The blue locomotive of the Great Eastern Railway streaked through the countryside of Cambridgeshire. To a farmer nearby, the train s cars was a rumble of teak and steel plowing through his fields, where seedlings of barley, wheat, and oats etched their own green tracks in the springtime loam. It was early May in 1900, and the earth, like the new century itself, pulsed with possibilities.

Among the train s passengers was William Bateson, a don at St. John s College, Cambridge. Bateson, who was a zoologist, was stoop-shouldered and large. His tweed vest strained at the buttons, his handlebar mustache gleamed only his droopy eyes saved him from looking self-satisfied or smug. He had just turned forty, and was one of Britain s chief combatants in the controversy over evolution and the theory of natural selection, still the source of strident debate more than forty years after Charles Darwin first proposed it.

When Bateson boarded in Cambridge, he had no idea that in the next sixty minutes he would read a paper that would change the course not only of his own career, but of mankind s understanding of its place in the great cacophony of nature.

Out the windows of Bateson s velvet-and-leather compartment were mazes of hedgerows to the left, a pretty little river to the right. A tan stucco pub, looming beyond a hillock just past Harlowtown, marked roughly the halfway point on the familiar trip from Cambridge to London. But, according to the legend that has persisted for a full century, Bateson spent most of that train ride immersed in an old article from a small journal out of Austria. He was not gazing idly at the scenery.

The article, written by an obscure Moravian monk named Gregor Mendel, described the elegant botanical experiments Mendel conducted in a modest monastery garden in the old Hapsburg empire of Austria. Mendel had painstakingly crossed and back-crossed pollen and egg cells from the common pea plant to reach a better understanding of inheritance. After working on peas and other plant species for seven long years, he recorded and analyzed his findings in a two-part lecture delivered in 1865. That lecture was published as a forty-four-page journal article and then was all but ignored for the rest of Mendel s life.

What brought Bateson to that journal article on the morning of May 8, 1900 was the work of three other scientists, one of them the subject of his lecture that very afternoon. All three had cited Mendel s forgotten paper almost simultaneously in their own separate publications. Uncannily, like a field of oat stalks that somehow know to erupt in unison, all three articles had appeared within two months of each other, during the same strange spring of 1900.

As he read, Bateson realized that what he was trying to do in his own experiments was almost precisely what Mendel had already done thirty-five years before. He was both shocked and elated. As his wife put it, using a metaphor that prettily evoked Mendel's garden, it was as though, "with a very long line to hoe, one suddenly finds a great part of it already done by someone else. One is unexpectedly free to get on with other jobs."

By the time the Great Eastern Railway train pulled into Liverpool Street Station, Bateson knew he would have to rewrite the paper he was about to deliver. As he pushed through the crowd in search of a carriage to Buckingham Gate, he re-thought the afternoon s lecture to the Royal Horticultural Society on the topic of "problems of heredity as a subject for horticultural investigation." He had planned to focus on the work of Hugo De Vries, the great Dutch botanist whose new "mutation theory" could account for the large-scale variations that Bateson believed were necessary to propel Darwin s natural selection, the underlying mechanism of evolution. But now Bateson was suddenly more interested in describing the work of this unknown monk, whose findings resonated so beautifully across the span of thirty-five years and the eight hundred miles separating London from the hilly recesses of southern Moravia.

Settling into a carriage, absent-mindedly fingering his vest to be sure it was still buttoned his wife accused him of being so indifferent to his attire that he would wear gardening clothes to town, and town clothes in the garden Bateson began to mull over his opening lines. How should he introduce this forgotten genius to the English-speaking world?

In a drafty space in Drill Hall situated along the curving street known as Buckingham Gate, Bateson gave the lecture that for the rest of his life would demarcate a turning point in his evolution as a scientist. "An exact determination of the laws of heredity will probably work more change in man s outlook of the world, and in his power over nature, than any other advance in natural knowledge that can be foreseen," he began. "There is no doubt whatever that these laws can be determined."

Bateson spoke for more than an hour. Whatever the exact words he used that afternoon all we have now is a text printed two years later, no doubt edited and amended to include more references to Mendel we can surmise, based on a report published that week in the RHS s official journal, Gardeners Chronicle, that there was not much discussion. But the die was cast, either on May 8 or shortly thereafter. William Bateson had aligned himself irrevocably with the legacy of Gregor Mendel.

Within a few more years, Bateson saw how far the sweep of Mendel s contribution extended. He made a pilgrimage to Brünn, the town where Mendel lived and worked; had Mendel s paper translated into English; coined the word genetics; and became the chief apostle of a new scientific discipline that represented the very apotheosis of the twentieth century. He became embroiled in a scientific controversy that pitted him against some of the greatest biologists of his day, including one who had been his best friend when they were both undergraduates at Cambridge. Indeed, the controversy would become so bitter and so personal that, when this former best friend died unexpectedly in 1906, some accused Bateson of killing him.

So much about gardening feels like a metaphor. Take weeding. The ubiquity of the weeds, their thorny tenacity, the hardiness of their buried roots -- all seem to symbolize the pitfalls of life itself, the temptation of settling for the superficial fix when we know that deep-seated problems will just re-erupt later, or elsewhere, in other and hardier forms. It makes sense, then, to look to the garden for metaphors regarding who we are, who our ancestors were, and where we and our descendents are headed.

Part of the allure of Mendel as a hero of modern science is that we can picture him puttering in his garden, seeking answers to universal questions in his crops of peas. To some extent, Mendel s story is primarily the story of a gardener, patiently tending his plants, collecting them, counting them, working out his ratios and calmly, clearly explaining an amazing finding and then waiting for someone to understand what he was talking about. It is the story of a gentle revolutionary who was born a generation too soon.

The myth that has grown around Gregor Mendel mirrors our contemporary understanding of scientific progress, discovery, and the nature of genius. It casts him as a tragic figure whose brilliance was unappreciated in his own lifetime. The legend is a familiar one think of the creative geniuses who died unrewarded, from Melville to van Gogh and it resonates reassuringly for those of us who also feel our brilliance goes unnoticed. The story is this: Mendel worked tirelessly in his garden for seven years, presented his findings of "certain laws of inheritance" during a two-part lecture in the winter of 1865, and then passed into scientific obscurity only to have his work "rediscovered" and resurrected simultaneously by three different scientists (one of whom was the Dutchman Hugo De Vries), working in three different countries, in the spring of 1900. The explanation usually given for this curious turn of events is that the world wasn t ready for Mendel s laws in 1865, and by 1900, it was.

But like a vine-encrusted garden path of crumbling bricks, the myth has been picked apart and slowly dismantled. Mendel was not even looking for the laws of inheritance, some scholars now say; he was just trying to find a way to breed better, more reliable flowers, fruits, and vegetables. His work did not fall into obscurity, say others; it was cited no less than twenty-two times, sometimes in prominent publications, before its raucous rediscovery thirty-five years later. And, most damaging to the traditional story, a few believe the priest was not a genius at all just a conscientious amateur botanist with a special talent for cross-breeding who, like so many of us, stumbled into nearly every good thing that happened to him: his university training; his membership in a community of scholarly monks; his affiliation with a progressive scientific society; his position as high school teacher, for which he never earned certification; even his obscurity, which allowed him to persevere in his crossbreeding far longer than he might have with a reputation to uphold. The Mendel legend, say these revisionists, was created by biologists who in 1900 were already locked into a heated debate about the mechanism and pacing of evolution. Those who, like Bateson, needed Mendel s laws to support their position turned a modest, meticulous, clever, and rather lucky monk into a scientific giant.

The truth, as it so often does, lies somewhere in between.

Impressions of Mendel seem to have come full circle, from the original revisionism of thirty or forty years ago to the more respectful attitudes of today. The question now is not so much whether the man was a genius, but where exactly his genius lay. He seems to have been the plodding, hardworking, single-minded kind of genius, the one for whom discovery was, as Thomas Alva Edison put it, "one percent inspiration and ninety-nine percent perspiration," and not the playful, intuitive genius that a man like Picasso was. The great painter once said, "I do not seek I find," an attitude that describes many of the men and women we now think of as geniuses.

In the conventional use of the word, genius is something you are born with, something that sets you apart from ordinary humans with our typically linear, categorical way of looking at the world. Mendel s genius was not this flamboyant, touched-by-an-angel kind. He toiled, almost obsessively, at what he did. But still he had that extra one percent, that inspiration that helped him to see his results in a way that was just slightly askew. This flash of insight even if it follows long, dull stretches of routine labor is what made Mendel great. It allowed him to perform a feat of genius: to propose laws of inheritance that ultimately became the underpinning of the science of genetics. Even if he as a man was subsequently over-lionized by men with their own agendas, even if he was not in fact the heroic father of genetics he was made out to be, that should not diminish what Gregor Mendel truly was: a man with a vision, and the dedication to carry it to its brilliant, radical conclusion.

To tell the story of Mendel s life and intellectual flowering requires some educated deduction. We have so little real, specific information about him barely more than three short published papers, seven letters to a botanist in Munich, and a brief autobiography written when he was twenty-eight years old. Almost nothing exists that puts Mendel in his garden, or his monastery flat, or his church, or his beloved orangery on any particular day.

This is, in certain ways, a good thing. "We are lucky to have limited information and are completely free," said one scientist. "We can speculate to our hearts content because nobody can say we are wrong. They can only say, `I do not agree with you. " What it means for this book is that we need occasionally to indulge in speculation not quite "to our hearts content," because this is not a novel, but more than most nonfiction writers are accustomed to. We have no way of knowing with certainty what our protagonist was doing or thinking at any particular time. But, based on circumstantial evidence and the sifting through of options, we can tell his story in the way it most probably occurred.

Most of the myth-making about Mendel was a direct outgrowth of the bitter fight between Bateson and the Mendelians on the one hand, and Bateson s erstwhile best friend and the so-called Biometricians on the other. Both sides were playing for the highest stakes: the right to claim a truthful insight into the workings of the natural world. What they uncovered turned out to be the foundation of a science that has taken us to the very brink of human possibilities.

What Mendel provided was the observation that traits are inherited separately, and that some characteristics that seem to be lost in one generation crop up again a generation or two later, never having been lost at all. He gave us a theoretical underpinning for this observation, too: that the traits were carried as discrete, individual units that passed from parent to offspring in a matter that was consistent, predictable, and mathematically precise.

In Mendel s wake followed a steady string of discoveries: that these hereditable units can be found in the genes, which in turn are found on the chromosomes, which are in turn found in the nucleus. By the 1940s, scientists knew that the meaningful information of the genes was packed into a molecule called DNA; by the 1950s, they could build a physical model of the DNA molecule, and interpret the code through which DNA talks to the cell. The intervening years have been consumed with using that code to see where genetic disorders originate, and to map out, in order, all the pieces of DNA that fit together to make up entire organisms: first a bacterium, then a worm, and in quick succession a fruitfly, dog, rat, plant (one of the simplest species, Arabidopsis thaliana), and ultimately a human.

As this knowledge, especially about the human genome, has unfolded, geneticists have started to tinker with our natural inheritance. Sometimes the tinkering has been disturbing, as with the eugenics movement that advocated selective breeding to improve the gene pool by prohibiting misfits from marrying; the drive for human cloning, which would enable people most likely the richest ones first to duplicate their own, younger identical twins; and research into genes that supposedly carry the traits a society values, and for which it would pay dearly to pass on to the next generation: tallness, thinness, or protection against going bald.

The same impulses that sparked eugenics, cloning, and the search for the ideal child also have led to some of the worst atrocities of the twentieth century. Barely thirty years after the word "genetics" was coined, Adolf Hitler, a product of the same region of the world as Gregor Mendel, was coalescing a new Nazi party and mastermiinding a massive genocide program that would be his "final solution." Hitler s holocaust cast its long, ferocious shadows into the end of the century as well, bringing forth "ethnic cleansing" campaigns in Rwanda, for instance, and the breakaway regions of the former Yugoslavia. We have never gotten far from the belief that our genes are our destiny.

At the same time, of course, the tinkering made possible by the century s explosion of scientific knowledge has been nothing short of miraculous. How astounding that we can now screen for defective genes through DNA analysis, help build families through genetic counseling, exchange good genes for bad through gene therapy. And how awesome as well as morally complex that we can soon, if we want to, manipulate the fate of future generations, so that every newborn is guaranteed to be, at least as geneticists define it, perfect.

Nearly a century after the ferocious debate over Mendelism that set the stage for contemporary genetics, almost every part of our modern understanding of how the world works the relationship between parents and offspring, the delicate interplay between identity and individuality, the underpinnings of nature and the commonalties among all living things -- can in large measure be traced back to that startling spring of 1900, when anything was possible.