For most of his adult life, Charles Darwin was mulling over the earthworm question. In the years before his famous adventure on the H.M.S. Beagle, he began investigating how these creatures creates soil-enriching mold during their digestive process. Off and on in the decades after that, he continued to ponder earthworm behavior and publish papers about it. And finally, in the last few years of his life, Darwin set down everything he'd learned about earthworms over 44 years — all his experiments, observations, and somewhat tedious labwork — in a book called The Formation of Vegetable Mould Through the Action of Worms, with Observations on Their Habits. It was an instant bestseller.

Now, science journalist Virginia Hughes has read the e-book version and written about it on the science e-book review Download the Universe:

Some of his experiments are fun to read about, like when he exposed his potted household worms to the noise of a metal whistle, a bassoon, piano banging, and shouting, all to prove that the critters were deaf. Other observations are not so fun, like the 23 pages describing which end of a leaf a worm pulls into its burrow (for English plants: 80 percent were tugged from the tip, 9 percent from the base, and 11 percent from the middle). Even in the tedious sections, though, the narration has a satisfying intellectual payoff. For example, Darwin uses the worm's leaf-pulling methods-which are neither random nor instinctual-to argue that the animals have some level of intelligence.

After 86 pages of worm habits, Darwin finally gets into the meat of the theory, describing in detail the soil observations that he made at his uncle's house and in the decades since. The next chapters are more historical and thoughtful, asking how worms may have played a part in the "burial of ancient buildings" and the "denudation of the land." It may have been no coincidence that Darwin chose decomposition as his final scholarly subject. By that time he was old, sick and beginning to talk a lot about his own death. He died in April 1882, six months after Worms was published.

While slogging through the book, I kept wondering how it could have been so popular, selling thousands of copies within weeks. Not only that, but Darwin apparently received a lot of fan mail. Readers sent him all sorts of their own stories and questions about earthworms.