Scientist of the Day - Charles Darwin

Stone at Stonehenge, partly sunk into the ground, wood engraving in The Formation of Vegetable Mould, through the Action of Worms, by Charles Darwin, p. 156, 1881 (Linda Hall Library)

Charles Darwin, 72 years of age, published his last book on Oct. 10, 1881. The Formation of Vegetable Mould through the Action of Worms.  It would be easy to dismiss the work as a trivial exercise by an old man who had run out of original ideas to pursue. You would be excused for thinking that, but you would be dead wrong.

Title page, The Formation of Vegetable Mould, through the Action of Worms, by Charles Darwin, 1881 (Linda Hall Library)

Darwin wondered in this book where topsoil comes from (what he calls "vegetable mould").  His proposal was that it is the byproduct of the feeding habits of earthworms, who take in dirt, extract the bits of organic matter present, and excrete the remainder on the Earth's surface in the form of casts, which break down to form the mould in which plants love to grow.

Tower of worm castings found near Nice, wood engraving in The Formation of Vegetable Mould, through the Action of Worms, by Charles Darwin, p. 107, 1881 (Linda Hall Library)

Darwin demonstrated the inexorable nature of earthworm activity by observing what happens to objects left on the surface of the Earth over the course of years and decades. Stones slowly sink and disappear at a rate that he measured at about a quarter of an inch per year. He found a variety of ways to determine this rate. His uncle, Josiah Wedgwood, had strewn lime on a field at his estate at Maer, to increase its productivity. As it happened, the field went unused. Eight years later, some square holes were dug in the field, and about two inches down, there was a thin white line everywhere they dug, marking the descent of the lime.  He found other sites where building tiles from decades ago were found a foot or more below the surface, covered by a layer of vegetable mould. His proposal was that this fine surface soil was the inadvertent gift of earthworms, who ever-so slowly plowed and aerated the Earth's surface.

Darwin supported his hypothesis by determining that there are indeed enough earthworms down there – hundreds of thousands per acre – to recycle the Earth's surface soil, and he gathered many examples of objects that must have started out on the surface, like the bluestones at Stonehenge, which are being slowly swallowed by the Earth, thanks to the  lowly worm (first image).

Section from the ruins of the basilica at Silchester, with 9 inches of vegetable mound lying atop layers of building tiles and even concrete, wood engraving in The Formation of Vegetable Mould, through the Action of Worms, by Charles Darwin, p. 206, 1881 (Linda Hall Library)

All this is fascinating, and plausible, but why write an entire book about it?  Well, we guess that Darwin had more in mind here than worms and building tiles. His worm book is really a discourse on time – geological time, and evolutionary time.  Geologists like Charles Lyell had been arguing for 50 years that the Earth's rocks are slowly formed and reworked by small forces like erosion and sedimentation operating over eons of time, and Darwin himself had argued for the natural selection of small variations that accumulate over those same eons to form new species. But those slow changes were difficult to demonstrate to humans who, with their brief lifespans, cannot see what time can accomplish, if you have enough of it.

Enter the earthworm, which we can see at work, and whose rate of soil production we can measure.  This is a form of uniformitarianism we can observe and understand, and Darwin wants us to do just that, and then go back and read his Origin of Species, and Lyell's Principles of Geology, with a new appreciation of what time, given millions of years to work, can accomplish.

John Murray, the publisher of most of Darwin's other books, dutifully printed 3000 copies of Vegetable Mould, probably not expecting much in the way of sales, but wanting to accommodate an author who had made him a lot of money. As it happened, the first printing sold out quickly, and it went through two more printings ("fourth thousand" and "fifth thousand") before the year was out, and was regularly reprinted after that; Darwin's death and funeral in the spring of 1882 probably gave a boost to sales. 

Caricature of Charles Darwin and his recent book on worms, by Linley Samboune, in Punch, Oct. 22, 1881 (darwin-online.org.uk)

Just 12 days after Vegetable Mould was published, cartoonist Linley Sambourne published a caricature of Darwin and his earthworms in Punch (fifth image). Note the title on the spine of the book on the ground: Diet of Worms. That would have been a clever title for Darwin's book, although my guess is, he preferred the one he chose. 

We have, I believe, all the various editions, printings, and “thousands” of The Formation of Vegetable Mould in our rare book collections, including 5 American printings, some 18 volumes in all. Darwin would have been pleased that his last book was such a best-seller. We are happy that he had the extraordinary prescience to write it.

William B. Ashworth, Jr., Consultant for the History of Science, Linda Hall Library and Associate Professor emeritus, Department of History, University of Missouri-Kansas City. Comments or corrections are welcome; please direct to ashworthw@umkc.edu.