The last book written by Charles Darwin, one year before his death, is, as he himself confused it, on an insignificant subject. But he was writing back to a discovery made shortly after his voyage on the Beagle about the effects of earthworm activity, which was described as "a new geological force." In the more than forty years since that discovery, he collected data in his characteristic way about the importance of earthworms in soil erosion, soil fertility and the preservation of ancient buildings. And he delved into the intelligence of worms. Contrary to his expectations, the book was a great success. In the months after publication, more copies were sold than his 'Origin' when it was published!