The world is doomed—except when it's not. In fact, the world is roaring on toward brightness, getting better and better—except when it's not.

In other words, we really do intuit one sure truth: We are careening toward disaster. And yet, we really do intuit another sure truth: Things are better than they've ever been. On one side we find the angry sibyls, howling of our fall into abyss unless we live small. On the other side we see the happy crystal-gazers, nattering of our rise to the heavens if only we reach for the stars.

The science writer and historian Charles C. Mann calls them prophets and wizards, and in his latest book, he looks at a pair of them from the 20th-century—a pair who symbolize the enduring modern tension between the convincing rightness of the Chicken Littles and the obvious correctness of the Pollyannas. His happy Pollyanna is Norman Borlaug, the Iowa farm boy who jumpstarted the Green Revolution that transformed agriculture across the globe. His sad Chicken Little is William Vogt, the author of the influential 1948 environmentalist tract The Road to Survival. Together, they form The Wizard and the Prophet of Mann's title—the figures on either edge of the curious chasm, progress and decline, that slashes through the middle of the modern worldview.

The excellence of The Wizard and the Prophet shows most of all in Charles Mann's sense of detail and story. He wants us to like both these men, to appreciate their strange biographies, regardless of whether we feel personally drawn to one side or the other of the perennial debate that they represent. The weakness of The Wizard and the Prophet . . . well, interestingly, that derives from the same source. Mann wants us to like both these men, and despite his gift for incisive phrasings, he cannot bring himself to use much sharp criticism on his central figures.

After a pair of opening chapters of biography for Borlaug and Vogt, The Wizard and the Prophet examines the consequences of their work in four fields: food growth, water distribution, energy production, and global climate. (The attempt to match these to the four classical elements—earth, water, fire, and air—gives the book a more confusing architecture than it needs.) Mann conducted a considerable amount of archival research and interviewed scientists and activists everywhere from Iowa to India. He's a little over-concerned to let the reader know about his travel adventures, but it's a forgivable fault in his scene-setting.

Through it all, Vogt and Borlaug genuinely emerge as the archetypes of their differing kinds. Vogt was, for example, the talented amateur. A young birdwatcher of some renown and boundless energy—he almost singlehandedly forced into print Roger Tory Peterson's subsequently famous bird guide—he worked for a while as director of the Jones Beach State Bird Sanctuary in New York. It was there that he noticed the baleful effect of the attempt to eliminate mosquitoes: the draining of marshland and spread of chemicals lessened the mosquito population while also lessening the bird population that was the reason for the sanctuary.

Rising to become an editor for the Audubon Society magazine, Vogt wrote a pamphlet called Thirst on the Land that stressed the cyclical nature of ecosystems in cleaning water and recycling nitrogen: work that human replacements don't do anywhere near as well. Mann insists we owe to that text the idea that "the environment" is itself a being, an entity that needs to be given primacy in our deliberations about the world.

Vogt was not a man who played well with others—has there ever been a prophet who did?—and the Audubon Society soon fired him. He managed to land a gig from a commercial company worried about its guano supplies on a Peruvian island, and after studying the problem he told the company that nothing would "augment the increment of excrement" from the birds. Conservation alone—"the balance between species continually sought by nature"—would allow guano mining to continue.

After the Second World War, he worked as conservation director of the Pan American Union, where he wrote The Road to Survival, which Mann calls, in one of his nice phrases, "the first modern we're-all-going-to-hell" book. Preaching population control, he eventually became national director of Planned Parenthood. Squabbles with Margaret Sanger drove him out of that job, as squabbles had driven him out of so many jobs. Vogt killed himself in 1968 at age 62, convinced he had failed to persuade anyone—although only a few years later, America would see such gains for his worldview as the bestselling status of Paul Ehrlich's The Population Bomb and the rise of Earth Day.

Meanwhile, Norman Borlaug had an even less prepossessing start. Born in 1914, he studied forestry at the University of Minnesota in the 1930s. A doctorate in plant pathology got him a stint at a Rockefeller Foundation program in Mexico that sought some marginal increases in wheat yields.

Borlaug gave them what they wanted, and then some. He was almost fired when he rejected the era's notion that breeds were climate dependent and needed rest for germination. But he persevered, undertaking the "shuttle breeding" of moving new strains back and forth from hot lowlands to colder highlands till he had hardy varieties that could be grown almost anywhere and thrive through multiple planting seasons.

When combined with factory-produced fertilizers, drawing nitrogen from the air, Borlaug's seeds provided an exponential increase in productivity, and his reduced famine to levels never seen before. The man saved more lives than anyone else in the 20th-century—perhaps more lives than all other life-saving heroes combined. His 1970 Nobel Peace Prize was only the capstone of the innumerable honors he received for being willing to imagine that modern science could simply step over the hurdle that Malthusian math seemed to have placed in humanity's way.

By refusing to take sides, Charles Mann is able to recognize that neither the prophets nor the wizards—the environmentalist doomsayers and the science-besotted cheerleaders—have a faultless Eden to promise us. His interesting pair of earlier books on the era of Columbus, 1491 and 1493, allowed him to say both that the European arrival in the New World had catastrophic consequences and that the New World was populated with some nasty populations that damned well needed to be invaded. In The Wizard and the Prophet, he applies a similar method to the idea of modern progress, recognizing that nothing we do is without some negative consequences.

The Greens, for example, would let people starve and suffer in the name of their semi-religious prophesy. Environmental lobbyists, Borlaug once complained, have "never experienced the physical sensation of hunger. They do their lobbying from comfortable office suites in Washington or Brussels. If they lived just one month amid the misery of the developing world, as I have for fifty years, they'd be crying out for tractors and fertilizer and irrigation canals and be outraged that fashionable elitists back home were trying to deny them these things."

Meanwhile, the Green Revolution helped diminish the family farm across the globe, absorbed water needed for drinking, and created dangerously vulnerable monocultures in agriculture. Borlaug believed that future science would help solve the problems that past science had created, but to his credit he never denied the more serious of the problems that the Green Revolution created on its way to saving a billion people.

The Wizard and the Prophet is not a book to make anyone happy, except the general reader willing to be drawn into Mann's tales of Borlaug and Vogt. Arguing that "the clash between the Vogtians and the Borlaugians is heated because it is less about facts than about values," the book offers no answer to the dilemma of the modern worldview. No bridge across the chasm. No path to unite the Chicken Littles and the Pollyannas.

I think of that, however, as the great honesty of The Wizard and the Prophet. From the Romantic turn against the Rationalists down to our own day, modern thought and modern emotion—all the elements of the modern worldview—have drawn us toward rival extremes, both at the same time. William Vogt and Norman Borlaug are only 20th-century symbols of a divide that rips through culture, politics, and philosophy—mostly because it isn't just a social divide. It's a fault that runs even through our own minds, as we contemplate this imperfect world.