His “Queries About Expression” consisted of 17 questions, remarkable for their scope and detail: Is astonishment expressed by the eyes and mouth being opened wide, and by the eyebrows being raised? When low in spirits, are the corners of the mouth depressed? Do the children pout when sulky? How is fear expressed? It covered the shrugging of shoulders, nodding of the head, uplifted palms and raised eyebrows. Darwin would not only seek to establish a universal bond among humans but throughout the animal kingdom, as reflected in the title of his eventual book on the subject: Expression of the Emotions in Man and Animals (1872). The Grays furnished detailed observations from Italy and Egypt, sent near the end of their tour in May of 1869. Jane even made observations of Renaissance paintings while she was at it: “I thought you would have been interested in seeing an old picture here of Fra Angelico’s of the deposition from the cross,” she wrote Darwin from Florence; “The Madonna has the distress muscles very carefully painted.”15
Jane Gray’s letter from Italy was written on the very same day—May 8, 1869—as another letter that brought earthworms to the surface of Darwin’s mind once again. Gardener and horticultural writer David Taylor Fish had reservations about Darwin’s theory of the efficacy of worms in generating “vegetable mould”—what we call topsoil. He thought topsoil was simply decayed plant matter, and published an open letter in the Gardeners’ Chronicle saying so. Surely so much plant matter decomposing “annually, and hourly, in fact, over the surface of the world” provides “sufficient energy, force, and persistency to cover the earth with vegetable mould.” This, Fish asserted, “would account for all the elevations of surface so graphically described in the admirable paper of Mr. Darwin.” Furthermore, he wrote the editor, he was sure that “it would be gratifying to many of your readers to know if this distinguished geologist [yes, geologist] still adheres to his original estimate of the usefulness and power of worms as earth-lifters and surface-elevators.” You bet he did. Darwin immediately wrote the magazine: “As Mr. Fish asks me in so obliging a manner whether I continue of the same opinion as formerly in regard to the efficiency of worms in bringing up within their intestines fine soil from below, I must answer in the affirmative.” Here was an opportunity to provide some updated information: he had not made actual measurements of late, he said, but had
watched during the last 25 years the gradual, and at last complete, disappearance of innumerable large flints on the surface of a field with very poor soil after it had been laid down as pasture. I have also purposely covered a few yards square of a grass-field with fine chalk, so as to observe the worms burrowing up through it, and leaving their castings on the surface, which were soon spread out by the rain. The Regent’s Park in early autumn is a capital place to observe the wonderful amount of work effected under favourable circumstances by worms, even in the course of a week or two.16
Regent’s Park, home of Emma’s sister, was regularly visited by the Darwins, so his mention of the locale here underscores his incessant observing. You can imagine him stealing away for solitary walks in the park, or perhaps taking after-dinner strolls arm-in-arm with Emma, to admire the industrious worms of Regent’s Park. His observations told him that Fish could not be correct. Judging from how quickly he observed the surface to become covered with fine soil, he reasoned that if mere decay of grass was responsible, soil many feet in thickness would form in the course of just a few centuries. On the contrary, “In ordinary soils the worms do not burrow down to great depths, consequently fine vegetable soil is not accumulated to any inordinate thickness.”17
Rising to the defense of worms that May was probably a welcome diversion for Darwin: the year already had had its ups and downs, and it wasn’t even half over. For starters it took him a month and a half to complete the editing for the latest (fifth) edition of his “everlasting old Origin”; some of the continuing comments and criticisms were well taken while others were maddeningly out of left field. “I begin to understand your sufferings over the Origin,” Huxley wrote him on March 17th; “a good book is comparable to a piece of meat & fools are as flies, who swarm to it, each for the purpose of depositing and hatching his own particular maggot of an idea.”18 Huxley did not suffer fools gladly, but Darwin acknowledged that some of his critics, notably Swiss botanist Carl Nägeli and Scottish engineer Fleeming Jenkin, made good points that he had to address.
That same month, too, Wallace managed to elicit both deep gratitude and profound shock and disappointment in Darwin: he might have made himself a case study for his own Queries on Expression. Wallace’s book The Malay Archipelago came out in early March to much acclaim, bearing a warm and heartfelt dedication to Darwin. “The dedication is a thing for my children’s children to be proud of,” Darwin wrote appreciatively to Wallace.19 But Wallace soon wrote him a letter that mentioned, incidentally, that he would soon come out with an article of which he feared Darwin would disapprove. Darwin was apprehensive. Wallace’s bombshell of a paper, dropped just a few weeks later, included an about-face over human evolution, arguing that natural selection could not explain the evolution of the human brain after all. Late March 1869 found a shocked and dismayed Darwin bitterly declaring to Wallace that he hoped his friend had not “murdered too completely your own & my child.” He followed up with another letter, still upset: “As you expected I differ grievously from you, & I am very sorry for it.”20 Right about then Darwin was thrown from his horse, injuring his back and leg. While incapacitated, Horace helpfully pulled him along on a wagon so he could make the rounds of his botanical observations in the greenhouse. He was still not quite recovered by June, and he and Emma decided it was time to get away. They headed to northern Wales, spending most of June and all of July near Barmouth, amid the salubrious mountain scenery and atmosphere. While there they visited with family friends Lawrence and Mary Ann Ruck, the parents of Frank’s future wife Amy, who would become another one of Darwin’s earthworm field assistants. At the moment, however, her interest lay more with Horace than Frank.
Wales did Darwin some good, but he was slow to recover. He wrote Wallace about his condition, their friendship strained but still strong: “I can say nothing good about my health, & am so weak that I can hardly crawl half a mile from the House . . . Oh dear what [would] I not give for a little more strength to get on with my work.”21 Somehow he found the strength, and then some: all through 1870 he worked at Descent, which came out at last in February 1871. That January he started on Expression and, in a tremendous burst of writing, finished the manuscript in April . . . only to take on the editing for what he resolved would be the final edition of the Origin. That project took another half year, punctuated by visits with family in London and Southampton, and the happy occasion of Etty’s marriage to Richard Litchfield in August of 1871.
The Earthworm Returneth
Earthworms cropped up again in 1870 and 1871, this time with more staying power. Visiting his sister’s family at Leith Hill Place in October of 1870, Darwin and Lucy, his ever-ready research assistant, got to speculating on the vast amount of earth the worms must bring to the surface annually. They had an idea: why not quantify it? They soon set up two square-yard plots, one in a partially shaded grassy spot near her home, and the other on unenclosed common land near Leith Hill Tower (an eighteenth-century Gothic-style landmark atop Leith Hill). The plan was for Lucy to collect worm castings over the course of a year, and they would weigh them at the end. She was diligent in her collecting, and when the Darwins came for a visit the following November she had two bags full of castings for her uncle. When he got home Darwin excitedly dried the castings and wasted no time getting hold of the most accurate scale he could find, which he borrowed from the local pharmacist.22 The more shady plot yielded exactly 3.5 pounds of castings, scaling up to 7.56 tons of soil per acre, while the open hilltop one yielded 7.45 pounds for a whopping 16.1 tons per acre, close to the quantity he had guessed for the site. That’s a remarkable quantity of earth moved annually by such humble organisms. Darwin sent
the results to a delighted Lucy. “My dear Uncle Charles,” she replied, “Thank you very much indeed for sending me such a full account of the worm castings which I was deeply interested to read. What a wonderfully accurate guess of yours about the amount! The 16 tons an acre is the most astonishing part.” (The rest of her letter, true to form, concerns observations she was just making on male turkey displays.)23
Around this time Darwin resolved to produce a thorough study of the effects of worms, but he couldn’t do it alone, or quickly—carnivorous and climbing plants were competing for his attention at precisely this time, not to mention correcting page proofs for both the sixth edition of the Origin and his new book Expression. When he wrote Gray for information about North American worms, his friend admonished him: “Now, pray don’t run off on some other track till you have worked out and published about Drosera and Dionaea”24—the carnivorous plants he had been working on since 1860 (see Chapter 7). Yes, he knew he had to buckle down, but he had a hard time working on just one project at a time, and the worms were growing more insistent. He took a page from their play book, a uniformitarian strategy that Lyell would appreciate: marshal his field assistants, enlist friends, family, and colleagues, and gather data little by little. As with the worms’ castings, it adds up.
Darwin—and what became a small army of assistants and informants—proceeded with worms on several fronts through the 1870s. As with his Queries on Expression, Darwin drafted instructions to distribute among his contacts explaining what he was looking for. He wanted to extend his analysis of the quantity of soil brought to the surface by worms, document their effects in terms of the burial of objects and landform erosion, and, more and more, he wanted to better understand their mental qualities.
On the erosion front, he sought help from far and near. He wrote Archibald Geikie, newly appointed Professor of Geology and Mineralogy at the University of Edinburgh, who not long before had written on “denudation,” the nineteenth-century term for erosion. He had to first acknowledge his Glen Roy blunder of some 30 years prior, when he assumed that turf-covered slopes were resistant to erosion even over thousands of years. “This I now see as a complete error,” he admitted.25 Not only was such erosion possible, but it was also aided by worms. Based on the fineness of the mineral soil defecated by the worms, Darwin was sure that they played a role in breaking down bedrock, not by directly attacking it, but by ingesting and grinding down small stone particles. It was well known even then that worms, like certain birds, have muscular gizzards in their alimentary tract that function to mechanically break down tough food items as an aid to digestion. They augment the action of the muscle by ingesting small stones as grinders. Naturally occurring acids from decaying vegetation help break down the rock substrate, but once rock particles reached a certain size the earthworms could ingest them along with soil, further wearing them down.
The end result, literally and figuratively, was very finely triturated mineral soil mixed with organic waste. Geikie agreed, and pointed out that his fellow Scot John Playfair, the eighteenth-century Edinburgh geologist who communicated James Hutton’s uniformitarianism to the geological world, had written that the permanence of the soil furnishes “a demonstrative proof of the continual destruction of the rocks.”26 His idea was that despite continuous erosion by rivers and streams the soil seems never to diminish, and concluded that this was because it was being constantly renewed by the disintegration of bedrock. The role that worms might play in this process was unappreciated until Darwin posited it. Darwin related his and Lucy’s worm-casting study to Geikie, and his interest in the persistence of ridges and furrows in old pastures plowed in the distant past. He enclosed a copy of his questionnaire, worth reproducing here in full to appreciate how minutely he was thinking about the action of worms:27
I have seen (some 45 years ago!) in North Wales land, showing by ridges & furrows that it had been ploughed apparently centuries ago. Do such exist in your neighbourhood; or can you find a field with ridges & furrows which has been laid down in grass for about half a century or more. In all such cases if the surface slopes I want much to learn whether the ridges & furrows run straight or obliquely down the slope, or transversely across it. In the former case it [would] be of the greatest service to me if you [would] observe whether the ridges & furrows are as distinct near the base of the slope as at its top. By far the best way to observe [would] be to stretch a string in 2 or 3 places from ridge to ridge & measure the depths of the furrow in the upper & in the lower part of the slope. If the ridges & furrows runs transversely across the slope, the only point to observe [would] be whether the two slopes of the ridges have become different to what they are in a recent stubble field.— In any field observed please to notice whether there are many worm-castings about.—
My object is to trace the manner of gradual obliteration of ridge & furrow in old pasture-land, as far as such obliteration is ever effected | C. Darwin
The Scotsman readily assisted, sending details of furrowed fields and old fortifications in the Edinburgh area. Closer to home Darwin’s sons George and Horace were enlisted. The two were at Trinity College, Cambridge, where George was a Fellow and Horace was a student. In early December the younger Darwin passed his first exam for the B.S. degree—the Litte Go as it was called—an event that occasioned some reflection on his father’s part. “I have been speculating last night,” Darwin mused in a congratulatory letter, “what makes a man a discoverer of undiscovered things, & a most perplexing problem it is.— Many men who are very clever—much cleverer than discoverers,—never originate anything. As far as I can conjecture, the art consists in habitually searching for the causes or meaning of everything which occurs. This implies sharp observation & requires as much knowledge as possible of the subjects investigated.”28 That’s the voice of experience speaking, though doggedly might have been a better term than “habitually” searching for causes or meaning, as persistence certainly was a hallmark of Darwin’s working method—and procuring assistance.
Just after Christmas, George and Horace made observations of a field near Biggin Hill, a couple of miles down the road from Down House (a locale later famous for its RAF air field and the heroic role it played in defending London during World War II). They measured the depth of soil in furrows and ridges running transversely across the hill by probing with a stick. “The furrows appeared slightly more filled up on the hill side than on the flat,” George reported. “There were a good many earth castings about tho’ I should say that they were by no means very numerous.”29 On New Year’s Eve William took more precise measurements of a field in Wiltshire, near Stonehenge, braving the winter cold to measure the slope and the furrow depths at different points along a steep hill. His father’s idea was that worms hasten the disappearance of old ridges and furrows, especially on hillsides, by helping to erode the former and fill in the latter. Frank sent further observations, and his future wife Amy was even initiated into the family fieldwork. In early February 1872 she sent observations on earthworm activity in old hillside furrows looking for little ledges on the sides of steep, grass-covered slopes which, Darwin suspected, resulted from the washing down of worm castings. Her future father-in-law dubbed her his “geologist in chief for N. Wales.”30 Of course, Lucy was in on it, too, sending observations of a cow-trodden hill while on holiday in the Swiss Alps.
The idea of worms hastening the erosion of ridges and furrows along hillsides may sound reasonable to us today, but for his skeptical contemporaries, unaccustomed to appreciating the finer points of uniformitarian worms, it was a stretch. He was determined to marshal irrefutable data. No detail was to go unexamined, as when he asked Lucy to probe wormholes with a knitting needle on steep, grassy slopes to determine whether or not they come out at right angles to the slope. “We have no steep grass-covered slopes here,” he lamented, and he acknowledged that “It is not easy to probe the holes.” Lucy was not deterred. Two weeks later, she sent her findings: “Out of 25 worm-holes probed with a
blunt wire between Jan 6 & 14 on different slopes for a few inches, 8 came to the surface nearly vertical to the slope; the rest at various angles.” She promised to probe some more, and to dig trenches for data about furrows. Her uncle was thrilled: “My dear Lucy,” he wrote right back, “You are worth your weight in Gold.” He shared what he was after: “If worms would be so good as to come up generally at right angles to slope, it would bring the earth down grandly.” His idea was that since earthworms leave their castings piled up at the mouths of their tunnels, if the tunnels are at right angles to the sloping surface then gravity and rain will more easily draw the castings downslope.31
Darwin was fixated on gaining enough data to prove his theory. Later that year he tested the idea while on one of his enforced holidays. In late October 1872, people strolling at beautiful Knole Park, the 1000-acre deer park south of London, were met with a curious sight: there was Mr. Darwin flat on his belly, carefully observing earthworm castings on the steepest hillsides. After several wet days he found that “almost all the many castings were considerably elongated in the line of the slope.” At the mouths of worms’ tunnels where earth had been ejected, there was always more earth below (downslope) than above the tunnel. He continued his observations back home, seeking suitable hills in the neighborhood. At Holwood Park, outside nearby Bromley, he measured slopes between 8 and 11 degrees where “castings abounded in extraordinary numbers,” creating a uniform surface coating of fine material extending downslope.32
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