some of his interpretations of barnacle anatomy and development did not stand the test of time, but they give us insight into Darwin’s early evolutionary thinking and working method. Janet Browne pointed out that the barnacle period marked a sea change in Darwin’s awareness of the ubiquity of individual variation. Prior to poring over barnacles he seemed to have had a conception of variation mainly induced by environmental change and other factors acting on the reproductive system. As a result of scrutinizing not just individuals, but populations of individuals of many species (albeit populations occupying museum drawers and jars), Darwin gained a new appreciation for the sheer abundance of variability in virtually all traits, great and small. “You ask what effect studying species has had on my variation theories,” he wrote Hooker from Malvern, where he was trying to combat his latest bout of illness with the water cure. “I have been struck . . . with the variability of every part in some slight degree of every species: when the same organ is rigorously compared in many individuals I always find some slight variability.”38 He joked that working on systematics would be easy were it not for “this confounded variation,” but acknowledged it was a boon for his species theory. It was, after all, the very raw material that selection acted upon, and for selection to shape species as completely as he envisioned, variation must abound in even the most trivial of traits.
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In this period, too, Darwin refined a research method that stood him well through all of his subsequent investigations: first, systematic and thorough study, sometimes more experimental and sometimes more observational, but always marked by noticing unnoticed phenomena, asking unasked questions, and connecting dots where others could not see the patterns. Crucially, his method included immersion in the available literature and cultivation of a worldwide network of correspondents: academics and amateurs, aristocrats and humble museum men, ship’s captains and army officers—anyone and everyone expert in an area of interest to Darwin or otherwise able to help him was approached to bounce ideas off of and to ask for specimens, observations, and information.
Gooseneck or pedunculated barnacles. Drawing by Leslie C. Costa.
By the time Darwin finished his 8-year barnacle study, his children had gotten used to his scientific work at home. He had seven kids ranging from a teenager of 15 years to a toddler of 3 (there had been eight, but the couple suffered the devastating loss of their eldest daughter, Annie, in 1851). His barnacle work was going on for most to all of the children’s lives, depending on the child. Sir John Lubbock, a family friend and neighbor, told an anecdote about this period: when visiting a friend who lived nearby, one of the Darwin boys looked around and, seeing no microscope or dissecting equipment, asked of his friend’s dad, “Then where does he do his barnacles?” For all the kids knew, everyone’s dad worked on barnacles.39
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