by Janet Browne
At the same time, the firm of Smith & Elder, which still held the copyright to his early geological books, encouraged Darwin in 1874 to bring out a revised version of Coral Reefs, taking account of James Dana’s theories of reef formation and Alexander Agassiz’s remarks. They also wished to produce an updated version of Geological Observations on the Volcanic Islands and Parts of South America in 1876. These books had been his first scientific writings in the post-Beagle years. To him it seemed as if he was turning a full circle. Without hesitation, he agreed. He had been proud of them once, and was still.
If he had paused to consider the matter, Darwin might have quailed at such a punishing self-imposed schedule. Every year from 1860 onwards he had published at least one book, sometimes two, either a new title or a revised edition, and since that date a total of some fifty-five articles serious enough to be listed in the Royal Society Catalogue of Scientific Papers. He persisted in this regime until his death, except for five individual years when he was either too ill or too busy collecting facts. Folder after folder of notes piled up as he worked his way through these publishing obligations. “I have no news about myself, as I am merely slaving over the sickening work of preparing new editions,” he moaned gently. “I wish I could get a touch of poor Lyell’s feelings, that it was delightful to improve a sentence, like a painter improving a picture.”2
In between he rewarded himself with a remarkable array of botanical projects. Where he had once used plants simply to explore evolutionary theory and as a pleasant mental diversion, he now reversed the process and made the theory serve as a powerful engine to break into the secrets of physiology. He shifted gears. He desired the real work of day-to-day experimental investigation, not so much the search for so-called “crucial” experiments to which individuals might give their name but the patient probing of nature’s mechanisms with no particular way of telling where any line of exploration would end up, the unsung feature of a scientist’s life that all researchers know to be authentic. He wanted to pick up problems as he went along, to use the techniques he understood best, to out-think the ingenuity of living organisms.
He was at long last able to give insectivorous plants his full attention, asking Hooker and Gray, and then an army of correspondents, to send specimens to Down so that he could feed them increasingly unusual diets and observe their cellular activities under his microscope. No project in this sense was ever finished. Darwin had waited for ten years or more for this treat. He spent hours studying the leaves and trap mechanisms, still unsure exactly how the plants could digest insects. With Francis, well versed in the new physiology, at his side, he started a major research programme limited only by the ad hoc nature of his household resources. He had to write apologetically to Hooker to say that Lettington had destroyed a prime specimen from Kew. “I am very much vexed.… I am sure it was an oversight of Lettington’s & not carelessness, as he was very proud of the state of D. capensis.”
He bought himself a bigger and better microscope, a Smith and Beck compound-lens apparatus, so that he could study the movements of protoplasm inside cells. Hooker surprised him by saying that the accepted wisdom of the day was that plant leaves could not absorb any substances. Absorption was a function confined to roots alone. “Have you tried Begonia leaves?” Hooker asked. “Shall I look out for you some plants with hyaline bladdery epidermal cells for you to operate on?” Puzzled, the two old friends consulted William Thiselton Dyer, a younger man who had trained as a physiologist with Foster and Huxley and was assistant director at Kew (soon to become Hooker’s son-in-law, too), much closer to the cutting edge of physiological science than either of them could pretend to be. In his spare time Dyer was translating Julius Sachs’s important work on plant physiology from German into English. Dyer “would be up to the latest discoveries,” Hooker assured Darwin.3
And he enjoyed the macabre ingenuity of the plants themselves: the traps, lures, and snares that they devised to capture their prey. The sticky-leaved sundew “is a wonderful plant, or rather a most sagacious animal.” A deep-seated appreciation of the sundew’s murderous course in life formed the central theme of his inquiries. In this he was helped immeasurably by Edward Frankland of the Royal College of Chemistry, in London, and John Burdon Sanderson, the new professor of physiology at University College London, who both answered his inquiries patiently. When one of Darwin’s homespun experiments proved too tricky to handle in his kitchen, Frankland volunteered to macerate Drosera leaves in his laboratory and analyse the juices for traces of pepsin and hydrochloric acid, the principal digestive acids of animals.
John Burdon Sanderson had studied under Claude Bernard and then succeeded Michael Foster in London University in 1870.4 At this point he was beginning his life’s work on the fundamental properties of living tissue, and his particular concern was the way muscles work. To that end he specialised in identifying the small electrical changes that took place during contraction and relaxation. He also interested himself in histology and cellular pathology, especially the structure of blood corpuscles. All this was music to Darwin’s ears. In June 1873 he contacted Burdon Sanderson (through George Romanes, who worked in the same laboratory) to ask if he would look over his investigations into the cellular activities of Drosera. Darwin was still bothered by the process he called protoplasmic “clumping” or “aggregation” inside the hair cells when the leaves were stimulated into feeding. Hooker told him that chlorophyll in plant cells did the same thing when exposed to sunlight. Darwin’s new microscope, with the better magnification, told him only that the process remained incomprehensible.
Burdon Sanderson was not at first convinced that the plants could even move—Darwin could tell that he thought he was imagining the whole thing. So Darwin set out to demonstrate the sensitiveness of Drosera to both Burdon Sanderson and Huxley, asking them to visit Down House. They peered at one of Darwin’s Drosera specimens until “Mr. Huxley cried out, ‘It is moving!’ ”5 Burdon Sanderson agreed to make the experiments. Within the month Darwin also persuaded him to investigate the movements of a Venus’s fly-trap. Could it be possible, he asked, that the leaves might possess some botanical equivalent to nerves and muscles? So Burdon Sanderson attached his electric probes to individual fly-trap leaves and stimulated them with minute bursts of current. The leaves responded “like animal muscle,” he reported at a meeting of the British Association in 1873. It had been a stunning example of laboratory dexterity. Moreover, this announcement was a revelation to the experimental physiologists in the audience who scarcely gave a thought to plants. “Not merely then are the phenomena of digestion in this wonderful plant like those of animals,” Hooker said admiringly, “but the phenomena of contractility agree with those of animals also.”6
Before long, Darwin told Thiselton Dyer he was “in that state in which I would sacrifice friend or foe.”
I fear that you will think me a great bore, but I cannot resist telling you that I have just found out that the leaves of Pinguicula possess a beautifully adapted power of movement. Last night I put on a row of little flies near one edge of two youngish leaves; and after 14 hours these edges are beautifully folded over so as to clasp the flies.… I have ascertained that bits of certain leaves, for instance spinach, excites so much secretion in Pinguicula, and that the glands absorb matter from the leaves.7
The whole business was a sensational story of surface innocence and hidden guile, just the kind of thing Darwin enjoyed in his recreational reading. Who could believe that the pretty common butterwort, with its golden star of leaves, had such a sinister nature? Or that under the quiet surface of a pond there were plants casting sticky nets, like Dictynna in Greek mythology, to trap their prey? Some plants specialised in drownings, others in knocking out victims with anaesthetics, still more in smothering or gluey entanglement.
Intrigued by his evident excitement, Darwin’s botanical friends rallied round. Lady Dorothy Nevill sent him an unusual bladderwort from her conservatory, a specimen water-plant that us
ually drifted on the surface of rivers in South America. Darwin had seen it only in dried form. When he and Francis opened up the bladders they found plenty of animal remains inside. A specimen of Genilisea ornata animated them even more. Nothing could escape from the trap it set, for the fly’s route to the death chamber was lined with a grid of spikes. Once inside, the victim could not retreat.
The great solid bladder-like swellings almost on the surface are wonderful objects, but are not the true bladders. These I found on the roots near the surface, and down to a depth of two inches in the sand.… I felt confident I should find captured prey. And so I have to my delight in two bladders, with clear proof that they had absorbed food from the decaying mass. For Utricularia is a carrion-feeder, and not strictly carnivorous like Drosera.… I have hardly ever enjoyed a day more in my life than I have this day’s work; and this I owe to your Ladyship’s great kindness.8
Messages across the globe rang with the same delighted claims. “Your magnificent present of Aldrovanda has arrived quite safe.… You are a good man to give me such pleasure.”
Amused by Darwin’s enthusiasms, Asa Gray sent a practical joke in the post—an article describing a carnivorous plant in Madagascar that subsisted on humans. Darwin confessed that he “began reading … quite gravely.” He did not perceive it was a hoax until he came to the woman who was lunch.
Early in 1875, Darwin felt ready to dispatch a manuscript on insectivorous plants to Murray. Inevitably he kept finding new things to observe. “You ask about my book,” he said to Hooker in February that year. “All I can say is that I am ready to commit suicide; I thought it was decently written but find so much wants rewriting that it will not be ready to go to printers for two months.” In May he was still covering his proof sheets with an inky forest of alterations. Faced with the crisis of placing actual words on paper, he decided that insectivorous plants must possess what he could only call “nervous matter,” analogous to animal nerves. He felt unable to describe this nervous matter further without the authority of hundreds of detailed experiments.
Emma despaired of getting him away for the customary break in London.
F. jibbs a good deal & I am afraid I shall not get him to move, at least not more than for 2 days at Q.A. [Queen Anne Street, Erasmus’s house] which he thinks might rest him, & so I dare say it would for a few days. However the proof sheets are coming in at a great rate.… F. has had prosperous news today from Dr B. Sanderson.… Lady Dorothy Nevill is coming to lunch on Tuesday! It is rather serious. F. will have to be so friendly & adoring (if possible).9
The book was nicely illustrated with diagrams by George and Francis and published by Murray in July 1875 under the title Insectivorous Plants. It proved far too specialised for a general audience and was not printed again while he lived. To a public accustomed to reading about apes and religious dissent, this Darwin seemed a very different author from the Darwin discussed in the newspapers.
II
At the same time, Darwin initiated another burst of investigation into orchids. “They are wonderful creatures, these Orchids,” he told Bentham. “I sometimes think with a glow of pleasure, when I remember making out some little point in their method of fertilisation.”10
And he plunged again into a full reevaluation and extension of his earlier work on cross-fertilisation and self-fertilisation in plants. This was perhaps something more than his usual urge to complete projects left dangling many years before. “I cannot endure doing nothing,” he told Jenyns in 1877. It was almost as if he feared the moment when his mind might be empty, when his work might be done; and to stave off this abyss constantly found old and new topics to pursue. If not dread of idleness, then dread of decrepitude. He often said that his work made him feel alive, helped his mind sing, was the one thing that blotted out his cares. Although he called himself a “a kind of machine for grinding general laws out of a large collection of facts,” the truth was he only felt himself when immersed in some demanding new project.
Francis Darwin helped with the researches. Later, Francis explained that his father had been driven by the idea that cross-fertilised plants would produce offspring that would be more successful in the competition for survival, and any mechanisms developed for ensuring the transfer of pollen from flower to flower therefore gave the species a significant adaptive advantage. This conviction had formed part of Darwin’s belief in evolution by natural selection for more than thirty years. The whole of the living kingdom, as he regarded it, was dedicated to ensuring sexual reproduction—man and woman, flower, and beast. Now, in particular, he wished to probe more deeply into aspects of plant fertilisation, heredity, and sterility.
Underneath, it seems entirely possible that Darwin was also becoming anxious about his own family’s reproductive future and that his botanical experiments echoed this concern. His seven children were grown up. William, the oldest, was thirty-five in 1874. Horace, the youngest, was twenty-three. Five of these children were as yet unmarried, and the two that were married, Henrietta and Francis, were childless. Even leaving aside any worries he might have felt over hereditary malaise, he probably wondered whether his own family was to prove a biological cul-de-sac. For a man whose intellectual life was structured around reproductive success, it must have been disturbing to consider the possibility of having no issue. His children were not doing what came naturally to most living beings. In biological terms, after all, he was convinced that reproduction kept species actively evolving. Self-fertilising organisms were probably on the road to extinction. Francis Darwin recorded that his father “once remarked to Dr. Norman Moore that one of the things that made him wish to live a few thousand years, was his desire to see the extinction of the Bee-orchis—an end to which he believed its self-fertilising habit was leading.”11 To muse about grandchildren and to investigate the origins of incipient sterility in plants or the advantages of cross-fertilisation were topics likely to resonate at a fundamental level with his private existence.
Once more, he lay in wait in the flower beds to observe bees pushing their way into a nectary, in search of the co-adaptations that had grown up between flowers and their insect pollinators. Once more, he isolated experimental plants under billowing nets of gauze and delicately pollinated them by hand with a camel-hair brush. As the summers passed, he sowed trays of compost with the seeds of particular crosses and counted seedlings. “I have taken every kind of precaution,” he assured Gray, telling him how he germinated plants in a pot on his chimney-piece to avoid contamination. Throughout, he intended to demonstrate that the offspring of out-crossing individuals were more vigorous, and more numerous, than the offspring of self-fertilised plants. “Nothing in my life has ever interested me more than the fertilisation of such plants as Primula and Lythrum, or again Anacamptis or Listera,” he told Hermann Muller in 1878.12
Actually, the investigation was tedious in the extreme and revealed Darwin at his most patient, foot-slogging best. “It is remarkable,” reminisced Francis, that this work “owed its origin to a chance observation.”
My father had raised two beds of Linaria vulgaris—one set being the offspring of cross- and the other of self-fertilisation. These plants were grown for the sake of some observations on inheritance, and not with any view to cross-breeding, and he was astonished to observe that the offspring of self-fertilisation were clearly less vigorous than the others. It seemed incredible to him that this result could be due to a single act of self-fertilisation, and it was only in the following year, when precisely the same result occurred in the case of a similar experiment on inheritance in Carnations, that his attention was “throughly aroused,” and that he determined to make a series of experiments specially directed to the question.13
“I am experimenting on a very large scale,” he confessed to Bentham on another occasion. “I always supposed until lately that no evil effects would be visible until after several generations of self-fertilisation; but now I see that one generation sometimes suffices.” If vigour could be l
ost in the course of one generation, the chances of survival were correspondingly reduced.
Principally he recorded “vigour” by measuring the height of the seedlings. “Lyell, Huxley and Hooker have seen some of my plants, and have been astonished.” But Wallace told him, too late, that weight would be a much better criterion than height—the total weight of seed produced by a plant was probably a better indicator of productivity, he said. So Darwin repeated the experiments the following year, this time calculating weights. He germinated the seedlings all over again, he labelled ripening seed-heads and then weighed the seeds in packets, one for each plant, on his old chemical balance. For a change, he tried planting some of the seedlings outside in the autumn (a tough test for the greenhouse varieties) and counted the ones that made it through the winter. After a good hard frost one Christmas morning, he was there in the garden with his notebook taking a roll-call of the survivors. As he hoped, he did ultimately substantiate his point. The crossed offspring were more vigorous than the self-fertilised.
Father and son found these days together rewarding. They sat side by side with their microscopes and plant trays, exchanging comments as necessary. Francis said that Darwin’s manner was bright and animated.
His love of each particular experiment, and his eager zeal not to lose the fruit of it, came out markedly in these crossing experiments—in the elaborate care he took not to make any confusion in putting capsules into wrong trays, &c. &c. I can recall his appearance as he counted seeds under the simple microscope with an alertness not usually characterising such mechanical work as counting. I think he personified each seed as a small demon trying to elude him by getting into the wrong heap, or jumping away altogether; and this gave to the work the excitement of a game.14