It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organised creatures.
—CHARLES DARWIN, The Formation of Vegetable Mould, Through the Action of Worms, With Observations on Their Habits, 1881
THE FIRST TIME I held a worm in my hand, I was surprised at how light it was, how harmless. It didn’t slither around or try to get away. Instead it lay curled in a near-perfect circle, as if it had already accepted its fate.
The worm I held was a red wiggler, Latin name Eisenia fetida. It is in many ways a quintessential worm, small and reddish pink, with faint stripes between each segment. It is a master composter, preferring a heap of rotting garbage to just about anything else. Dig around in pig slop, barnyard manure, or a mound of damp leaves, and you’ll probably find red wigglers, eating and laying cocoons in the mess. But the worms themselves are not messy; this one slipped out of its pile of rubbish perfectly clean.
It came out of my worm bin, a small composting operation on my back porch in which I deposit scraps from the kitchen. I don’t know how many of them live in the bin—ten thousand, maybe. Sometimes when I dig around in there, the worms are so thick that they look like ground beef set in motion, a mass of churning bodies. It is hard to think of them as individuals, but when it came time to pull one out of the bin and set it on my palm, I did spend a minute looking down at them, trying to choose the right one. A good sturdy specimen was working its way up the side of the bin as if it was ready for adventure.
The reason I was choosing a worm to hold was that it had occurred to me that in all the years I’ve kept them, I’d never actually touched one. Strange that I would have such an aversion to letting one get next to my skin. How was I to learn anything about the dark and damp place where the plants in my garden put down roots, if I wasn’t ready to get intimate with an earthworm?
With one finger, I poked at the worm in my hand. It was completely limp. I could see a purplish vein running along the length of it, just beneath the skin. I curled my palm around the worm, folding it in half and in half again. It didn’t react. I began to wonder how a creature this weak could do anything, even move through dirt. Then a few seconds later, it seemed tired of this expedition. It raised one end up—the head, I suppose—and extended one segment at a time into the air. Now, finally, it moved and left a little slime in my palm. I shuddered but didn’t drop it. This slime, this worm mucus, was its way of reacting to stress—stress that I had brought on by pulling it out of its bedding and exposing it to light. The worm moved to the edge of my hand, and this time pointed its head down towards the bin, towards home. It was intent on getting back. Just then it looked as if it were capable of doing something after all. It moved with purpose, seeking to escape, trying to return to its familiar habitat. I dropped it into the bin, where it ducked under a layer of damp newspaper and disappeared.
I held worms quite often after that—not just from the bin, although I did get into the habit of pulling four or five out at a time and letting them wiggle through my fingers. I also started handling worms I found in the garden, particularly the enormous nightcrawlers, Lumbricus terrestris, that stretched the length of my hand. A nightcrawler, I learned, would press its tail against my wrist as if it was seeking traction, then point its head out beyond the end of my middle finger. On a rainy day, I might handle a half-dozen nightcrawlers like this. It is both fascinating and a little disturbing to pull something out of the ground and stare at it, something that does not belong up here with the rest of us.
WHEN I STAND OVER a patch of earth and wonder about the subterranean activity taking place underfoot, I am not alone. Gardeners are inquisitive by nature; we are explorers; we like to turn over a log or pull up a plant by the roots to see what’s there. Most of the gardeners I know are, like me, quite interested in earthworms, in the work they do, churning the earth, making new dirt. We hold soil in our hands, squeeze it and smell it as if we are checking a ripe melon, and we sift through it to see what inhabits it. Ask a gardener about the earthworm population in her garden, and I guarantee she will have something to say on the subject.
It seems strange, then, that most scientists before Charles Darwin didn’t consider worms worthy of study. Very little was known about them in the nineteenth century, when Darwin emerged as a sort of champion of worms, devoting his last book to a painstakingly detailed research of their physiology and behavior. The Formation of Vegetable Mould, Through the Action of Worms, With Observations on Their Habits was published in 1881. He was an old man when he wrote the book, but the subject had interested him for decades.
How could such an insignificant creature capture the attention of a distinguished scientist like Darwin? He knew from an early age that earthworms were capable of far more than most scientists gave them credit for. He recognized, in a way that no scientist before him had, that they possessed an ability to bring about gradual geological changes over decades, even centuries. This notion—that the smallest changes could result in enormous outcomes—fit perfectly with his work on evolution and the origin of species.
The story of Charles Darwin and his worms begins in 1837, when Darwin was not yet thirty years old. He’d just returned from a trip around the world on a British sailing ship called the Beagle. He was offered passage because the captain, Robert FitzRoy, wanted a gentleman on board who could share the captain’s table with him. The boat would travel to the coast of South America, where Darwin would have ample opportunity to do the work of a naturalist, collecting specimens and recording his observations. Darwin could not resist the opportunity—he’d only just then been trying to find a way out of the career path his father had laid down for him, that of a parson in a country parish where young Darwin would have plenty of time to chase butterflies and beetles in between his duties to the parishioners. It was not the ideal career for the man who would come to be known as the father of evolution. As one biographer put it, “There was, needless to say, the small matter of his faith.” A journey around the world would put off these troublesome questions for a while, and his father agreed to the expedition. But once on board, Darwin realized that it would not be the idyllic adventure he had hoped for: the crew encountered more than its share of dangerous weather, the captain suffered some sort of breakdown midway through the voyage, and Darwin himself was often sick and discouraged. Still, he worked steadily, collecting artifacts and taking notes.
He was away for five years, longer than he could have predicted, and he came back with far more new discoveries than he could have imagined. He arrived in port with over two thousand journal pages, fifteen hundred preserved specimens, and nearly four thousand skins, bones, and dried specimens. It would take him years to organize it all, and even longer to realize the full impact of what he’d collected, for it was here, in this collection of fossils and insects and bird skeletons, that he would begin to see the patterns that would point him towards a theory of evolution. The idyllic vision of a country parsonage was long forgotten. Darwin had now chosen a life for himself as a scientist.
But this was no easy path, and there was no steady employment for a man of his talents. He arrived home from his Beagle voyage exhausted, overwhelmed by the work that lay ahead of him, and uncertain of his future beyond that. At first he worked furiously on his collection of notes and field journals, but it was not long before his health was so compromised that friends persuaded him to spend a few weeks in the country. He traveled to Shrewsbury to recuperate at the home of his uncle, Josiah Wedgwood. Upon arriving at Wedgwood’s home, he scarcely had time to set down his hat before his uncle had him out in the pastures, where he pointed to cinders and pieces of brick that had been spread on the ground years before and had since become buried some inches beneath. Wedgwood was convinced that the objects had become buried through the actions of earthworms, a feat that would require far greater strength and single-mindedness of purpose than had previously been believed possible of the lowly worm.
In spite of all that he had seen on his voyage around the world, Darwin was impressed with the discovery that his uncle had made in his own backyard. Darwin made a presentation on the subject to the Geological Society of London later that year. At the time, scientists were asking such seemingly simple questions as, Where does dirt come from? Why does dust fall on ships at sea? (Darwin addressed the latter question in a paper that he called, in his typically straightforward way, “An Account of the Fine Dust Which Often Falls on Vessels in the Atlantic Ocean.”) After his visit to his uncle’s home, he began to believe that earthworms, and earthworms alone, were responsible for the rich uppermost layer of soil, which was, in his day, referred to as vegetable mould.
Although he made some revisions to his first paper on earthworms and saw it published in the Geological Society’s journal again a few years later, by this time he was focused on the publication of the account of his voyage on the Beagle, and he’d already begun a number of other projects, including the manuscript that would become On the Origin of Species. Over the next few decades, he would publish books on the habits of climbing plants, the expression of emotions in humans, the fertilization of orchids by insects, and the variations among domesticated animals. During that time, he would also continue to revise his most well-known works, The Descent of Man and On the Origin of Species. If earthworms occupied his thoughts during those years, they did not make much of an appearance in his published writings.
Still, when he returned to earthworms in his old age, the book he wrote on the subject would prove surprisingly popular. “As far as I can judge, it will be a curious little book,” he wrote prior to publication of The Formation of Vegetable Mould. “The subject has been to me a hobby-horse, and I have perhaps treated it in foolish detail.” Nevertheless, the book attracted nonscientific readers who enjoyed its clear and vigorous writing and its surprising conclusions.
He described the volume of soil that earthworms swallow and eject as castings, or earthworm manure, reporting that an acre of garden soil could contain over fifty thousand earthworms and yield eighteen tons of castings per year. He studied earthworms’ ability to bury objects in soil, from handfuls of chalk scattered on the ground to Roman ruins that had, he believed, come to be buried and preserved for archaeologists by an industrious earthworm population. Most of all, though, he credited them with the transformation of the soil itself. “Their chief work is to sift the finer from the coarser particles, to mingle the whole with vegetable debris, and to saturate it with their intestinal secretions . . . no one who considers the facts . . . will hereafter, as I believe, doubt that worms play an important part in nature.”
At the time, people thought his estimates were grossly over-inflated and his claims exaggerated. No scientist before Darwin had taken such an interest in the creatures living underfoot. Earthworms were still largely considered a garden pest that damaged plant roots and spoiled clean green lawns with their castings. At best, they were thought to provide some small service by perforating the earth and allowing water to penetrate. At least one reviewer of Darwin’s early papers insisted that they were too small and weak to carry out the massive movements of soil to which Darwin assigned them. Another critic dryly observed, “In the eyes of most men . . . the earthworm is a mere blind, dumb, senseless, and unpleasantly slimy annelid. Mr. Darwin undertakes to rehabilitate his character, and the earthworm steps forth at once as an intelligent and beneficent personage, a worker of vast geological changes, a planer down of mountainsides . . . a friend of man.”
Darwin wasn’t deterred by the criticism of his colleagues. “The subject may appear an insignificant one,” he admitted, “but we shall see that it possesses some interest.” He could hardly restrain himself before laying out his central thesis: his remarkable conviction that “all the vegetable mould over the whole country has passed many times through, and will again pass many times through, the intestinal canals of worms.” It is a stupendous achievement for a blind and deaf creature with no spine, no teeth, and a length of only two or three inches. Scientists of the day could scarcely believe it, and they were quick to express their skepticism.
Darwin had heard these criticisms before in response to the earlier paper he had presented to the Geographical Society, and he did not waste the opportunity to both refute his critics and remind them whom they were up against. After all, he’d fought most of his life to win acceptance for his theory of evolution, and he saw parallels between his work on evolution and his work with worms.
A scientist looking back over Darwin’s work wrote that “the key to his genius was the ability to stretch his imagination to encompass geological time—thousands of years, thousands of centuries.” He understood that tiny, incremental changes in the environment could bring about the evolution of a species. It was this same approach that led him to understand that soil could, over time, be transformed through the efforts of earthworms.
“Here we have an instance,” Darwin wrote of his detractors, “of that inability to sum up the effects of a continually recurrent cause, which has often retarded the progress of science, as formerly in the case of geology, and more recently in that of the principle of evolution.” He dispatched a French scientist who disagreed with his conclusions about the abilities of earthworms, making the calm statement that the Frenchman “must have thus argued from inner consciousness and not from observation,” for Darwin’s own observations bore out the truth. The power of earthworms, then, came not from their individual, but from their collective strength. It is a surprisingly egalitarian conclusion to reach about earthworms, one that could only come from a man who had great vision and also great affection for the creatures themselves.
Today, among earthworm scientists, Darwin is a kind of touchstone, a muse. He looked belowground with real interest and treated the dark earth as the mysterious unexplored territory that it is. He lived at an exciting time for scientists: in every corner of the world, exotic plants and birds and fossils awaited discovery. But he chose to look underground, to seek out the earthworm. Now we know that Darwin had only glimpsed the potential power of worms: his conclusion that over fifty thousand worms could inhabit an acre of soil was in fact quite low. Scientists have shown that figure to be one million. Earthworms in the Nile valley can deposit up to a thousand tons of castings per acre, helping to explain the astonishing fertility of Egypt’s agricultural land. As Darwin had only just begun to suspect, earthworms pass the top few inches of soil through their guts every year. This makes them beings to be reckoned with, a force for change in more ways than even he could have guessed.
Over the last one hundred years, earthworm scientists (called oligochaetologists, named after the taxonomic class in which earthworms fall, Oligochaeta), have come to quantify what farmers have always known: that worms, through their actions, substantially change the earth. They alter its composition, increase its capacity to absorb and hold water, and bring about an increase in nutrients and microorganisms. In short, they prepare the soil for farming. They work alongside humans, extracting a life from the land. They move the earth, a remarkable accomplishment for a creature that weighs only a fraction of an ounce.
AN EARTHWORM TRAVELS through the soil, pushing some particles aside and ingesting others. Although its food choices may look alike to the casual observer, the worm is actually sorting through the soil in search of tiny bits of decaying organic matter, which it will swallow along with some clay or sand particles. It builds a permanent burrow as it goes. At night it rises to the surface of its burrow, ejecting a small mound of castings around the entrance. It searches for food, tugging leaves, pine needles, and other detritus into its burrow. This simple routine is enough to ingratiate it to the farmer or gardener. On its nightly forage for food it acts like a small, very efficient plough.
The body of an earthworm is perfectly designed for life underground. Sight is unnecessary in the subterranean world; a sensitivity to light is all a worm needs to avoid straying out of its habitat. Lungs are not much use in the tight confines of a burrow; instead, the earthworm breathes through its skin, exchanging oxygen for carbon dioxide, relying on damp conditions to help absorb the oxygen in the same way that the damp interior of a mammal’s lungs facilitates the passage of air into the body. The earthworm’s shape allows it to be an extraordinary vessel for soil—the perfect container for holding, transporting, and transforming earth.
“The plough is one of the most ancient and most valuable of man’s inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earth-worms,” wrote Darwin. Although he studied many aspects of earthworm biology and behavior, the august scientist was especially intrigued by its ability to sift the earth. He watched them emerge from their burrows at night and draw in twigs and leaves or even drag small stones over a gravel walk until they formed a pile at the mouth of the burrow. He crept outside and unplugged enough of these burrows to know that the worms rested just inside, their heads readily visible just below the surface. Were they hiding from predators? Trying to keep rainwater out? Perhaps they were just protecting themselves from the cold night air. Whatever the reason, this nightly gathering of materials and systematic drawing in of leaves and plugging of burrows was certain proof of their unlikely physical strength and engineering abilities.
If a person were to pull leaves or twigs into a hole, Darwin reasoned, they would grab the object by its narrowest end and pull it in. If the object was long and skinny like the hole itself—say, a twig or stem—they would probably pull the thickest, heaviest end in first. Surely instinct alone could not account for the manner in which a worm selected material for its burrow. Intelligence, he declared, had to be the guiding factor. When the worms reached for fallen leaves and twigs around their burrows, they were selecting the best material available. They evaluated, they experimented, they made decisions.
Let me say that again: they made decisions—actual decisions, made after trying several alternatives and choosing the one that seemed best for the situation. This is perhaps the most surprising revelation in Darwin’s book. Although earthworms had undoubtedly been making such decisions for centuries, they had a new and unlikely advocate in Charles Darwin. He had the time, the resources, and the scientific methodology to prove that what earthworms did was more than mere chance.
I THOUGHT OF DARWIN and his worms when I was out in the garden, digging a new vegetable bed for the three dozen asparagus crowns that arrived by mail. A layer of fog had descended over Eureka, covering the hills around Humboldt Bay I can usually see from here. The earth was damp but not muddy, just right for planting.
I pushed a pitchfork into the soil and leaned back on the handle just enough to raise the tines of the fork and disturb the ground. My days of double-digging—of scooping out the top layer of earth and the one beneath it, filling in the trench with compost, and placing a mixture of soil and compost on top—are over. The soil is an intact system, a community of microorganisms that lives and breathes, and it will function best if I don’t disturb it too much.
Once the ground was loosened I spread a layer of compost on top. The microbes—the bacteria, the protozoa, the fungi—could work their way into the earth gradually, and the earthworms would rise to the surface and take the compost down with them. Down the center of the bed, I pulled apart the soil with a hand spade and created a narrow trench to bury the crowns. A layer of compost went in the bottom, and then I pulled the crowns out of the box, and spread the roots so they straddled the compost. I knocked enough dirt back into the trench to cover the crowns, but a shallow depression remained. I planned to fill it in slowly over the next few months as the first asparagus shoots appeared. The extra soil around the newly formed shoots would make them pale and tender, at the same time providing enough nutrients to encourage them to grow tall and robust.
There were easily a few dozen earthworms inhabiting the newly dug asparagus bed. Each worm holds less than a teaspoon of earth in its body as it moves through the soil. In a day, they will eat about a third of their body weight in soil, maybe more. This doesn’t sound like much, but even Darwin’s conservative estimates showed that over the course of a year, a healthy earthworm population can move almost twenty tons of soil per acre.
I leaned against my shovel, calculating that I’d spread about thirty pounds of compost over my asparagus bed. Over the next year I could expect earthworms to add another thirty pounds of castings around the roots of the plants. If conditions are right, they’ll supply another thirty pounds—maybe more—the following year, and the year after that. These asparagus crowns will produce for over twenty years. In that time, if the earthworms flourish, they’ll contribute about six hundred pounds of nutrient-rich castings to this small space, taking care of my vegetable bed far more efficiently than I ever could.
DARWIN IS RESPONSIBLE for putting these kinds of thoughts in my head. My gardening chores take significantly longer now that I slow down to count worms, now that I sit in the garden path, chin in hand, calculating the volume of castings. I have slowed down, it seems, to Darwinian time. He had that luxury in his later days; he could spend hours out in the fields around his house, watching earthworms and collecting their castings after they had disappeared into their burrows, making guesses about how they spent their time after they vanished from sight.
He also had the good fortune to know scientists around the world, and those colleagues sent him specimens and castings in the mail. He weighed and cataloged them, made a note about the area where they were collected, and organized the results into tables. Thanks to his meticulous approach, his work today remains some of the best data on earthworm activity. He wrote this in his autobiography: “I think that I am superior to the common run of men in noticing things which easily escape attention, and in observing them carefully.”
There is no doubt that he took some pleasure in his work. He had a genuine fondness for the worms and seemed to enjoy the painstaking effort that his research required. I can only imagine that his experiments on their habits were a daily delight in his old age. One biographer wrote that Darwin “became in the end what he had always been in his heart, almost a part of nature himself, a man with time to lean on a spade and think, a gardener.” I like to imagine him as a dabbler, a homebody, a man who explores his most intimate surroundings with both deliberation and wonder. In the waning years of his life, he was sometimes weak and infirm, but that only turned the attention of his scientific mind away from the wider world and towards his home, his garden, and the earth.
The approaches he used to evaluate earthworms were, by this time, classic Darwinian methods. Throughout his career, he took an ingenious, almost playful approach to experimentation. Like most naturalists, he was a tinkerer, interested as much in nature’s minutiae as in its grandeur. He liked the inner workings, the tiny springs and gears of the natural world. Perhaps he felt that nature’s true power rested there, in the movement of pebbles and seeds, and in the commerce of ants and worms.
Think of him in his laboratory, with his notebooks and specimens. One day he becomes interested in the mechanism that allows climbing vines to climb, and he ties small weights to the tendrils of plants to see how they respond. They hang on the vines like miniature Christmas tree ornaments, forcing the plant to reveal its tricks. He marvels at plants whose leaves roll tightly shut after dark. How could a plant act so deliberately, with such intent? He forces their leaves open so that they cannot close at night, hoping to lay bare their secrets.
And now, when the old man turns his attention to worms, picture him stealing outside on wet mornings to pull leaves out of burrows and observe how they had probably been tugged inside. He gathers a handful of pine needles and scatters them around burrows to see how worms will handle them. Eventually his curiosity about their mental capacity leads him to cut out irregularly shaped paper triangles and set them among active burrows, then chart the number of times the triangles are drawn in by the apex, the middle, or the base.
Darwin was meticulous with his research. Since this was to be his last book, he seemed determined to get it right, to document every element of earthworm life. He pulled not a few leaves out of burrows; he pulled 227 out and reported that 181 of them, or eighty percent, had been drawn in by their tips. The others had been drawn in by their bases or seized in the middle, causing the leaf to crumple once inside the burrow. The image of the elderly scientist pulling 227 leaves out of burrows and cataloging them to prove the intelligence of earthworms in his backyard is amusing, even surprising, but he didn’t stop there; he went on to reconstruct pine needles by breaking them apart and rejoining them at the base using glue or thread. He aimed to prove that worms knew to drag them into their burrows by the base where the needles were joined, rather than by one end, which would surely result in the needle getting stuck midway. He wanted to demonstrate that they were not acting out of instinct, because of a pine needle’s particular taste or feel. He created 271 of these artificial sets of pine needles and observed that eighty-five percent of them were drawn in by their bases, noting that worms were slightly more likely to draw pine needles in by the base if they were attached with thread as opposed to being attached with glue, which might have smelled or tasted unpleasant to the worms. He wondered if the worms naturally avoided the sharp points of pine needle ends and chose the base because it was rounder. To test this, he carefully trimmed off the sharp ends and found that worms drew them in by their base regardless.
As for the paper triangle experiment, he did not simply cut a few triangles and leave them lying around. He cut 303 triangles of various sizes, coated them with fat to keep them from going limp in the night dew, and established some baseline data by drawing triangles into small tubes using tweezers to determine the most efficient method that he would employ if he, rather than the worms, were given this task. (He chose the apex, as opposed to the middle or the base.) Even working with this unfamiliar material, they drew the paper triangles in by their apex sixty-two percent of the time. He went on to observe that the triangles pulled by their apexes had been drawn in cleanly, with very little evidence of fumbling around or trial-and-error first. “We may therefore infer—” he writes, “improbable as is the inference—that worms are able by some means to judge which is the best end by which to draw triangles of paper into their burrows.”
ONE OF DARWIN’S most extraordinary qualities was his ability to recognize when a scientific question could not be answered due to the limitations of the science of his day. He knew, for instance, that during his lifetme, no significant progress would be made on the question of how life first began. Near the end of his life he wrote to a colleague, “You expressed quite correctly my views where you said that I had intentionally left the question of the Origin of Life uncanvassed as being altogether ultra vires [beyond the powers] in the present state of knowledge.” The same could be said of Darwin’s insight into the role of earthworms in the soil. The technology that would allow scientists to understand the complex relationships between soil microbes, plants, and earthworms would not be advanced for several more decades.
When The Formation of Vegetable Mould was published in 1881, it was a novel idea that an earthworm could possess enough intelligence to judge how to best pull objects into its burrow. No scientist had paid so much attention to this seemingly trivial matter, nor devoted so many pages of published work to it. But even Darwin could not grasp the importance of the earthworm’s impact on the soil ecosystem. Scientists in his day knew that bacteria and other microorganisms lived in the soil, but the ideas were quite new. Louis Pasteur initiated the science of microbiology during the last few decades of Darwin’s life. A strain of bacteria was first identified as the cause of a plant disease in 1878. Still, the relationship between the microscopic world of soil and the macroscopic ecology—the earthworms and other visible creatures that inhabit the earth—was largely a mystery. Over the next several decades, the study of earthworm behavior was eclipsed by the study of its role in the soil. To understand what’s happening underground, we have to know more about this creature that lives below our feet, selectively drawing organic matter down from the surface, creating pockets of air everywhere it goes, sifting and digesting particles of earth.