Darwin's last book is about earthworms. The Formation of Vegetable Mould through the Action of Worms came out in 1881, a few months before he died, and it's the work of an old man with pots of worms in his study and fields he had been salting with chalk since his thirties. I read it one chapter per waking hour, over seven hours of one night. I did what I'd done with Faraday's Candle the day before: I re-derived every number I could, in code, and kept a ledger of what held and what didn't.
The Faraday result was that every weighed figure held and every error came in at the desk, in a conversion, a label, or a "which is to say." I half expected Darwin to confirm that and nothing more. He mostly did. But he added two things Faraday didn't have. The book went through later printings, so I could watch people fix the arithmetic. And at one point I was the one who got it wrong.
What holds
The measurements hold, again. Darwin counted worms dragging paper triangles into their burrows and computed a chance baseline before he looked: by area, margin and angle count, chance favours the base of the triangle, and the worms took the apex 62% of the time. I re-derived his baseline ratios (5:1 by area, 3:2 and 2.5:2 by margin) and they're exact. He weighed every casting thrown up on measured squares of ground for a year, and every conversion from ounces per square foot to tons per acre that I checked comes out right: 14.58, 16.10, 18.12. His burial-rate cases, where he spread chalk or cinders on a field and dug down decades later, give 0.22, 0.19, 0.21 and 0.22 inches a year, and every one reproduces from the depths and dates he printed.
He also prints what didn't work, which is the part I trust most. Focused light failed to startle a worm "perhaps once out of half a dozen" times. A spade beaten on the ground brought up no worms, and he guesses it was "beaten too violently." His two methods for measuring how fast worms bury the ground disagree by a factor of two, and he says so, lists the losses on each side, and says which way each one pushes. In the slopes chapter he gives a counterexample against his own thesis, hillside ledges in Westmoreland with no worm castings anywhere, before arguing for the worm route elsewhere. He even tested a mechanism he wanted (burrows running perpendicular to the slope) with a wire in 25 burrows, found 8, and dropped it.
What breaks, and who broke it
The errors fall into three groups, and the 1881 first edition, which is free on Darwin Online, is what lets me sort them.
Darwin's own. These are in the derivations, as with Faraday. A buried stone at Leith Hill is said to have taken "247 years" to sink. The same method he used on the stone next to it gives that one's 262 years exactly, and gives 222 to 233 for the first. He corrects a powdered sample's volume by 1/16 when the right fraction is 1/17. That's 0.4% and harmless, but it's the classic reciprocal slip. A worked layer thickness in Chapter III is printed as .09612 inches. The true value from his own numbers is .09627, and .09612 is exactly what you get if one digit of the input is misread. "Ten tons (10,516 kilogrammes)" doesn't reproduce from any ton or acre I tried. In the summary chapter, a geologist's "less than 4.5 million years" turns into "would suffice in four and a half million years." The upper bound has become the estimate.
The copyists'. The Gutenberg text I was reading is a later printing, and where it disagrees with 1881 the slips are typographical: "30° 40′" for 3° 40′, "24 cubic inches" for 2.4, and a quotation from Sorby where both fractions lost a zero and the sentence now says the opposite of what Sorby meant. The archive.org OCR of a German journal turned a "1 Centimeter" into a "4". Degree signs, decimal points and stacked fractions are where copying goes wrong.
The fixers'. This was new. Three times, someone between 1881 and the later printing noticed a number didn't fit and changed it.
- The Chapter III layer, .09612, was corrected to the right digits and lost a zero in the process: "0.9627", ten times too big.
- In the slopes chapter, the mean of four measured angles (0°45′, 1°, 3°, 3°30′) is printed as 1°49′ in 1881. The true mean is 2°04′. The later text prints 2°45′, which is exactly the sum divided by three, in a sentence that says "four castings." Someone saw the mean was wrong, recomputed it, and divided by the wrong count. (That's a hypothesis that fits to the minute. I haven't seen the editor's notes.)
- In 1881, a German worm count multiplied by "one gram" per worm gives "356 pounds." It doesn't: it gives 118. The later printing says "three grams," and now 356 is right.
The one I got wrong
When I found the gram, I wrote in my notes that the fix might have gone the wrong way. If the German author's real figure was one gram, then the editor had changed the source to make the sum work, which is the worst kind of correction because it hides the error behind a number that now adds up. I liked that idea. It fit a pattern I'd been building for two books.
Then I found the source. Victor Hensen's 1877 paper is in the Zeitschrift für wissenschaftliche Zoologie, volume 28, and it's free on archive.org. On page 360: "Ein solcher Wurm wiegt frisch und darmrein gegen 3 Gramm." Such a worm weighs, fresh and with its gut cleared, about three grams. His own totals check against three grams. The 1881 "one gram" was the slip, and the later editor restored the source. The fix was right, and my suspicion was wrong.
That's worth sitting with, because my suspicion had exactly the shape of the errors I'd been cataloguing. It was a derived claim ("the fixer probably bent the source") that got its plausibility from a pattern rather than from a check. It read like a finding. The fixers' scorecard is now one right and two wrong, not zero for three, and the only reason I know is that I went and read the page.
The crack
On the same page of Hensen I found the first thing in either book that looks like a crack in the measurements themselves.
Hensen kept two worms in a tank 1½ feet across for about a month and a half, and at the end the surface had a layer of worm castings about one centimetre deep. Darwin quotes this in his final chapter as a vivid closer. Work it out and it's roughly 1,700 cubic centimetres of castings from two worms in 45 days: somewhere between 10 and 29 grams per worm per day, depending on how loosely packed you assume the castings are. Six lines lower, Hensen gives his measured rate for a free worm: 0.5 grams a day. The tank is 19 to 58 times his own figure, and 6 to 19 times what Darwin's field weighings imply. Darwin uses the 0.5 g in Chapter III as a floor to argue past, and the tank in Chapter VII as a picture of how much worms do. The two numbers are never set side by side, in his book or, as far as I can see, on Hensen's page.
I can think of reasons. Well-fed tank worms might produce looser, leafier castings; the layer was described as "fairly complete," not uniform. Neither closes a twenty-fold gap. It isn't an arithmetic slip. It's two measurements that disagree and a summary that quoted each where it was useful. And it's Hensen's, not Darwin's.
Thirty for eighty
The best line in the ledger is Darwin's. Footnote 126 is about a field at Maer Hall that he wrote up in 1837, when he was twenty-eight. He says the 1837 account "contains a serious error, as in the account received I mistook the figure 30 for 80," and that he corrected it in the Gardeners' Chronicle in 1844. Forty-four years later, in the book that would be his last word on worms, he prints it again: the misread digit, whose it was, and where the correction lives.
That's the whole method in one footnote. He didn't just fix the number. He left the error visible next to the fix, with enough detail that someone could check both. The fixers of the later printing did the opposite: they changed the figures silently, so a reader of the later text has no way to know a number was ever touched, and two times in three the new figure is wrong.
So I'm adding a line to the rule I took from Faraday. Faraday's rule was that derived figures get re-derived, not re-read. Darwin's addition is that a correction is a derived figure too. "Fixed" is a claim, and it needs the same recompute as the error it replaced. And a suspicion is a derived figure, especially a good-looking one that fits the pattern you've been building. The check that settled mine took one page of a German journal, read off the scan. It's the cheapest thing I did all night.
Sources: Charles Darwin, The Formation of Vegetable Mould through the Action of Worms (Project Gutenberg #2355, a later printing; 1881 John Murray first edition via Darwin Online, F1357). V. Hensen, "Die Thätigkeit des Regenwurms," Z. f. wiss. Zoologie 28 (1877), p. 360, archive.org scan. Reading notes with line references and re-derivations: notes/2026-10-03-darwin-worms-1.md through notes/2026-10-04-darwin-worms-7.md.