Darwin's Backyard

by James T. Costa

  This creature, rooted to one spot through life and death, was in its infancy a free swimming animal, hovering from place to place upon delicate ciliae, till, having sown its wild oats, it settled down in life, built itself a good stone house, and became a landowner, or rather a glebae adscriptus, for ever and a day. Mysterious destiny!6

  Mysterious indeed, these barnacles—so like diminutive glebae adscriptus, the medieval laboring “adscripts of the soil” permanently attached to the land. But a deeper mystery was how the identity of these humble organisms eluded naturalists until just a few years before Darwin’s walk on that beach in southern Chile: before the 1830s they had been classified as mollusks.

  Barnacles are arthropods, classified today in the infraclass Cirripedia of the subphylum Crustacea, meaning their closest relatives are crabs, shrimp, lobsters, and their ilk. They take their name from their elongated, slender and gracefully curved legs, or cirri (“cirripede” is derived from the Latin cirrus, “curled” or “tufted,” like cirrus clouds, and pede, “foot”), tipped with fine hairs for sweeping food particles from the water column. The shell-like armor and two-part valves of common encrusting barnacles, together with the erroneous belief that they did not undergo metamorphosis, may well have misled naturalists since time immemorial into thinking that they were odd relatives of limpets or clams. It was not until 1830 that John Vaughan Thompson (1779–1847), a British army surgeon accomplished in zoology and botany, revealed their remarkable structure and metamorphosis in a memoir published in his collection Zoological Researches, and Illustrations; or Natural History of Nondescript Or Imperfectly Known Animals. Seen through a good microscope, these organisms were far from nondescript, but they certainly were imperfectly known. Thompson worked out the surprising life history of barnacles, and how their anatomical structure pointed to a relationship with arthropod crustaceans. Piquing the interest of other naturalists, studies confirming and extending Thompson’s findings appeared in due time, but so too did studies arguing against Thompson’s findings. The matter was not fully settled until the early 1840s, but most naturalists acknowledged the organisms’ true identity well before then.

  Thompson showed that barnacles start out as a free-swimming one-eyed larva he called the nauplius, compact with odd projecting structures for swimming, resembling a menacing alien spacecraft of the kind you might see in a Star Trek movie. After six months or so these mature into an adult affixed to one spot. In between is a short-lived transitional stage of the barnacle, mobile but nonfeeding, which Thompson called the cyprid, which plays the role of scout, feeling out potential real estate for permanent settling using specialized antennae. Once a suitable spot is found, the cyprid attaches itself headfirst by producing a glue-like substance from the antennae. The head-standing critter soon metamorphoses into an adult, and an even stronger substance cements the deal, literally, as the adult is permanently bound to the substrate by its head. It also begins to secrete protective calcareous plates—a fortified cell.

  Now the barnacle takes on its familiar form: little cones or volcano-shaped structures stuccoing rocks, boats, shells, even whales and turtles. At least, that’s the form of the common sessile or “acorn” barnacles; a whole other group of them are “pedunculate,” or goosenecked, perched atop a flexible stalk attached to the substrate. The adults can continue to grow for a time, molting within the safety of their shelter as all good arthropods do, and expanding the protective plates as they get larger.

  Thompson’s well-thumbed memoir on barnacles was among the many books in the Beagle’s reference library (which was at Darwin’s fingertips, lodged in the same cozy 10 × 11-foot poop cabin where he worked by day and slung his hammock by night).7 With Thompson’s book at hand and his keen marine zoology interests, inspired by his Edinburgh mentor (and evolutionist) Robert Grant, Darwin was familiar with barnacles and their habits. Yet none that he knew of drilled holes. His cataloging and dissecting done, however, he packed the shot-through Concholepas shells away and moved on to the next intriguing collections as the Beagle made its way northward along the coast.

  The sessile (acorn) barnacle Balanus tintinnabulum encased in its protective shell. Note the modified legs, or cirri, which serve as the food-gathering structures of these suspension-feeders. From Darwin (1854), vol. 2, plate 25.

  Cuidado

  Barnacles were on Darwin’s mind again in March 1837, the month that saw him turn into a transmutationist. He had just moved to London, renting a place on Great Marlborough Street in Bloomsbury for easy access to the British Museum, and conveniently just up the street from his brother ‘Ras. The parallel Darwin perceived between geography and geology—the geographical distribution of related species and its uncanny correspondence with the relationship between living and extinct species of an area—got his adrenalin pumping, as he realized this correspondence spoke of species changing. The epiphany fired his imagination, sending him seeking more information, new observations, to test his hunch. Questions abounded, but among Darwin’s earliest notebook entries as a transmutationist we find him especially concerned with reproduction, metamorphosis, and the unity of life, all somehow bearing, he was sure, on the nature of species, varieties, variation, and change—and barnacles were front and center.

  In March 1837 he pulled out his old marine zoology notebook from Edinburgh, flipped it over, and commenced a series of notes from the back. The first of these, under the heading “Zoology,” referenced the recent debate over barnacle metamorphosis.8 John Obadiah Westwood, the Oxford arthropod specialist, challenged Thompson’s interpretation of barnacle structure and development, and therefore their classification. His critique was based on the prevailing view of relationships between animal classes and phyla, which held that crustaceans are more closely related to vertebrates than to insects. Westwood reasoned that since many insects and no vertebrates exhibited metamorphosis—no vertebrate experienced anything at all like the radical transformation of caterpillar to butterfly, say—by analogy crustaceans should not be so insect-like as to metamorphose either. Westwood was not alone in this belief—odd as it may seem today, notable naturalists of the day could not see the evidence for metamorphosis that Thompson pointed out; such is the power of preconceived ideas, perhaps. Westwood concluded that the weight of evidence pointed to barnacles as some sort of anomalous mollusk, somewhere between the “articulates” (arthropods) and vertebrates. Thompson and Westwood read dueling papers at the Royal Society, published back-to-back in the Society’s Philosophical Transactions in 1835. It was these papers that Darwin referenced in his notebook.

  Darwin’s money was on Thompson’s interpretation, maybe owing to the stock he put in the argument based on the clear segmentation and other arthropod-like features of the barnacle’s early stages. This may be the earliest indication of Darwin’s interest in the value of development to inform relationships. In this case early stages were used to make decisions about classification, but later he would see how early stages, especially embryos, could give clues to evolutionary diversification. That idea, too, was in its infancy in the 1830s, and Darwin was abreast of the latest discussions. Richard Owen (1804–1892) would have impressed the utility of early stages upon the younger naturalist in the many conversations they had in his post-Beagle years. Darwin might have even heard Owen, a rising star and first permanent Hunterian Professor at the Royal College of Surgeons, hold forth on the subject in his acclaimed Hunterian Lectures in the spring of 1837. On May 9th of that year Owen declared to a rapt 400-plus audience that studying the structure of fully developed animals is inadequate for diagnosing true relationships. Early developmental stages were key: just as entomologists now gain insight into insect relationships by studying their immature stages, “What should we know of the zoological relations of the Barnacle, if we were acquainted only with its organization in the last fixed stage of existence?”9 In this very same lecture, however, Owen took a dim view of such discreditable ideas as transmutation—a public condemnation that would have
reinforced Darwin’s resolve to keep his evolutionary speculating to himself had he been present.

  Reproduction became as central to Darwin’s transmutational musings as geography and geology. In July 1837, during a visit to the family home in Shrewsbury, he started a fresh notebook with copious notes on his transmutationist grandfather’s work Zoonomia. In it he speculated on the nature of species relationships over time: a branching pattern captures the essential idea of change within lineages as well as links between lineages, and he showed that he understood the dynamic of lineages diverging over time, as in one comment pointing out that gaps between groups like arthropods and vertebrates will grow over time, as extant species become more divergent from their common ancestor. “Heaven know[s] whether this agree[s] with Nature,” he wrote, literally ending on an underscored cautionary note: “Cuidado”—caution.10

  And proceed with caution he did, at least publicly—that was only prudent, as he was becoming something of a rising star. But in the privacy of his notebooks his speculations and questions ran rampant as he immersed himself in literature of all kinds, from hard-core geology, botany, and zoology to practical agricultural improvement and breeding, with philosophy, religion, and literature thrown in. He boned up on the prevailing arguments against transmutation, clarifying the key issues but also arming himself. Darwin knew only too well Lyell’s extensive anti-transmutation arguments in the watershed Principles. The fifth edition came out in 1837, the very year of Darwin’s conversion to transmutation; reviewing Lyell’s arguments he wrote in the margin of his copy: “If this were true, adios theory.”11

  Another Volcano Erupts

  From time immemorial those inclined to see transmutational change cited links throughout the organic world as evidence: species or their taxonomic groupings can be arranged in something of a sequence, it was thought, generally in order of increasing “complexity” (however that was defined). Schemes varied, but their common denominator was continuity, the idea that all forms could be linked through a chain of intermediates—versions of the ancient Greek Scale of Nature idea that became codified in some strains of Christian thought. With increased knowledge of geological strata and the fossil record in the eighteenth and especially nineteenth centuries came the realization that to a large extent this chain seemed to map onto a temporal sequence. The Chain of Being and its development over geological ages were thus easily interpreted in terms of an unfolding divine plan marked by successive bouts of creation (and extinction), a teleological view that inevitably had as its aim the arrival of humanity, the pinnacle of creation made in the very image of the Creator. But such chains of relationship are just as easily interpreted in transmutational terms, as Lamarck (and later Darwin) came to believe.

  One of the most serious arguments against transmutation, in contrast, was based on supposed discontinuities, morphological gaps between major animal groups. If there were such gaps, the rug was pulled out from under any possibility of transmutation. Thus was born the concept of embranchements, distinct, separate, decidedly unlinkable taxonomic groups reflecting distinct body plans, an idea introduced by the illustrious and formidable Georges Cuvier in Paris. Cuvier (1769–1832) recognized four basic body plans—Radiata (sea stars, jellies, and relatives), Mollusca (bivalves, slugs, cephalopods . . . and barnacles), Articulata (arthropods and relatives), and Vertebrata (all animals with backbones: fish, reptiles, amphibians, birds, and mammals)—each fully independent of the others. Sure, recognizing the diversity of forms within these categories, certain taxa in one embranchement could “approach” others in another embranchement, but there was no possibility of a real relationship or link between embranchements, let alone a passage—barnacles might have some traits that apparently “approach” insects among the articulates, for example, but they were essentially mollusks no matter how much “articulateness” they may exhibit, and one was certainly not derived from the other.

  Cuvier’s system was devised in explicit refutation of his (unfairly) reviled colleague Lamarck, with his notorious transmutational ideas. No linkage between the fundamental taxonomic groups, no possibility of some kind of “passage,” no transmutation. Why transmutation was seen as such an affront is a bigger issue than we have space for here; suffice it to say that this became a signal issue for a larger battle over materialism and natural law versus divine providence, and thus over atheism versus received religion. In important respects this was a false dichotomy, not least because then, as now, some simply saw transmutation by natural law as itself a divinely ordained process. But the readiness with which the anticlerical revolutionaries of France and their hopeful counterparts in Britain in the late eighteenth and early nineteenth centuries brandished the banner of change, transmutation in all things—from societies to species—and the ensuing horrors this unleashed in the Terror would only harden the position of conservatives defending the social order through the central authority of church and state. Thus it was that transmutation was itself transmogrified into a byword for atheism and even sedition by Darwin’s day.

  That was certainly the way that Richard Owen saw it. He embraced Cuvier’s system of embranchements with gusto, and his later term archetype (ideal generalized form of a given species or group) was inspired by Cuvier. Fervently opposed to Lamarck and transmutationism, Owen was determined to stamp out such heresy in England. Lyell’s polemic against Lamarck and transmutation in the Principles was written in the same conviction, though not the same mean-spiritedness. Owen’s antipathy and Lyell’s disapproval would have been abundantly clear to Darwin, and concerns over the social implications of his theory were behind his note of cuidado.

  Transformation was thus very much on Darwin’s mind in the late 1830s—not least his own transformation as his thinking developed. In the fall of 1837, not long after his brief notes on the Thompson-Westwood debate over crustacean metamorphosis, he delved into another and more famous debate, one that took place back in 1830 and had implications that struck at the very heart of his transmutation theory. That was the year that Georges Cuvier squared off against his younger colleague and sometime protégé, Étienne Geoffroy Saint-Hilaire at the Académie Royale des Sciences in Paris. The issue was no less than the unity of life. For years Cuvier and Geoffroy argued, mostly in private, over the fundamental question of relationship between Cuvier’s four embranchements. Geoffroy (1772–1844) was as brilliant an anatomist as Cuvier, and while not an out-and-out transmutationist, he was a friend and supporter of Lamarck, who had died the year before the debate. Geoffroy subscribed to an idea that might be described as a zoological uber-plan: he saw a single generalized body plan uniting all animals, all four embranchements, allowing that some creative force had fashioned each from the same basic starting point. Perhaps some had even been derived from others, rather than being created de novo. Geoffroy’s evidence was anatomical structures that he traced through development—essentially what we call homologies today. Through a series of acclaimed studies he had built up a body of work establishing not only unity of the animal body plan, but also principles of connection between seemingly disparate forms, the significance of rudimentary structures, and his “law of balancement of organs.” It was Geoffroy who discovered that the mammalian skull could be understood in terms of fusion of various bones, separate in so-called “lower” vertebrate forms and, significantly, early embryological stages of “higher” forms. “Nature constantly uses the same materials,” Geoffroy maintained, “and is ingenious only in varying their forms.” Early on Cuvier applauded his younger colleague’s discoveries, but when Geoffroy sought to derive from them a visionary new philosophy of morphology proclaiming the unity of all animal forms, Cuvier balked.

  The trigger of the great debate was a paper Geoffroy encouraged, read by a pair of younger naturalists presenting anatomical evidence that cephalopods form a linking group between mollusks and vertebrates. Cuvier did not take it well. The senior naturalist’s vehement rejection of the paper provoked Geoffroy to enter the fray. The argum
ent soon became public as the savants presented dueling papers at the Académie Royale through the spring and summer of 1830. This all unfolded against the backdrop of another drama developing in Paris at the same time: the political upheaval that culminated in le trois glorieuses, the Three Glorious Days in late July 1830 when the people rose up and, manning the barricades, forced the abdication of the repressive King Charles X. It was a matter of perspective which was the more significant battle: one Freédeéric Soret, of Geneva, had occasion to visit the aging polymath Johann Wolfgang von Goethe in Weimar, Germany, just as news of the Paris uprising reached the city, setting “everyone in a commotion,” as Soret recalled. He recounted his meeting with the German savant:

  “Now,” [Goethe] exclaimed as I entered, “what do you think of this great event? The volcano has come to an eruption; everything is in flames, and we no longer have a transaction behind closed doors!” “A frightful story,” I replied. “But what else could be expected under such notorious circumstances and with such a ministry, than that matters would end with the expulsion of the royal family?” “We do not appear to understand each other, my good friend,” replied Goethe. “I am not speaking of those people at all, but of something entirely different. I am speaking of the contest, of the highest importance for science, between Cuvier and Geoffroy Saint-Hilaire, which has come to an open rupture in the Acadeémie.”12