Rob Hardy: Darwin and earthworms

November 10, 2009 11:32:00 AM

Rob Hardy -


Charles Darwin''s name is so firmly linked with evolution that it is often forgotten that he was interested in specifics of biology. For instance, while he was fretting for 17 years over whether to publish about evolution, he was busy investigating barnacles. He was to publish an authoritative work on them. He also wrote about the geology he had seen on his travels in the "Beagle," and did experiments on whether eggs or seeds could travel the oceans to get to new lands. He was constantly busy on other projects, constantly enquiring and doing his own research simply because he had an exemplary curiosity.  


That his curiosity would fix upon lowly earthworms might seem a condescension from the man who had the great idea that is the principle of all biology, but Darwin thought earthworms were important. "Worms have played a more important part in the history of the world than most persons would at first suppose," he writes in "The Formation of Vegetable Mould through the Action of Worms, with Observations on their Habits," which was published in 1881, Darwin''s last book (he wanted to get out his book on earthworms, he joked, "before joining them").  


For this sesquicentennial of Darwin''s birth, I wanted not to read his most influential works again, but to take up "Worms" just to listen to Darwin the natural historian writing about a subject he obviously loved and which can never inspire condemnation from religious enthusiasts. The book is a delight. Always serious, and often with the stuffiness that is simply to be expected from scientific writing of the 19th century, Darwin is keen on earthworm behavior, even making informed speculation on earthworm mentality. The earthworm may be tiny, but Darwin brings up the next fact or the next experiment, one after another, to show how worms have affected geology, his other great interest. 


Earthworms had been a longstanding interest for Darwin. In 1837, he had read to the Geological Society of London his paper on the role of earthworms in soil formation. Some of the experiments, too, required long times of monitoring. Darwin had a field near his home spread with broken chalk in 1842, and dug it up 29 years later to see what had happened to the chalk layer. The chalk nodules did subside under the mould cast up by the worms, at a rate he could calculate at 0.22 inches per year. He went on an excursion to Stonehenge to see how its monoliths had subsided into the soil. By digging in the earth, the worms create voids, which a huge stone''s weight gradually compresses; and also they cast up soil around the stone, so that it sinks. He reports on the visit by telling of the measurements he took, always writing in the objective observer''s third person.  


The weight, however, did not seem to him to make a difference; he noted that bits of cinder, chalk and quartz, all of which have different specific gravities, all seemed to sink to the same depth at about the same rate. Everything left on the ground, if the ground has earthworms, is affected the same way. "Archaeologists are probably not aware how much they owe to worms for the preservation of many ancient objects," Darwin writes, but he takes pains to distinguish such action from the "enormous beds of rubbish, several yards in thickness, which underlie many cities, such as Rome, Paris and London, the lower ones being of great antiquity" because city rubbish has not been acted upon by worms. Darwin always admired the ability of the earthworms to sift the soil; he wrote, "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." In a larger view, Darwin looked at the geological processes which worms cause, the denudation (removal of disintegrated rocks and soil to lower levels). The land may be sculpted by the sea, volcanoes and earthquakes, but the little earthworms played a big role, too. 


Darwin''s delightful experiments with individual earthworms are fun to read about. Some people like to have an aquarium, but Darwin had pots of earthworms so he could experiment on them. They have no eyes, and he thought that they could not respond to light, but found that somehow worms knew to withdraw from a bright light shone for a long while. They do not have any sense of hearing, as this whimsical passage makes clear: "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 notes of a bassoon." They were indifferent to the piano, except if their pots were placed directly upon it, whereupon the musical vibrations would be sent through the solid pot and soil, causing them to retreat into their burrows instantly.  


They are, of course, sensitive to touch. They were indifferent to the scent of such things as tobacco juice or perfume (although upon the nearby application of acetic acid, "the worms appeared a little uneasy"). They did have a preference for onions and cabbage leaves, and Darwin judged that their eagerness for certain foods meant they enjoyed the pleasure of eating. They dug burrows and pulled leaves into them, and Darwin was astonished to find that they did this in a methodical and even intelligent way. He gives a table of how leaves of different plants or triangles of paper were drawn into earthworm holes, statistically showing that the worms deliberately went for the narrow part of the object to pull in, rather than trying to get a blunter part in first. He gives evidence that although pulling leaves into a burrow might be instinctually commanded, the earthworms are capable of showing some degree of intelligence in how they orient the leaves for the job. 


Among Darwin''s other great gifts was his ability to imagine changes over geological time. Evolution is mentioned exactly once in this volume. He takes note of the words of a critic who could not believe that earthworms had done as much work as Darwin proposed because they were too small and weak and the work was too stupendous. "Here we have an instance," said Darwin, "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." Clearly Darwin had no such inability. I have no doubt that researchers have since his time brought out bigger and more specialized volumes on the earthworm, and that earthworm science has greatly advanced in the century-and-a-quarter since this book came out. This one, however, is a delightful marker of the beginning of the scientific valuation of the earthworm, and a reminder of the broad yet deep interests of its accomplished author.  

