The Formation of Vegetable Mould, through the Action of Worms was published on October 10, 1881, to instant acclaim—3,500 copies sold within a month. “I must own I had always looked on worms as amongst the most helpless and unintelligent members of the creation,” his old friend Hooker wrote, declaring that he was now “amazed to find that they have a domestic life and public duties!”47 Lady Derby, who had long corresponded with Darwin on scientific subjects, was equally impressed: “I have read your book with the greatest interest. You said once, laughing—that you were finding that “Worms” could revolutionise the world—you have succeeded in proving the greatness of their power.”48
Darwin came full circle with his worms: they were the subject of one of his first published studies and represented his earliest scientific interest, geology. Now, 40 years later he completed the chapter he opened with them by writing his last book on worms. He would die 6 months later, on April 19, 1882.
Caricature of Darwin as the sage of worms, by Linley Sambourne for Punch (December 6, 1881).
Earthworm Reprise
Formation of Vegetable Mould presaged if not prompted several lines of research that came into their own in the century to come. Pedology, soil ecology, bioturbation, ethology, and even erosion and archaeological and forensic stratigraphy all owe something to Darwin’s little book, though recognition of the significance of worms has sometimes been slow. This brings to mind Darwin’s admonishment to David Fish, the Doubting Thomas of his worm gospel: “Here we have an instance of that inability to sum up the effects of a recurrent cause, which has often retarded the progress of science, as formerly in the case of geology, and more recently in the principle of evolution.”49 Note his rhetorical device of drawing a parallel with the early resistance to Lyell’s geological vision, and to the cumulative effects of selection acting over vast time periods. You will come to see the error of your ways, he is saying. But there have always been those who, like Lady Derby, appreciated that Darwin well and truly succeeded in proving the greatness of the power of worms.
Some researchers have returned to Darwin’s own landscape laboratory over the years to revisit the effects of worms there—descendants, no doubt, of the very worms that Darwin knew and loved. Sir Arthur Keith, Scottish anthropologist, revisited Darwin’s fields in the early 1940s, digging 70 trenches to see how far Darwin’s flints had sunk. Publishing his findings in Nature in 1942, Keith reported the important observation that objects are buried only to a point, where they collect in a layer—often on a denser stratum of clay or rock below which the worms can’t go. Just as a stone, thrown into a pond, comes to rest on the bottom, so too do objects on the surface come to sink to a point where they cannot sink any farther. Understanding more precisely the dynamics of object burial by earthworms is as important for forensic investigations as archaeology.
The most recent (and ongoing) study was initiated in 2007 by ecologist Kevin Butt, who heads the Earthworm Research Group at the University of Lancashire in the UK, in collaboration with US Forest Service soil ecologists (one of whom happens to be my brother-in-law, Mac Callaham), English Heritage, Down House staff, and dedicated volunteers. The team set up a long-term field experiment to replicate Darwin’s 1842 flint experiment in Great Pucklands Meadow—Darwin’s own replication of his Uncle Jos’s observations at Maer, and the same meadow in which he later undertook his biodiversity studies with Miss Thorley (see Chapter 3). This time an experimental design informed by modern statistical approaches was implemented: a randomized 2 × 2 factorial design, with “large” and “small” flint size categories and “low” and “high” flint densities distributed among 16 1-square-meter (m2) plots. At the same time 10 0.1-m2 plots were established nearby for quantifying worm-casts, and a parallel study revealed 19 earthworm species in the greater area, 9 at Down House alone. The star of the show proved to be Aporrectodea longa, which belongs to the largest and most widespread temperate zone earthworm genus. A copious caster, A. longa abounds in Great Pucklands Meadow, and is the main species responsible for transforming Great Pucklands from the “stony field” remembered by the Darwin children to the stone-free and well-vegetated meadow it became in Darwin’s later years, and remains today.
The plots were revisited in 2013. In the intervening 6 years the flints were buried at an average rate of 0.96 cm per year—a good deal faster than Darwin’s estimate of 0.21 cm per year. The greatest proportion of buried flints were small ones at high density, but the larger flints at both densities were buried deeper—over 5 cm on average. This differs too from Darwin’s finding that smaller stones tend to get buried more quickly than larger ones, but it’s worth bearing in mind that the difference in depth between small and large flints in the contemporary study amounts to a centimeter or less on average. It will be interesting to see how well the worms of Great Pucklands Meadow get on with their uniformitarian work in the years to come; the plots are next scheduled to be revisited in 2019. Reflecting on the countless earthworms that are incessantly working away unseen, can we possibly look at the landscape in the same way again? As Darwin so eloquently put it in the final paragraph of his earthworm book:
When we behold a wide, turf-covered expanse, we should remember that its smoothness, on which so much of its beauty depends, is mainly due to all the inequalities having been slowly levelled by worms. It is a marvellous reflection that the whole of the superficial mould over any such expanse has passed, and will again pass, every few years through the bodies of worms. The plough is one of the most ancient and most valuable of mans inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earth-worms. 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.50
Darwin’s fields and meadows are a microcosm of the wider world. As with all of the naturalist’s investigations over the 40 years he lived in Downe, what he learned locally—in his study and greenhouse, field and garden, and woodland and lawn—had global implications. It is resonant that Darwin’s researches on worms spanned his entire life at Down House; he worked away above ground as persistently as they reworked the soil beneath. That 40-year earthworm odyssey was classic Darwin: seeing common things with new eyes, intuiting new significance; asking with childlike wonder why or how; always experimentising to find an answer. He was as revolutionary as his worms, with his knack for asking questions and the determination to follow out and reinforce his evolutionary ideas through subject after subject, book after book. Those books were like worm castings, slowly burying received wisdom and old prejudices, making the world anew.
Experimentising: Get Thee to a Wormery
With the publication of his earthworm book, Darwin had come full circle. He not only established these humble organisms as a uniformitarian geological force, but as animals with some measure of sense and sensibility. Seen in this light, the sensibilities of worms are not far removed from the sense perception of climbing plants, or the appetites of carnivorous plants. Here we’ll follow Darwin’s lead in testing for choosiness in worms.
Obtaining Your Worms
Collect or purchase a dozen or more common night crawlers, Lumbricus terrestris, available from most live-bait shops. Avoid the common “red wiggler” worms often associated with compost piles; these do not exhibit the burrowing and surface-foraging behavior that we are interested in.
Mighty earthmover: Lumbricus terrestris, the common earthworm. Drawing by Leslie C. Costa.
A. Materials
• 6–8 Lumbricus terrestris worms
• Terra-cotta flowerpot with drainage hole (medium-sized, large enough to hold 3–4 L of soil), OR 2-L clear plastic bottle (a soft-drink bottle with the label removed will work), cut near the top at its widest diameter and with a drainage hole cut in the base.
• Garden trowel
• Soil, locally dug and preferably not organically enriched
• Cle
ar plastic wrap
• Rubber band, large enough to stretch the width of the flowerpot or bottle
• Masking tape
• Ruler or tape measure
• Spray bottle with water (preferably bottled, rain, or well water)
• Lard, butter, or oil
• Construction paper (black)
• Plain white paper
• Scissors
• Spoon (teaspoon size)
• Assorted fresh leaves (e.g., oak, maple, dandelion, pine needle, lettuce, spinach)
• Corn meal, shredded cabbage, and fresh grass clippings for food
B. Procedure
You have the option of making a terra-cotta flowerpot wormery, much as Darwin did, or to make one out of a clear plastic bottle, which has the advantage of permitting burrow observations should any worms construct a burrow against the bottle wall.
1a. Flowerpot wormery: Fill your flowerpot ¾ full of moistened (not saturated) potting soil and place two or three small to medium 1½–2 in. (4–5 cm) L. terrestris worms on the surface. Sprinkle a bit of cabbage and corn meal over the surface for food, spritz with water, and secure with plastic wrap and rubber band. Allow your worms to settle in for a week, giving a daily spritz of water and replacing cabbage bits and corn meal as they disappear.
1b. Plastic bottle wormery: Carefully cut the top off of the 2-liter plastic bottle, leaving most of the length of the cylindrical bottle. Cover the cut edge with masking tape. If the bottle has a label, remove it if possible so that you can easily observe what’s inside, and punch a few small holes in the bottom for drainage. Fill the bottle with ~1.5 L of soil, and place one or two L. terrestris on the surface. Add food, water, and cover as above, and tape black construction paper around the bottle in such a way that you can easily remove it later for observations. Place this wormery in a cool and dark place (although covered with black paper, some light will get in and disturb the worms).
2. Note the number and size of wormholes at the soil surface of your wormery. These mark burrow entrances. Are the holes surrounded by mounds or other materials?
3. If you made a plastic bottle wormery, remove the black construction paper and see if any burrows were constructed along the wall, permitting you to see into the burrow. If so, what is its length and diameter? Do you see a worm in the burrow? Is there any evidence of cabbage or corn meal in the burrow?
A flowerpot “wormery” like those Darwin used to house his earthworms. Drawing by Leslie C. Costa.
4. Using the white paper, cut out paper triangles, squares, disks, and diamonds, each about 3 cm in length. Darwin rubbed lard on his paper cutouts to repel water and prevent them from becoming soggy and limp on the soil surface. If necessary, do this with lard, butter, or oil.
5. Place two of each shape (eight total) randomly onto the soil surface of your wormery, and re-cover it with plastic wrap.
6. Check the wormery twice daily and note the position of the paper cutouts. Are they moved between observations?
7. After 3 or 4 days, how many cutouts remain on the surface? Were any plugging the mouth of a burrow? Any unaccounted for were drawn into the burrows, out of sight. Did the worms prefer some shapes over others? Why do you think this was the case, or not?
8. Repeat with three kinds of cutout triangles: (1) equilateral triangles, 1 cm to a side; (2) isosceles triangles, with 1-cm base and 2-cm sides; and (3) right triangles, with 1-cm sides meeting perpendicularly. Prepare and place three of each on the wormery surface. Check daily, and note position, type, and number of visible triangles.
9. After a few days of observations, try carefully excavating a burrow to note the orientation of the paper cutouts within. Use a spoon to remove soil around the burrow until you come to the paper cutouts. Note the orientation of each triangle you excavate.
10. Did the worms prefer some triangles over others in drawing them into their burrows? Of those drawn in, how were they oriented? Darwin noted that nearly ⅔ of the triangles he provided to his worms were drawn in by one of the pointy ends.
11. Test the worms’ preference for different natural materials: small leaves, pine needles, leaf petioles, and so on. Do the worms exhibit any preferences? Darwin also did taste tests with the leaves of garden vegetables. Try cabbage, tomato, turnip, and green onion leaves and see which ones they have an appetite for.
See also:
“Earthworms” at the Darwin Correspondence Project: www.darwinproject.ac.uk/learning/universities/getting-know-darwins-science/earthworms.
ACKNOWLEDGMENTS
“I cannot express too strongly my obligations to the many persons who have assisted me.” So wrote Charles Darwin in the first volume of The Variation of Animals and Plants Under Domestication, words of gratitude that I heartily echo. A number of friends, colleagues, and family members have directly or indirectly assisted me with this project, beginning with the very direct contributions of my wife, Leslie Costa. I am deeply appreciative of Leslie’s keen editorial eye, and her deft artistic hand with the illustrations.
It has been my good fortune to have learned a great deal from David Kohn and Randal Keynes, who share an infectious enthusiasm for Darwin’s experiments. The Darwin Manuscripts Project, directed by David and hosted by the American Museum of Natural History, is an invaluable treasure trove of Darwin manuscript material, and I am grateful to David for facilitating my use of this wonderful resource. Randal has curated another invaluable treasure trove: thematic compilations of letters, manuscripts, and other documents bearing on Darwin’s varied research interests. Deepest thanks to Randal for so generously sharing his compilations, and, indeed, more—it is difficult to adequately express my gratitude to Randal and Zelfa Hourani for their warm hospitality during my visits to London. The Wissenschaftskolleg zu Berlin, where I spent a blissful fellowship year in 2012–2013, also helped advance our experiments project by kindly supporting Randal’s and Zelfa’s visit to Berlin, enlivening those short winter days with Darwin-inspired brainstorming.
Exploring Darwin’s “backyard”—Down House and environs, managed by English Heritage—is always inspirational. I thank the Down House manager and staff for kindly hosting our meetings, and especially gardeners Rowan Blaik and Kristyna Slivova for sharing their enthusiasm and knowledge of Darwin’s garden and grounds, and presenting Darwin’s experiments. One year my family and I spent a magical day with Randal and Irene and John Palmer exploring verdant Orchis Bank, dotted with beautiful orchids descended from those that so enthralled Darwin. Icing on the cake that day was a slow worm caught by the kids—also descended, I fancied, from ones that Darwin surely rejoiced over when his own kids turned one up. Many thanks to Irene, warden with the Kent Wildlife Trust, for sharing her deep knowledge of orchids and Orchis Bank.
I have been gratified by the enthusiasm and encouragement of several friends and colleagues for the idea of the pedagogical—and inspirational—value of Darwin’s experiments. Kefyn Catley, Louise Mead, and I co-taught the Evolution in the Blue Ridge course for science educators at Highlands Biological Station in the summer of 2011, in which we used Darwin’s experiments as a new approach to teaching and learning about Darwin and his method. We learned as much from the teachers participating in the course as they learned from us: thank you Megan Cassidy, Tom Copeland, Alyssa Fuller, Jena Gladden, Hannah Grimm, Paula LaPoint, Mark Lonac, Randi Neff, Gloria Painter, Leslie Schoof, and Susan Steiner.
Special thanks to Leslie Costa for her comments, corrections, and suggestions on multiple earlier drafts of the book; our children Addison and Eli for conducting some of the experiments; Mac Callaham for assistance with the earthworm chapter; Nuala Caomhanach for sharing ideas and enthusiastically developing the first Darwin’s Dangerous Experiments program at the AMNH; and Richard Milner for kindly providing the Beagle illustration. The wonderful librarians at Western Carolina University’s Hunter Library were ever-ready to assist with all manner of literature-hunting, and I would be remiss if I failed to acknowledge too th
e tremendously useful online resources of the Darwin Correspondence Project, Wallace Correspondence Project, and Biodiversity Heritage Library.
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