Gardening columnist Stewart spends far more time digging through soil than most of us, so she has a keen eye for the delightfully nuanced behavior of the earthworm. ''I should probably leave the worms in my own garden undisturbed, but I can't resist,'' she writes. ''Earthworms are the custodians of the planet.'' She's not kidding. Using Darwin's research as a starting point, she delves into their incredible abilities and offers useful tips for the green thumbs in her audience. Did you know that wasabi and mustard ward off worms? Or that, given millions of years, their dutiful work can refill a strip mine? And what about their jarringly familiar mating habits? ''When a worm is looking for a sexual partner, its primary criteria is length.''