Darwin's Backyard

by James T. Costa

  The soil-burrowing endogeic and anecic earthworms literally eat their way through the earth, grinding with their muscular gizzards the tiny particles they ingest into tinier particles still, digesting the organic matter as they go. They often expel the indigestible soil with their waste products at the surface in the form of small mounds called castings. On closer inspection the mounds can be seen to consist of long, convoluted tubes of extremely fine particles—like extruding the contents of a very narrow-mouthed tube of toothpaste into a pile. Or, better, think of spraying a little mounded pile of that scariest of processed foods, aerosol cheese in a can. You get the picture. The tubular shape is that of the worm’s intestine. Uncle Jos posited that by ingesting soil below and defecating it at the surface, earthworms in their multitudes are constantly turning the soil, in the process undermining and slowly burying surface objects and helping create the organic humus or topsoil layer (called “mould” at the time, to use the British spelling). Darwin’s imagination was fired immediately by the idea.

  He examined several fields with his uncle, one of which had been plowed, harrowed, and covered in cinders and marl 15 years earlier, and left undisturbed since. They carefully dug a number of test pits and trenches, confirming that the sunken material occurred in a distinct layer—not haphazardly at varying depths. Different-sized objects seemed to have sunk at the same rate, too, which would not be expected if rainfall and gravity were responsible. No, clearly something was acting upon the scattered granules and cinders simultaneously, and that something was humble earthworms. It was precisely the kind of explanation that most excited the Lyellian in Darwin: a mundane and modest force constantly applied that added up to great effects over time. Not only soil turnover and burial of objects, but also their intestinal processing of coarse soil particles, grinding them into ever-finer particles: why, these worms were a veritable erosional force, unrecognized though literally beneath our feet.

  Consulting with his uncle, Darwin prepared a paper for the Geological Society. It was read on November 1, 1837, and published in the society’s Proceedings the following year. He described the observations from the Staffordshire fields and pointed out similar observations, such as the elevated marine shells he noticed high in the mountains of Chile, covered by a shallow layer of earth in areas where rain was unlikely to have washed it over the shells. “The explanation of these circumstances,” he told the Society, “which occurred to Mr. Wedgwood, although it may at first appear trivial,” was the digestive process of worms. Darwin proceeded to speak to the abundance of worms and their soil-eating habit. The quantity of soil moved is evidenced by their ubiquitous castings—there was “scarcely a space of two inches square without the cylindrical castings of the worms” throughout his uncle’s fields. Their digestive process is a veritable geological power, he urged them to realize.2 With coral reefs fresh in his mind he pointed to a similar process going on in the oceans of the world. In reefs the incessant excavations of boring and tunneling mollusks and marine worms is helped along by coral-crunching fish like seabream (genus Sparus), converting great quantities of calcareous coral into fine mud. The accumulated calcium-rich sedimentary mud they produce eventually becomes limestone. He thus argued that a significant proportion of the chalky limestone formations of Europe was produced from coral, “by the digestive action of marine animals, in the same manner as mould has been prepared by the earth-worm”3 from long-disintegrated rock. Today, scientists believe that the calcium carbonate shells (called tests) of near-microscopic foraminifera—amoeboid protists occurring in untold scintillions, which incessantly accumulate on the sea floor as they expire and drift down like a never-ending calcareous snowstorm—are the single most important source of limey ocean sediments (and therefore limestone). Darwin’s coral-crunching fish and burrowing marine worms do make a contribution, albeit a minor one. In any case, the image of great limestone formations owing their existence to the accumulated action of humble organisms, whether foraminifera or marine worms, is certainly evocative. Returning to the terrestrial world, he declared it probable “that every particle of earth in old pasture land has passed through the intestines of worms.”

  The paper caused a stir—no less a figure than the eccentric and distinguished geologist William Buckland of Oxford (Charles Lyell’s old professor) strongly urged that the Geological Society publish Darwin’s paper not just in the meeting proceedings, but also in the more prestigious Transactions of the society, with illustrations to boot. Darwin had described nothing less than “a new Geological Power,” Buckland declared.4 Darwin was delighted, though probably not with Buckland’s recommendation to strike his hypothesis about the creatures that made the chalk formations. The earthworm argument was so compelling, Buckland reasoned, why distract from it with a disputable conjecture that, if it ends up being well supported, would deserve a paper in its own right? It was dropped from the Transactions paper, but Darwin stubbornly included it in later writings. His family cheered him on. Cousin Elizabeth Wedgwood wrote to say that Uncle Jos “desires me to tell you he is very much struck with your hypothesis of chalk being made by fishes—if fish made Chalk Hill I don’t see why worms may not make a meadow.”5 Buckland’s reservations proved prescient, however. A few years later, Berlin-based naturalist Christian Gottfried Ehrenberg astounded the scientific community with a series of masterful microscopic analyses of various kinds of rocks and sediments, including the chalk. Ehrenberg showed that microorganisms were the more likely source of the chalk and other limestone formations, and coined the name foraminifera, “hole-bearers,” a reference to the perforated tests of these ubiquitous marine protists.

  Cross-section of a field at Maer, showing the stratum of cinders, burnt marl, and quartz pebbles buried by earthworms. From Darwin (1840), p. 506.

  Darwin’s earthworm ideas fared better, and back home his extended family shared his delight at the buzz he created in the London geological scene. “We are very much obliged to you for sending us a copy of ye ‘Maer Hypothesis,’ which you so handsomely give my father all the credit of,” Elizabeth wrote. “We shall all be exceedingly glad when you can come & philosophize again with us.”6 Darwin decided to expand the study, and asked Uncle Jos and Elizabeth to gather information on different fields in the parish for him. Others, too, volunteered their observations. When Charles Lyell sent a copy of Darwin’s paper to friends in Edinburgh it caught the attention of industrialist and farmer William Fullerton Lindsay-Carnegie, of Forfarshire, who eagerly wrote Lyell to say that not only was Darwin’s theory correct, but that beyond its geological insights “the discovery may do much in an economical sense.”7 He related how farmers in his district had misinterpreted how lime and other fertilizing materials spread on their unplowed fields had ended up below ground, thinking that the lime simply had a tendency to sink. Assuming therefore that these things would sink even faster in plowed fields, they held off applying the fertilizer until very late, “thus, losing much of the advantages to be derived from a thorough intermixture.” When Lindsay-Carnegie bucked convention and laid down lime and plowed immediately, the farmers grumbled that he had made a grave error. But the results said otherwise, and Darwin’s theory proved the error of their ways. “By means of Mr Darwin’s observation I think the prejudice will be entirely removed,” he concluded. Darwin was able to incorporate this account and more from Lindsay-Carnegie as well as fresh information from the Wedgwoods into the Transactions paper, which came out in 1840.

  By then much had transpired: Darwin had married, read a host of papers at the learned societies, begun serving as editor of the Beagle voyage zoological volumes, published his Beagle travel memoir to some acclaim, was hard at work on his coral reef book, and he harbored a burning secret, having become a closet transmutationist. A rising star in the British geological firmament, in some ways Darwin was a victim of his own success at that busy time: parrying an invitation to become secretary of the Geological Society in 1837, he succumbed the following year while also accepting the
post of vice-president of the new Entomological Society of London. Earthworms and the “Maer hypothesis” were put on the back burner, but were never taken off the stove altogether. When the March 16, 1844, issue of the Gardeners’ Chronicle reprinted in commentary form his Geological Society earthworm paper—making the point that “the poor despised ‘worm of the earth’ ” is in fact “one of the farmer’s best friends”—he took advantage of the opportunity to correct an error in the original paper and give refined estimates for the worms’ rate of burial. Published in the April 6th issue, Darwin’s note reveals that he had continued to work on worms, at least occasionally: he checked on the progress of Uncle Jos’s flints and cinders, and interviewed the local farmer, William Dabbs, whose nearby croft was the subject of data sent by his Wedgwood cousins in 1837. He wanted to verify when the Dabbs’s field was initially treated, and dug holes and trenches to measure the layer of marl, cinders, and broken pottery beneath the surface. The rate of burial in the field was nearly 3½ inches per year, much faster than the rate of ¼ inch per year found in his uncle’s boggier field. He also found a large clod of earth “penetrated by eight upright, cylindrical worm-holes, nearly as large as swan-quills, so that I could see through them.”8 Those cylindrical wormholes were, to Darwin, so many barrels of a smoking gun: where is all that soil missing from the long tunnels? Why, removed by the worms and deposited on the surface.

  Then, all was quiet on the earthworm front for over 20 years, at least as far as the documentary record goes. All the while Darwin was as industrious as his worms, constantly churning ideas day in and day out. Despite his sometimes-debilitating illness and through the joys and tragedies of his family life, we’ve seen how he was incessantly probing, writing, experimentising as reflected in the multitude of “research programmes” he undertook: the barnacle years of the late 1840s and early 1850s (see Chapter 2); his insights into the nature of competition and diversity (Chapter 3); the mystery of bees’ cells (Chapter 4); understanding the “grand game of chess” that is geographical distribution (Chapter 5); plumbing the secrets of pollination and flower morphs (Chapter 6); and the masterful case study of orchids (Chapter 7). Along the way his barnacle monographs appeared in this period, followed by the Origin, his orchid book, and his two-volume treatise on domestication. Then, too, carnivorous plants caught his eye (Chapter 8) and climbing plants beckoned (Chapter 9)—the stuff of books yet to come.

  And earthworms? They may have been out of sight, figuratively and literally, but they were not altogether out of mind. For one thing they played a role in some of Darwin’s other investigations, such as buried seeds: one February day in 1856 he collected some earth along the sandwalk consisting of worm castings and planted them under bell jars in his study. Three plants came up in one, and two in the other. Earthworms moved seeds in two ways, he realized: carrying them in their gut they moved seeds deep underground or up to the surface. Also, they buried seeds by constantly turning the soil. On balance they probably buried more than they brought to the surface, since two processes carried seeds down, while just one carried them up. Darwin was struck by just how deeply seeds can be moved underground, speaking from experience when he later described “how easily a botanist might be deceived who wished to learn how long deeply buried seeds remained alive, if he were to collect earth from a considerable depth, on the supposition that it could contain only seeds which had long lain buried.”9 The movement of seeds is important for understanding the struggles that plants endure, too: those seeds “in right climate & soil,” and “which got planted at right depth” by earthworms and other agents, will germinate. Then, however, comes the “battle for life with slugs & insects & other plants.”10 There seems to be no further mention of worms for the next dozen years after his seed burial observations; he was hot on the trail of other investigations.

  Fishing for Clues with Worms

  Worms made an appearance again, cropping up suddenly in Darwin’s notes like castings in a field after an autumn rain. His nieces Lucy, Margaret, and Sophy Wedgwood had begun helping their Uncle Charles with field collections and observations of flower morphs beginning in 1862. They were the daughters of his sister Caroline. She had married Josiah (Joe) Wedgwood III in 1833 (another Darwin-Wedgwood cousin liaison), and were now living at Leith Hill Place in Surrey. Their career as their uncle’s field assistants came about in typical Darwin fashion. Upon hearing that his sister’s family was heading to north Wales for a holiday, Darwin asked his nieces to check local purple loosestrife populations, and let him know how many of each of its three flower morphs they counted (see Chapter 6) so that he could compare the ratios found in different localities. (They did, and he included their data in the paper that eventually resulted.) Of the three young Wedgwood ladies, Lucy had the naturalist’s bent. It was likely on one of the Darwin family visits at Leith Hill that Lucy was introduced to the finer points of earthworms. She was soon sending her uncle questions and observations, one of which he forwarded to the Gardeners’ Chronicle: “As gardeners have much to do with worms,” he wrote the editor, “I think that you will find the enclosed little communication, written by my niece, worth insertion in Gard. Chronicle. I can vouch for her remarkable accuracy.”11 Lucy had noticed small stones arranged atop and around the entrance to wormholes. On several occasions she removed the stones and came out at night to see how the worms replaced them. She noticed the worms would hold their bodies in the holes with their tails and extend themselves until they felt one and dragged it to the hole with their mouth. Her query: What was the worms’ reason to make stone heaps over their holes?

  Lucy’s investigations got Darwin thinking—he long admired worms as irrepressible earthmovers, but now he was beginning to view their behavior as more sophisticated than he thought. To most people these blind and mute creatures wriggling around gardens have no behavior to speak of. He knew better, but Lucy’s experiments were intriguing. What was it about a worm’s life that would result in so curious a behavior as to gather stones? There was more to worms than even he realized, but just at the moment he was swamped with other projects. Having completed his domestication volumes and a paper on flower morphs, he had resolved to publish a book giving his views on human evolution, a topic he had glossed over in the Origin. In the 1860s Lyell and Huxley in Britain and Haeckel in Germany all came out with books bearing on human origins. Darwin had mixed feelings about their efforts: unsurprisingly, he had his own ideas about human evolution, and he simply couldn’t put off publishing them any longer. (This would become The Descent of Man, appearing just a few years later in 1871. But an awful lot of work lay ahead to get there.)

  On February 4, 1868, Darwin recorded that he “Began on Man & Sexual Selection.”12 But there was a month’s break in March to visit his brother Erasmus and then Emma’s older sister Elizabeth. They returned home on April 1st; it was quiet now, with just Etty (then 25) and Bessy (21) living at home. Lenny and Horace were off at boarding school, and Frank and George were at Cambridge. Darwin managed to do some work on Descent, but there were always the letters: in April 1868 alone they take up just over 100 pages in the collected Darwin correspondence volume for that year (albeit with endnotes).

  Darwin was unwell much of the summer, so the family took a month-long holiday on the Isle of Wight, renting the home of the noted portrait photographer Julia Cameron. There they met the poets Tennyson and Longfellow (a very distant Darwin relation) through Cameron’s good offices. It was restful, but he was impatient. “We have been here for 5 weeks for a change,” Darwin wrote in one letter, “& it has done me some little good; but I have been forced to live the life of a drone, & for a month before leaving home, I was unable to do anything & had to stop all work. We return to Down tomorrow.”13 That was late August, and they stopped in Southampton on the way home to visit William, now well established in banking. A few weeks later Darwin was cheered by a visit from Alfred Russsel Wallace and his wife Annie, along with ornithologist John Jenner Weir and all-around zoologist Edwa
rd Blyth, just back from Calcutta. The visit was a great success, notwithstanding that Blyth was “a dreadful bore” according to Bessy.14 That welcome visit was followed by another: his American botanist friend Asa Gray and his wife Jane and several nieces came to see the Darwins in October, on their way to southern Italy and Egypt.

  That visit occasioned an opportunity to recruit the Grays for yet another of Darwin’s ongoing investigations: as part of his researches into human evolution he became keenly interested in the physical expression of emotions. That is, the muscles involved in expressing happiness, anger, or grief, for example, as well as gestures and body language. By now you realize that seemingly odd investigations like this were no tangent for Darwin. He was nothing if not consistent, and his investigations were always undertaken to extend and reinforce his theory of evolution by natural selection. In this case, it was about the universality of expressing similar emotions in similar ways, by contracting the same sets of muscles. This line of research was consistent in another way too: it had deep roots with Darwin, dating back at least to 1839 when Willy was born and Darwin set about closely observing his behavior even as a tiny infant—the natural history of babies. The story of these investigations is beyond our scope here, so I will mention only that by 1868, with renewed interest in the subject, he had drafted and printed a questionnaire regarding facial expression and gestures in peoples of other races and cultures, which he distributed among friends and colleagues.