A review of Charles Darwin's wonderful final book, ‘The Formation of Vegetable Mould, Through the Action of Worms, with Observations on their Habits’.
Charles Darwin describing an earthworm taking a crap:
A worm after swallowing earth, whether for making its burrow or for food, soon comes to the surface to empty its body. The ejected earth is thoroughly mingled with the intestinal secretions, and is thus rendered viscid. After being dried it sets hard. I have watched worms during the act of ejection, and when the earth was in a very liquid state it was ejected in little spurts, and when not so liquid by a slow peristaltic movement. It is not cast indifferently on any side, but with some care, first on one and then on another side; the tail being used almost like a trowel. As soon as a little heap is formed, the worm apparently avoids, for the sake of safety, protruding its tail; and the earthy matter is forced up through the previously deposited soft mass. The mouth of the same burrow is used for this purpose for a considerable time.
This is pure Darwin: exquisite observation of the apparently trivial. Although his great theory of evolution by means of Natural Selection is central to our understanding of life's grandeur, Darwin had something of a soft-spot for the lowliest of creatures. He spent eight years studying barnacles; investigated how bees form honeycombs; and even took time to observe ants when he was supposed to be convalescing at his favourite hydropathy establishment.
But Darwin knew it was important to sweat the small stuff. As he is quick to point out in the introduction to his final book, The Formation of Vegetable Mould Through the Action of Worms:
the maxim “de minimis lex non curat,” [the law does not care about trifles] does not apply to science
Although, at face value, his earthworms book might seem charmingly eccentric, bordering on worm-obsessed at times, Darwin had a half-hidden agenda. Responding to a Mr Fish, who, writing in the Gardeners' Chronicle, had dismissed Darwin's earlier hypothesising about the contribution made by earthworms to the formation of the topmost layer of soil—the ‘vegetable mould’—Darwin writes:
Here we have an instance 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.
Darwin's theory of evolution, like his great friend and inspiration Charles Lyell's uniformitarian theorising about geological change, relied on the accumulation of a large number of small changes taking place over a very long time. Many critics argued (as some religiously motivated critics still do) that the small changes we can still observe taking place today (the movement of sand particles down a river, say, or the slight physical variations in an animal's offspring) do not have sufficient power to bring about far greater changes that we cannot observe directly over geological/evolutionary timescales (the formation of the Grand Canyon, say, or the evolution of new species). Darwin's earthworm book demonstrates small change writ large. If the humble earthworm, burrowing unnoticed beneath out feet every day, can, over mere tens or hundreds of years, alter our physical landscape, and bury beyond sight our ruins, who can deny what cumulative, small changes are capable of achieving over much longer timescales?
Small change writ large was a recurring theme in Darwin's work. In this, he was heavily influenced by Lyell's Principles of Geology, which he first read during the Beagle voyage, later writing to a friend:
I have always thought that the great merit of the Principles, was that it altered the whole tone of one's mind & therefore that when seeing a thing never seen by Lyell, one yet saw it partially through his eyes
In one of his first scientific papers, presented at the Geological Society in 1837, less than a year after his return from the Beagle voyage, Darwin correctly explained how the various forms of coral reef form slowly over time as the nearby land subsides. Later that same year, again at the Geological Society, he presented a paper ‘On the formation of mould’, in which he credited his maternal uncle and future father-in-law, Josiah Wedgwood II, with having drawn his attention to how various substances which had been spread over Wedgwood's fields years earlier were now to be found buried beneath several inches of soil—a phenomenon that Wedgwood attributed to the action of earthworms. It was an observation that triggered Darwin's long-standing interest in worms; an interest which was to culminate, over four decades later, in his final, hugely entertaining book.
As we have already seen, part of the joy of Darwin's earthworms book are his meticulous descriptions. Here he is, for example, describing a worm burrowing:
the worm inserts the stretched out and attenuated anterior extremity of its body into any little crevice, or hole; and then, as Perrier remarks, the pharynx is pushed forwards into this part, which consequently swells and pushes away the earth on all sides. The anterior extremity thus serves as a wedge.
and here he is describing worms sucking on ‘broad flat objects’:
The pointed anterior extremity of the body, after being brought into contact with an object of this kind, was drawn within the adjoining rings, so that it appeared truncated and became as thick as the rest of the body. This part could then be seen to swell a little; and this, I believe, is due to the pharynx being pushed a little forwards. Then by a slight withdrawal of the pharynx or by its expansion, a vacuum was produced beneath the truncated slimy end of the body whilst in contact with the object; and by this means the two adhered firmly together.
Darwin wasn't just a wonderful observer; he also loved to carry out what he self-deprecatingly referred to as ‘fool's experiments’. His earthworms book describes some characteristically surreal examples. Who else but Darwin would get his son to play the bassoon to pots of earthworms to establish whether they could hear? And who else would then, for good measure, get his wife, who had received tuition from none other than Frédéric Chopin in her youth, to play the piano to them? Then there were the tiny triangles of paper, representing leaves, that Darwin presented to his worms to assess their intelligence. The fact that they more often grabbed the triangles by their most sharply pointed corners, thereby making it easier to drag them down into their burrows, is an indication, Darwin claims, that they are more intelligent than we generally give them credit for.
Darwin dedicates a considerable proportion of this book to estimating the amount of soil shifted by worms, be it in levelling fields, eroding landscapes, or burying ancient monuments. The former archaeologist in me shuddered to read how he had arranged for a hole to be dug alongside one of the fallen ‘Druidical’ stones at Stonehenge to assess how deeply they had sunk into the soil, having been undermined by worms. He and his sons paid similar visits to recently excavated Roman villas and other ancient sites.
Darwin rounds off his last book with a typical Darwinian flourish, reminiscent of his justly famous closing ‘entangled bank’ paragraph from On the Origin of Species, in which he returns to his half-hidden agenda of small change writ large:
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 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. 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. Some other animals, however, still more lowly organised, namely corals, have done far more conspicuous work in having constructed innumerable reefs and islands in the great oceans; but these are almost confined to the tropical zones.
Darwin's The Formation of Vegetable Mould, Through the Action of Worms is a wonderfully entertaining book, and a worthy final volume from our greatest naturalist. Highly recommended.
- Darwin, C.R. (1837). On certain areas of elevation and subsidence in the Pacific and Indian oceans, as deduced from the study of coral formations. [Read 31 May] Proceedings of the Geological Society of London 2: 552-554. Available at Darwin Online
- Darwin, C.R. (1838). On the Formation of Mould. [Read 1 November, 1837] Proceedings of the Geological Society of London 2: 574-576. Available at Darwin Online
- Darwin, C.R. to Horner, Leonard (29 Aug ). Darwin Correspondence Database,
https://www.darwinproject.ac.uk/entry-771 accessed on Fri Oct 16 2015.
- Darwin, C.R. to Hooker, J.D. (6 May ). Darwin Correspondence Database,
http://www.darwinproject.ac.uk/entry-2269 accessed on Fri Oct 16 2015.
- Darwin, C.R. (1881). The Formation of Vegetable Mould, Through the Action of Worms, with Observations on their Habits. London: John Murray. Available at Darwin Online
- Healey, E. (2002). Emma Darwin: the inspirational wife of a genius. London: Review Books.