It was time to get more quantitative. He selected 11 castings on a hillside, and carefully measured their length in the direction of the slope (average: 2.03 inches). He then took a sharp knife and methodically cut each in two right across the mouth of the worm tunnel. Weighing the above-burrow and below-burrow portions, he found that the average weight of those occurring above (upslope) of the tunnels was 103 grains (1 grain = 64.8 mg), while the average weight of the downslope portion was twice that, at 205 grains. The idea was that fresh castings, while still a bit wet and fluid, slump a bit on the downslope side owing to gravity. George the mathematician then took over the calculations. Given this finding that ⅔ of the cast soil is found on the downslope side of worms’ burrows and ⅓ on the upslope side, George extrapolated to calculate the weight of the soil ejected by worms annually flowing down a slope with a mean inclination of 9° 26’. To be conservative he halved the downslope ⅔ of the worm casts. Based on an average casting size of 2.03 inches and using Lucy’s Leith Hill data (7.45 pounds of earth brought to the surface annually per square yard), he estimated that each year 1.1 ounces of dry earth would flow across each linear yard running across the slope, or nearly 7 pounds across a 100-yard long line drawn across the slope.33 Rain and wind would eventually erode these hills, but the worms help out by substantially accelerating the process, it appeared. Buckland hit the nail on the head back in 1837 when he declared that Darwin’s worms were “a new Geological Power.”
The Archaeologist’s Friend
If the worms were the geologist’s friend by helping create sedimentary strata, they were also the archaeologist’s friend by helping preserve ancient remains, as with the first observation that drew Darwin into the world of worms: those buried flints in Uncle Jos’s fields. Over the next decade Darwin’s field-workers—most notably sons William, Frank, George, and Horace—visited site after archaeological site armed with trowels, probes, and measuring rules. The list of sites, from druidical monuments and Roman villas to ancient abbeys and grand old manor houses, was impressive: Wroxeter, Stonehenge, Chedworth, Chirencester, Brading, Silchester, the Cloisters of Neville, Beaulieu Abbey, Gravetye Manor, and more. He did his own archaeological work too. His appetite for fieldwork was whetted in June 1877 when, on a visit to Leith Hill, he found a pair of large worked stones at the bottom of a hill—the remains of an old lime kiln that had been torn down 35 years prior, it turned out. He hired a local to help dig them out so he could inspect the nature of the soil beneath. Emma looked after him, running out with an umbrella one day, and fretting in a letter to Etty that she thought he would get sunstroke the next. Then Emma excitedly announced that he had bigger archaeological fish to fry: they would be going to Stonehenge, the awe-inspiring 5,000-year-old monumental stone circle rising from the Salisbury Plain. “I am afraid it will half kill [Father]—two hours’ rail and a twenty-four mile drive—but he is bent on going, chiefly for the worms, but also he has always wished to see it.”34
Five years before, William had visited Stonehenge to measure the depth of the humus at different spots and assess how deeply some of the smaller fallen blocks had sunk. There was no getting under the gigantic blocks, but by digging along the edges of several and measuring with a trowel and skewer he was able to gather some useful data.
George met them at Salisbury Station the next day and off they went to the site by carriage. Emma told Etty, who was away with her husband on holiday in Germany, that they were met by Stonehenge’s owner, Sir Edward Antrobus, who was “agreeable to any amount of digging, but sometimes visitors came who were troublesome, and once a man came with a sledgehammer who was very difficult to manage. . . .‘That was English all over,’ said he.”35 Some things never change—relic hunters are as destructive today as they were in Darwin’s day, though not so much at Stonehenge anymore, now a UNESCO World Heritage site well safeguarded by English Heritage. Charles and George dug trenches near some of the larger fallen stone blocks, recording the depth of the topsoil. With a bubble-level and rod they measured the contours of the land around the huge embedded blocks, determined how deeply they were buried, and measured the depth of the narrow sloping shoulders of turf surrounding the blocks. They concluded that worms, not simply the blocks’ enormous weight, were responsible for the sinking of the stones.
Later that summer Darwin received exciting news from one of his correspondents, Thomas Henry Farrer, who had an estate called Abinger Hall in Surrey, not far from Leith Hill Place. Farrer had been in touch with Darwin since the late 1860s, at first regarding their mutual interest in orchids. Soon, Farrer was drawn into making observations on cross-pollination and, with Darwin’s encouragement, published on the subject. He even became a relation by marriage when, in 1873, he wed Emma’s niece Katherine Euphemia Wedgwood. (In a few years he was to become a double relation by marriage, when his daughter Ida married Horace.)
Section through a nearly 8-foot fallen stone at Stonehenge, measured by Charles Darwin and his son George in 1877, showing its burial depth (given as 4" below the general ground level, for scale) and the narrow sloping shoulders at the rock’s edges indicative of worm activity. From Darwin (1881), p. 156.
Hearing of Darwin’s recent interest in worms and ancient structures, Farrer invited him to his estate to inspect the excavation of a Roman villa that one of his farm hands had recently discovered in a field adjacent to the house. Charles and Emma spent most of August 1877 at Abinger. Darwin and Farrer measured the slope of the field, and each day Darwin climbed into the pits to take measurements on the depth of worm activity and the various strata of humus and underlying materials. They departed for home on the 20th, but Darwin, curious about just how many worms lived under the newly exposed ancient mosaic floor, asked Farrer if he would continue to make observations for him. Farrer kept up the good worm work, sending Darwin his “worm journal” the following month with counts of worm castings and burrows over the next month or so, destroying the castings and burrows each day to count the new ones. On September 22nd, George came from Cambridge to help Farrer: “43 holes: defaced all” reads the worm journal. Day to day, Farrer counted as many as 61 fresh burrow openings in the 14 x 9-foot space.36 Once back at Cambridge, Emma kept George up to date: “[Father] is highly pleased about the worms. T. H. [Farrer] writes him word today that he counted 40 worm holes thro’ the absolute wall after heavy rain—I think it will come to be a wonder why ruins do not sink more.”37 A wonder indeed. Using the Roman coins found at the site to peg an approximate age, with George’s help Darwin estimated the rate of soil deposition by the worms over the centuries at about an inch per dozen years—worms had slowly but surely buried the Roman structure.
In early September, soon after Charles and Emma returned home, Horace presented his father with a device he invented to measure precisely the rate of stone subsidence. The soon-to-be cofounder of the Cambridge Scientific Instrument Company was at his inventive best. He unveiled his “wormstone,” a precision micrometer attached to a millstone of known weight and dimensions, with a metal bar driven down to bedrock through the center hole. His father eagerly helped him select a site, close to the house under a large Spanish chestnut tree. Later, a second one was installed at Leith Hill Place. Horace recorded data with the wormstone long after his father’s death, until it was accidentally dismantled in 1896—no doubt to Horace’s chagrin. (A reconstruction can be seen in approximately the original location at Down House today.) The device was sensitive enough to record the expansion and contraction of the earth owing to rain and temperature, and Horace even quantified this by testing with “artificial rain” (watering) in one experiment. In 1901 he published his data from 1878 to 1896; taking seasonal and weather factors into account he found that the stone sank 2.22 mm per year in that period, a bit less than 1 inch in 10 years. His father’s data from an adjacent open field gave an estimated rate of 2.2 inches in 10 years, a difference that could be attributed to field conditions (Darwin later lamented siting it under the tree, where he began to
suspect were fewer worms than in open fields), and the smaller-sized objects Darwin had used in the field, which sink faster than large stones.
Randal Keynes explaining the wormstone to Addison (right) and Eli (left) Costa in 2011. Photograph by the author. Inset: Horace Darwin’s “wormstone” micrometer, from H. Darwin (1901), p. 255.
Darwin must have been tickled by his son’s ingenuity. That fall also brought good news twice over: Darwin was to be awarded an honorary degree from Cambridge, and the bachelor William suddenly announced his engagement to Sara Sedgwick, originally from the other Cambridge, in Massachusetts. Both happy events took place in November 1877. The conferral of Darwin’s honorary Doctor of Laws degree at Cambridge was met with much fanfare, including the appearance of a stuffed monkey in robes and mortar board dangled from the gallery by mischievous undergraduates. Still, Emma “felt very grand walking about with my LL.D. in his silk gown,” she wrote William with pride.38 William and Sara were married on the 29th. When Thomas Farrer sent a note of congratulations, Emma, helping with the correspondence as she often did, relayed that Charles joked that the cream of his letter lay in the P.S. he appended about “the beloved worms, and not in any such trifles as marrying, &c.”39 They really were beloved worms, and not just to Darwin. Writing to his niece Sophy Wedgwood a few years later Darwin mentioned her sister Lucy’s “well-known affection for worms.” “I am also becoming deeply attached to worms,” he owned.40
Earthworm Serenade
It may sound like Darwin worked on little else besides earthworms through the late 1870s, but in fact they were a sideline—his books on cross- and self-fertilization in plants and the different forms of flowers came out in 1876 and 1877, respectively, and he completed his manuscript for Power of Movement in Plants in the spring of 1880. He then began his book on worms in the fall of 1880, which would appear the following year as The Formation of Vegetable Mould, Through the Action of Worms, With Observations On Their Habits, usually abbreviated simply as Earthworms. It was to be Darwin’s last book.
The subject of the last part of his title, the habits of worms, was also his last set of investigations, and they were just gearing up in 1880. Since Lucy’s observations of her pebble-gathering worms, he was intrigued by their behavior. By then Lucy’s question about what they were doing had been answered: Darwin and others, having excavated more than a few worm domiciles, knew the annelids often lined their burrows with pebbles and leaves, creating a wormy version of a snug nest. They expertly plugged the burrow’s mouth with leaves as well, but would resort to using the pebbles for this too if no leaves were at hand, so to speak. No mean feat for an animal with no eyes, no limbs, and not much of a mouth. How did they manipulate items, and evaluate size, shape, and suitability for their purposes? Their ability to do these things convinced Darwin that earthworms possess une petit dose de raison—a small dose of reason, as he quoted the celebrated Swiss entomologist Pierre Huber. Anything bearing on the intelligence and curious behavior of worms were subjects of great import to him, so it’s unsurprising that he put his findings front and center in the opening two chapters of his book.
But first, he took to bringing his worms indoors, housing them in glass-covered flower pots, eventually converting the billiards room into a wormery. They were probably Lumbricus terrestris, the common garden earthworm. Frank was his right-hand experimentiser now. As previously mentioned, Frank’s wife Amy had died in childbirth in 1876, and he moved in at Down. Their son, Bernard, survived, and was doted on by his grandparents—and was an occasional lab hand for worm experiments. The Darwins, father, son, and sometimes grandson, would sneak up on the worms to see what they were up to in their wormeries. The worms often lolled about on the surface, but always with the tip of their tail just inside their burrows, ever-ready to beat a hasty retreat. What were their daily rhythms? And what of their sensory world—they have no eyes, but can they see? The Darwins tested the worms’ reaction to bright light, dim light, sudden flashes of light, even colored light using red and blue glass filters that Leo procured for them. The worms proved indifferent to all except bright flashes, which induced them to zip into their burrows, “like a rabbit,” Bernard marveled.41 Darwin concluded that their thin skin admitted enough light to be perceived directly by their nervous system.
What other sense perceptions do worms have? The Darwins tested various stimuli. Radiant heat? Darwin waved a red-hot glowing poker over their pots, but the response varied. They reacted more decisively to concentrated heat from candlelight focused with a lens. Air movement? Gentle breathing had no effect, but strong puffs got the worms’ attention. What of odors? Darwin stuck cotton balls soaked in vinegar, perfume, or tobacco juice in his mouth (not a Darwin experiment advisable to replicate) and let the odors waft over the pots as he breathed on them. The worms were unfazed. Could they hear? Darwin resolved to serenade the earthworms to find out. The assembled musicians included 5-year-old Bernard on penny whistle, Frank on bassoon, and the accomplished pianist Emma on piano. It was a natural test to try, but may have been partly inspired by somewhat similar experiments carried out with ants a couple of years earlier by his neighbor and sometime protégé, John Lubbock, with Darwin’s cousin Francis Galton. Lubbock and Galton used a whistle that could produce very high-pitched sounds with forced oxygen or hydrogen, thinking maybe insects hear only ultrahigh frequencies. Having befriended Alexander Graham Bell, the following year Lubbock even borrowed a prototype telephone to test the ants’ ability to communicate via sound, holding a receiver and mouthpiece between different colonies in a nearby field. They riled up one colony to see if sounds transmitted by phone would agitate the other. In a notebook entry for November 24, 1879, Lubbock recorded that he made “some experiments on Ants with the telephone, but they had no result.”42 Darwin doesn’t appear to have tried the telephone, but in any case the worms seemed as indifferent to sound as the ants, being wholly unappreciative of the Down House Trio:
Worms do not possess any sense of hearing. They took not the least notice of the shrill notes from a metal whistle, which was repeatedly sounded near them; nor did they of the deepest and loudest tones of a bassoon. They were indifferent to shouts, if care was taken that the breath did not strike them. When placed on a table close to the keys of a piano, which was played as loudly as possible, they remained perfectly quiet.43
However, Darwin noted, when pots with two worms that had been “quite indifferent” to Emma’s playing were placed directly on the piano, they reacted: when “the note C in the bass clef was struck, both instantly retreated into their burrows,” he recorded. “After a time they emerged, and when G above the line in the treble clef was struck they again retreated.” It was the vibration, of course. He experimented further with the piano, creeping into the drawing room at night and suddenly striking a high note; one worm “dashed into its burrow” but another stayed until “C in the treble clef” was played. Darwin mentioned only these specific notes: did he try the entire scale? Or would they react to any note being played resonantly enough to send vibrations through their wormery? He noted that the worms were not touching the sides of the pots, so they were sensitive to dampened vibration coming through the soil.44
Although not all worm species show vibrational sensitivity, the phenomenon is widespread. In the southern United States there is a long tradition of “worm grunting” for fish-bait, inducing Diplocardia earthworms to surface by sending vibrations through the soil using a wooden stake driven into the ground, over which another object, often metal, is stroked rhythmically back and forth. As for why the worms surface in response to vibration, there are various hypotheses. Darwin thought that it might be to escape from burrowing predators like moles, which send telltale vibrations through the soil as they tunnel, alerting the worms to their approach. If so, there may be a cruel irony in the way some other predators apparently use the vibrational flight response of worms to their own advantage, including foot-stomping herring gulls (a behavior described by the distinguished e
thologist Niko Tinbergen in 1960) and wood turtles (described in the 1980s) that induce worms to surface to their doom.
These are adaptations that would have impressed but not surprised Darwin. What did surprise him was the intelligence of worms. Who would have thought that these humble creatures have much of a brain? Then again, he reflected, consider the remarkable repertoire of ants and bees, with a far smaller brain. But how much is instinct and how much “reason” with the worms? He tested their problem-solving abilities, giving them access to variously shaped leaves and other materials to see how and what they select for their burrows. There seemed to be an urgency with the worms in lining and, perhaps more importantly, plugging their burrows—maybe to keep predators or parasites out. They are choosy, and carefully select and position materials they deem suitable. In one slightly mean investigation he pinned the leaves to the soil of their pots, allowing him to make extended observations as the worms seemed to puzzle and struggle to move the leaves without success. They grabbed leaves with their mouths, but would latch on here and there as they maneuvered objects into place. It’s astounding, if you think about it—not seemingly trial and error, but rather more deliberate in grabbing hold after feeling the leaves all over. He made observations of the kinds of leaves found in burrows in the vicinity of the house, discovering that worms loved the lime tree leaves by the veranda, but were not as keen on rhododendron leaves. Long, narrow objects would be retrieved as well: they readily drew in pine needles and thin leaf petioles. Fixated, he set out to test further.
Commandeering some of Bernard’s coloring paper, Darwin cut out paper diamonds, triangles, and discs, varying the size and the acuteness of the angles, and then sprinkled them on the soil surface in the pots for the worms to evaluate. “The worms are drawing in my triangular card nicely,” he reported in one letter to William; “but enough have not yet been drawn in for any conclusion. I think that I shall try feathers.”45 Feathers were not a success, and he stuck with the paper cutouts. He could not only tell which shapes the worms preferred, but by excavating their paper-lined burrows he could also see the orientation of the cutouts. Most had been pulled in by their narrowest, most pointed ends—easiest to grab hold of, he surmised, but they managed to draw some of them in by the middle too. Flexible leaves were often drawn in from the center of the midrib, neatly folding each leaf as it was pulled into the burrow, making a more effective plug. Darwin was sure that the worms somehow acquire a “notion” of shape, most likely by feeling the objects all over. He likened them to a blind person with a heightened tactile sense, and felt they had some learning ability. Curious about their food preferences, he also did taste tests: cabbage, turnip, and onion leaves were worm favorites, but they weren’t keen on kitchen herbs. Darwin came to appreciate that his worms had personality, describing how they enjoy “the pleasure of eating” (based on their eagerness for certain leaves) and their sexual passions (“strong enough to overcome . . . their dread of light”), and even commenting on their feelings: “They perhaps have a trace of social feeling, for they are not disturbed by crawling over each other’s bodies, and they sometimes lie in contact.” He cited Werner Hoffmeister’s 1845 account of worms passing the winter “either singly or rolled up with others into a ball at the bottom of their burrows”—an incongruous image, perhaps, of cuddly worms snugly coiled in their burrows together.46
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