The Carson is Rachel and the ""factor"" is an awareness that in manipulating nature, we are dealing with life: a pretentious opening for a simple story that makes a point in spite of itself. In 1957, Oregon's Klamath Basin experienced the worst plague of field mice ever recorded in this country, and William Ashworth (Hell's Canyon, 1977; The Wallowas, 1978) describes mice overrunning houses; typical government hand-wringing; and a poison grain crusade that killed hundreds of geese instead of mice. His main concern, though, is why all this happened. It seems that after decades of intensive farming and irrigation, overly successful efforts to kill coyotes and other mouse ""predators,"" and overzealous spraying of crop-destroying insects, conditions were ripe when a drought hit the area in 1957 for parasite-free mice to breed, largely unchecked by their natural enemies. Moral: You tamper with one side of the balance of nature, and you must adjust the other or pay the consequences. If only Ashworth didn't throw in space-filling treatises on field mice (voles), coyotes, and predators around the world--plus what he calls ""a short digression into the disciplines of history and ecology,"" beginning with mouse troubles in medieval Europe. Would have been a great magazine article!