The Last Harvest: The Genetic Gamble that Threatens to Destroy American Agriculture.

Readers who peruse the recent bestseller The Hot Zone, whose flap copy suggests that a frightful virus accidentally set loose by a government lab near Washington, D.C. will "kill 9 out of 10 people" in the area within days or even hours, may notice that by the end of the tale - ah, no one's dead. The Hot Zone became both a hot book (deservedly: it's a good read) and the inspiration for two big-studio movies despite the fact that the very thing it concerns, the unstoppable runaway of a virus worse than bubonic plague, did not happen.

Paul Raeburn faces the same problem - writing about something that hasn't happened - in his new book The Last Harvest, and handles it well. Where The Hot Zone had Hollywood-esque aspects such as calls to the President and commando teams wearing biological protection suits, The Last Harvest has assistant professors in beat-up jeeps looking for potato roots in Mexico. Yet the disaster of which The Last Harvest warns is in some ways more compelling than an outbreak of a killer disease. It is Raeburn's thesis that modem farming, based mainly on cloned seed groups from a comparatively narrow genetic background, has put American agriculture in danger of "catastrophic losses" for which there may be no immediate antidote.

Raeburn, the science editor of the Associated Press, relates many chilling stories of the narrowing genetic base of modern farming. Fifty years ago, he writes, the Texas wild rice strain called Zizania texana was common around the San Marcos River near San Antonio. But development has altered most of the natural habitat for Z. texana, a plant some agronomists think holds tremendous genetic potential. Attempts to preserve the plant by breeding it away from the San Marcos River have not been successful.

This is a problem some researchers call "genetic erosion." Plant breeders need wild genes to generate crosses when a new blight or insect attacks crops. Though doomsday estimates for world species loss are almost certainly exaggerated, they need only be a little right - far less than half right - to mean that wild genes for plants are "eroding" at an alarming pace. Western agriculture is increasingly based on seeds cloned from a narrow inventory. For example, roughly 40 percent of the corn grown in the U.S. is now based on seeds cloned by a single company, Pioneer Hi-Bred. Use of cloned seeds has caused no meaningful problems so far, while helping bring about the unprecedented contemporary farm productivity that is, for the moment at least, keeping the world fed. But the fewer seed types in use, the greater the danger for genetic erosion.

Raeburn documents this problem well. It is to be admired that he has spent so much time producing a well-written, accessible book on the sort of seemingly esoteric problem often ignored until it becomes an emergency. My main objection to The Last Harvest is that it makes short shrift of genetic engineering, which may offer a counterbalance to narrowing agricultural gene pools. Gene engineering has produced no crop yield increases so far, but researchers have been looking at this technique for only about a decade.

And I wish Raeburn had made clearer the link between his work and this year's debate on the reauthorization of the Endangered Species Act. Most current research tends to suggest that plans for general habitat protection make more sense than the current system of lawsuits to pick and choose species for special safeguards. With elements of the Right now out to shred the Endangered Species Act and elements of the Left still clinging to the cumbersome species-by-species litigation approach, the progressive position is to advocate a new system based on general habitat preservation, yet allowing for natural species variations (including extinctions) within habitats. Raeburn's excellent The Last Harvest provides one of the many reasons such a system could be superior to present law.

Gregg Easterbrook is a contributing editor of The Atlantic Monthly, Newsweek, and The Washington, Monthly.