Darwin and the Barnacle
Page 14
Darwin, unlike the few barnacle collectors and systematists who had travelled this road before him, knew that in order to break this riddle of nature, in order to understand how barnacles had evolved the way they had, his journey must take him inside the horny shell – a journey into the heart of barnacle darkness, stalked or coned. In January, desperate to make the whole process easier for himself, determined to be able to see more, Darwin had invested in a new Smith and Beck microscope for £16, which made it possible to dissect under water much more easily. He wrote to describe it to Henslow: ‘If you are ever starting any young naturalist with his tools, recommend him to go to Smith & Beck of 6 Colman St. City for a simple microscope: he has lately made one for me, partly from my own model & with hints from Hooker, wonderfully superior for coarse and fine dissections than I ever before worked with. If I had had it sooner, it would have saved me many an hour.’11
Whilst he could see Ibla with his naked eye, he had no idea of the extent of this creature’s deviation from barnacle archetypes until he placed it under the new microscope. All barnacles are hermaphrodite, the zoology textbooks claimed. Hermaphroditism was what marked out barnacles from other crustacea: the barnacle hallmark. Darwin, though, had already discovered that Mr Arthrobalanus was all male and this female Ibla, to his astonishment, was all female, or as far as he could see – she only had female reproductive organs and no male ones. So what were the tiny creatures embedded in her flesh? More parasites? Many marine organisms ride pillion on larger organisms, living parasitically or symbiotically on their bodies. Darwin had seen thousands of tiny parasites in much of the marine flesh he had already dissected. The parasitical chain was infinite. Usually he ignored these interlopers or removed them so as not to obstruct his gaze, threw them away; but this time he didn’t. This time he eased them out of the flesh of the female Ibla and marked them up, sandwiched within their own pair of glass slides – worth checking, he thought.
He couldn’t understand what he saw under the microscope at first. The creatures living in the female Ibla’s flesh were, he found to his astonishment, the missing males – Ibla males. They were very primitive – little more than elongated tubes containing sperm (See Plate 14) – but they were very definitely male. How had this happened? Was it that food was deficient and that the creature had evolved separate sexes in order to divide up the workload of reproduction and food gathering? He couldn’t answer or even speculate about this question until he had examined other closely related stalked barnacles such as the Ibla quadrivalvis and the Scalpellum.
There were further surprises. Looking again at the Australian Ibla quadrivalvis specimens from the British Museum collection, he found that not all of them were female; some were hermaphrodite. But both the all-female and the hermaphrodite specimens had ‘complemental’ males embedded in their flesh. Although bigger in relation to the females, these complemental males were exactly the same as the Ibla cumingii males although ‘utterly different in appearance and structure’ from the female and from the hermaphrodite. They seemed to represent such ‘diverse beings, with scarcely anything in common, and yet all belonging to the same species’.12 There was no other case like it in the animal kingdom, no other instance of hermaphrodites and males ‘within the limits of the same species’, although, Darwin noted, ‘it is far from rare in the Vegetable Kingdom’.13 Another point of connection, then, between the apparently separate kingdoms.
Darwin was sure that he had here, arranged on his desk in front of the open window, a sequence of barnacles that each represented a stage in an evolutionary sequence, like a branching tree.14 ‘Gradation’, he had written in 1837; ‘prove animals like plants; trace gradation between associated & non associated animals. – & the story will be complete,’15 First he had the normal barnacles, the great majority by far, which were straightforward hermaphrodites, conforming to barnacle archetypes. Then the Ibla and a second, related stalked barnacle he had in his collection, Scalpellum vulgare, represented a transitional stage in a gradual shift towards separate sexes. These hermaphrodites still had male organs, but they were superfluous organs, now that the ‘complemental’ males had taken over their reproductive function. Eventually these male organs would drop away entirely or remain as much-diminished vestiges of their hermaphrodite past, like abortive stamens or pistils in flowers. Then came the completely separate-sex barnacles, representing the next stage in the evolutionary sequence: a stage in which the male was dwarfed in relation to the female and lived, as did the ‘complemental’ males, as nothing more than a sack of sperm, Ibla and Scalpellum were crossovers in the process of transition from hermaphroditism to separate sexes. For Darwin it provided a fascinating way of thinking about organs that were superfluous in higher animals – why men had nipples, for instance. They, too, might be vestiges of a hermaphrodite past. Curiouser and curiouser; gradation after gradation.
So by 1 April, when he wrote to Henslow, he could indeed claim that this was ‘better than castle-building’, but he was castle-building in his own way – not castles in the air but castles built up by the bricks and mortar of chiselled fact on fact. He wasn’t sure yet how he could explain the Ibla discovery, or what conclusions he could draw from it:
But here comes the odd fact, the male or sometimes two males, at the instant they cease being locomotive larvae 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 that nature should have made this one genus unisexual, & yet have fixed the males on the outside of the females…16
All this bizarre reproductive behaviour in the animal kingdom sometimes seemed to have a comic bearing on his own life, making him think about how human animals managed the reproductive process. Was it so very much superior to or more sophisticated than the barnacle way? The question was: did it work? At the end of his long April Fool’s Day letter to his Cambridge mentor, Henslow, Darwin added cheerfully: ‘We are all well here, & a sixth little (d) expected this summer: as for myself I have had more unwellness than usual.’17 This was their little joke: Henslow, amazed at the speed with which Darwin and Emma were producing children, always referred to Darwin’s as yet unborn children as little (d)s and Darwin, charmed, followed. Their code said something about inheritance and their shared assumptions about the male line (after all, it might have been a little (w) or a (d/w) to signal the Wedgwood inheritance). The bracketing of the baby ‘d’ within its protective, parenthetical shell also evoked simultaneously its as-yet-to-be-ness and its embryonic watery immersion within Emma’s curved belly. Did it also remind Darwin of the soft body of the barnacle, upside down within its curved shell, fishing with its feet through the bracketed opening?
None of the barnacle discoveries provided a very serious picture of the prehistoric emergence of maleness: thus fixed and half-embedded in the flesh of their wives they pass their whole lives and can never move again. Nor did it provide much to support an ideology of the absolute evolutionary superiority of the male. It didn’t shock Darwin, however – far from it – he found the whole barnacle sex sequence delightfully pleasing. After all, he had been thinking about all of this for some time, having worked out this theory of the gradual divergence of the sexes from an ancestral hermaphrodite in his Notebook D written in 1840. It had been just a hypothesis, then – a castle in the air. He’d been searching for and recording evidence from plants to confirm his theory ever since; but this find was more significant Here was evidence of the same sequenced branching pattern of sexual diversification in the animal world. It strengthened his commitment to wider theories about descent and modification – another piece had fallen into place, another brick of empirical evidence, not wild speculation. The evidence was all beginning to fit together.
What was newly exciting to Darwin was that his species hypothesis was now leading him, directing his questions; and it was like pushing at open doors. He would never have looked closely at those specks of parasites embedded in t
he Ibla if he hadn’t had a hunch, an instinct that they might provide a clue of some kind in relation to the divergence of sexes. What he had found was a polygamous animal, he wrote to Hooker a month later, in May, struggling to explain why this discovery was so important to his theory:
I never shd. have made this out, 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 already beginning to fail, & independent males ready formed. But I can hardly explain what I mean, & you will perhaps wish my Barnacles & Species theory al Diabolo together. But I don’t care what you say, my species theory is all gospel.18
For some time, perhaps even since his reading of German comparative anatomists in Edinburgh, Darwin had been convinced that embryos – of barnacles, of humans, of lizards – were important keys to many of these zoological puzzles about form and function and evolutionary history. He was intrigued by the fact that barnacles and crustaceans repeated the same patterns of transformation in their life cycles.19 Just comparing their adult forms seemed impossibly and blindingly limited to him by now. The clues of their origins and ancestry in deep time were evident as much in their embryological forms as in these sedentary adults.
He wasn’t the first to believe so. In St Petersburg the Estonian biologist Karl Ernst von Baer, in 1848 the Professor of Comparative Anatomy and Physiology at the Medico-Surgical Academy of St Petersburg, had been dissecting embryos for decades with similar ideas in mind. Darwin read his work enthusiastically – the questions were familiar ones. In the 1820s von Baer had shown that within an animal group – say the vertebrates – the embryos of mammals, birds, reptiles, for instance, were virtually indistinguishable from each other. The embryo literally grew into its reptileness or birdness – a movement away from the general form to the specific form.20 This was how nature had worked since deep time: a process of differentiation and of separation, from the general to the particular. Von Baer also believed that there were points of comparison to be made between the embryos of higher forms and the adults of lower forms. The adult fish, for instance, resembles the human embryo. This is because the human form is more specialized than the fish, and has moved further away from the basic structural plan. Divergence is the key to adaptability. So it is not that the fish hasn’t caught up with the human yet, rather that it has reached the point at which its own body plan fits it perfectly for its mode of existence. For von Baer, the natural world was a combination of diversity rooted in an underlying unity of form. Nature didn’t travel in straight lines. It was a branching world of constant separation from a common starting point.21
In 1846, just as he set out on his barnacle journey, Darwin had read an important essay by the Professor of Natural History in Paris, Henri Milne Edwards.22 Like von Baer, Milne Edwards believed that embryos were the keys to understanding nature’s branching forms. Dissecting hundreds of crabs and lobsters in his anatomy theatres in Paris, he argued that the changes that crustaceans make in the course of their life cycle take them further and further from the general characteristics of the group (the embryo stage) towards their distinctive lobsterness or crabness. So one must look to the lobster embryo to discover the general characteristics of vertebrates and look to the adult lobster to determine the common features of the species. By looking at the path from embryo to adult, Milne Edwards argued, it was possible to see that nature was not making straight lines from lower to higher animals but was more like ‘a tree which in rising from the ground separates into several stems each of which it then divides into secondary main branches and terminates in innumerable little branches; but like the leaves with which a tree is covered, the species of animals thus produced can never, without flagrant violation of their natural relationships, be ranged in a single line’.23
Everyone seemed to be talking about archetypes in the 1840s and Darwin was no exception. An archetype was the basic body plan that could be seen underpinning all crustaceans or all barnacles, the plan against which their variation could be measured. So, for instance, a crustacean had twenty-one body parts, according to Milne Edwards, and Darwin discovered that barnacles had seventeen of these. This was evidence that barnacles were a sub-group of crustaceans. Whenever he dissected a new barnacle he would check off the seventeen body parts; the mouth, for instance, would be small in some barnacles, large and elongated in others and virtually non-existent in the complemental males in which the mouth had aborted through non-use, but the seventeen body parts made up the archetype of the barnacle.
Zoologists did not agree on their interpretation of these archetypes and they used the notion of archetypes to argue for different things. For Richard Owen, these shadows and doublings and echoes between different species in adult and embryo forms could only be evidence of God, a divine architect who had designed all living forms from a single ideal pattern and he, Owen, was busy trying to discover and define that pattern. The idea that diversification had been brought about simply by progressive adaptation to environment was nonsense to him.
What could the barnacles tell Darwin about the beginnings of invertebrateness, or, for that matter, the beginnings of maleness? Darwin was finding that there were some fleshy discoveries that he could only share enthusiastically with a male friend, because their implications were both comic and chilling. The barnacle males of the Ibla genus, he wrote to Hooker in October, were even more grotesque and inventive than Mr Arthrobalanus. Their life cycle was entirely dedicated to reproduction: they had absolutely no other function or way of experiencing the world; at their prime they were ‘mere bags of spermatozoa’, in Darwin’s words.24 Once they had fertilized the giant female, they dropped away, to be replaced by other males. He wrote to Hooker, who was now in Darjeeling in the Himalayas collecting rare orchids, to tell him more about the males he had found:
I am glad to hear that you are struck with my case of the Supplemental males: I have lately reworked them most carefully. They have no mouth or stomach, but the natatory larva or rather pupa (for the larva in 2d stage in no cirripede, I find, has a mouth) fixes itself on the hermaphrodite, develops itself into a great testis! & then dies & is succeeded by a fresh crop of these temporary Supplemental males. I have caught one lately at right epoch & its entire contents were a great sperm-receptacle full of perfect zoosperms.25
Just as the older males dropped away, so they were replaced by a fresh crop of supplemental males. Nothing is indispensable: just as one creature dies, it is replaced by another, and for Darwin, visiting a dying Either, this was not an easy reflection. The massive Dr Darwin, six feet four, twenty-five stone and seemingly indestructible, a rock upon which a family and whole community depended for medical and financial support, advice and investment, was dying from an illness that swelled him up and brought out boils on his body as Darwin worked on this parasitic Ibla. Obese and immobile, Dr Darwin couldn’t turn himself over in bed and was confined to his bed or to a wheelchair, sitting for most days in the hothouse in his home in Shrewsbury, cared for by his daughters, Susan and Catherine. Darwin visited him in October and, suddenly aware of the closeness of his father’s death, he wrote to the heavily pregnant Emma: ‘Oh Mammy I do long to be with you & under your protection for then I feel safe.’26
Baby Francis was now due any day. At almost full term, his body had twisted and turned upside down in the increasingly cramped space of his mother’s womb, his wrinkled feet pressing on the springy walls of the uterus, his head fixed deep in Emma’s pelvis, his legs curled upwards, tucked under her ribs; he was fixed now, embedded in maternal flesh, waiting.
Darwin, as the months of the summer passed relentlessly onwards towards his father’s death and his child’s birth, was plagued with illness, digestive problems, ‘boils & swellings’ which, in the months after his father’s death, would become very acute indeed. His father was dropping away – how much longer before his time was up?
Parslow continued to bring him his daily cop
y of The Times newspaper with its alarming reports of continuing waves of European revolution. Darwin, though concerned primarily at present with the mutants pressed between his glass microscope slides, turned his mind to politics briefly and ordered Louis Their’s History of the French Revolution (1838) but discarded it as ‘dull & poor’ in comparison to Thomas Carlyle’s three-volume account, which he had read some years earlier. Emma read him Currer Bell’s Jane Eyre (1847) that summer and then Mary Wollstonecraft’s Vindication of the Rights of Women (1792), both of which made impassioned post-revolutionary pleas for the greater independence of women.27
Darwin and his family, like thousands of middle-class and landed gentry across the country, had steeled themselves for the impact of the revolutionary tide, but it had been felt only as the merest ripple in Britain – some rioting in Edinburgh and in London; even the Chartist demonstrations on Kennington Common on 10 April had passed peaceably enough. The Irish people, feared in England as another source of revolution, were weakened by famine.
The British government, committed to gradual political and social reform, played a non-interventionist waiting game.28 In Europe, however, the violence and revolution continued as the balance of power shifted back away from the revolutionaries. In May, Piedmont declared war on Austria, demanding independence from the Austrian Empire. In June, a hundred thousand Parisian workers, disillusioned with the ineffectiveness of the provisional government, took to the streets of Paris and were either killed, arrested or deported when government troops were ordered to suppress the uprising. In July, the Austrians defeated the Piedmontese, marched into Milan, and regained control of Vienna in October. The tide had turned.