"It's his last book. He wrote in in 1881, the year before he died, and usually we expect that in old age, just before death, that a great scientist will write a pontificating philosophical treatise on the nature of reality. And Darwin... wrote a book on worms. ...He was interested in worms because they were... a metaphor for his larger world-view. The worms that slowly churn the topsoil of England... that work literally beneath our feet, that we never notice, that we think are insignificant because they're so small and lowly, are in fact producing the very soil that is the basis of agriculture. And therefore Darwin uses it as a metaphor for the importance of apparently tiny things when you extend them over long periods of time. And that's what evolution is, the extension of small change (to Darwin) over vast periods of time. So the worms become a metaphor for evolution and for the whole process of temporal change, a very fascinating book."