We are all pebbles dropped in the sea of history, where the splash strikes one way and the big tides run another, and though what we feel is the splash, the splash takes place only within those tides. In almost every case, the incoming current drowns the splash; once in a while the drop of the pebble changes the way the ocean runs. On February 12, 1809, two boys were born within a few hours of each other on either side of the Atlantic. One entered life in a comfortable family home, nicely called the Mount, that still stands in the leafy English countryside of Shrewsbury, Shropshire; the other opened his eyes for the first time in a nameless, long-lost log cabin in the Kentucky woods. Charles Darwin was the fifth of six children, born into comfort but to a family that was far from "safe," with a long history of free-thinking and radical beliefs. He came into a world of learning and money—one grandfather, Josiah Wedgwood, had made a fortune in ceramic plates. Abraham Lincoln was the second of three, born to a dirt-poor farmer, Thomas Lincoln, who, when he wrote his name at all, wrote it (his son recalled) "bunglingly."

The obvious truths of 1809, the kind that were taught in school, involved what could be called a "vertical" organization of life—one in which we imagine a hierarchy of species on earth, descending from man on down toward animals, and a judge appraising us up above in heaven. Man was stuck in the middle, looking warily up and loftily down. People mostly believed that the kinds of organisms they saw on earth had always been here and always would be, that life had been fixed in place since the beginning of a terrestrial time that was thought to go back a few thousand years at most.

People also believed, using what they called examples ancient and modern—and the example of the Terror in France, which had only very recently congealed into Napoleon's Empire, was a strong case—that societies without inherited order were intrinsically weak, unstable and inclined to dissolve into anarchy or tyranny. "Democracy" in the sense we mean it now was a fringe ideal of a handful of radicals. Even in America, the future of the democracy was unclear, in part because of the persistence of slavery. Although many people knew it to be wrong, other people thought it acceptable, or tolerable, or actually benevolent, taking blacks toward Christianity. Democracy was hard to tell from mob rule, and the style of mob rule. Democracy existed, and was armed, but didn't feel entirely liberal; the space between reformist parliamentary government and true democracy seemed disturbingly large, even to well-intentioned people. In the 1830s, Tocqueville, sympathetic to American democracy, was still skeptical about its chances, writing that "until men have changed their nature and are completely transformed, I will refuse to believe in the duration of a government which is called upon to hold together forty different nations covering an area half that of Europe."

No era's ideas are monolithic, and the people of 1809 in England and America did not believe these things absolutely. The new science of geology was pressing back the history of earth; old bones would start turning up that threatened old stories; the new studies of the text of the Bible were pressing against a literal acceptance of biblical truth, too. And there were many Utopian democrats in both countries. We can find plenty of radical ideas in that day, just as we will find traces of the astonishing ideas of the next century somewhere on the fringes of our own time. But on the whole these ideas belonged to the world of what would have been called "fancy," not fact.

By the time Abraham Lincoln and Charles Darwin were dead—the American murdered by a pro-slavery terrorist in 1865, the Englishman after a long illness in 1882—the shape of history had changed, and the lives they had led and the things they had said had done a lot to change it. Two small splashes had helped to change the tide of time. Very different beliefs, ones that we now treat as natural and recognize as just part of the background hum of our time, were in place. People were beginning to understand that the world was very, very old, and that the animals and plants in it had changed dramatically over the eons—and though just how they had changed was still debated, the best guesses, then as now, involved slow alteration through a competition for resources over a very long time. People were convinced, on the whole, that democratic government, arrived at by reform or revolution, was a plausible and strong way to organize a modern nation. (A giant statue, one of the largest since antiquity, of a goddess of Liberty was under construction in once-again Republican France to be sent to a vindicated Republican America, to commemorate this belief.) Slavery in the Western world was finished. (Although racism wasn't.)

Most of all, people thought that the world had changed, and would continue to change, that the hierarchies of nature and race and class that had governed the world, where power flowed in a fixed chain on down, were false. Life was increasingly lived on what we can think of as a "horizontal," with man looking behind only to see what had happened before, and forward to see what he could make next. On that horizontal plane, we are invested in our future as much as in our afterlife, and in our children more than in our ancestors. These beliefs, which we hold still, are part of what we call the modern condition—along with the reactive desire to erase the instability that change brings with it.

The two boys born on the same day into such different lives had become, as they remain, improbable public figures of that alteration of minds—they had become what are now called in cliché "icons," secular saints. They hadn't made the change, but they had helped to midwife the birth. With the usual compression of popular history, their reputations have been reduced to single words, mottoes to put beneath a profile on a commemorative coin or medal: "Evolution!" for one and "Emancipation!" for the other. Though, with the usual irony of history, the mottoes betray the men. Lincoln came late—in the eyes of Frederick Douglass, maddeningly late—and reluctantly to emancipation, while perhaps the least original thing in Darwin's amazingly original work was the idea of evolution. (He figured out how it ran; he took a fancy poetic figure that his granddad, Erasmus Darwin, had favored and put an engine and a fan belt in it.) We're not wrong to work these beautiful words onto their coins, though: they were the engineers of the alterations. They found a way to make those words live. Darwin and Lincoln did not make the modern world. But, by becoming "icons" of free human government and slow natural change, they helped to make our moral modernity.

The shared date of their birth is, obviously, "merely" a coincidence—what historians like to call an "intriguing coincidence." But coincidence is the vernacular of history, the slang of memory—the first strong pattern where we begin to search for more subtle ones. Like the simultaneous deaths of Thomas Jefferson and John Adams on July 4, 1826, the accidental patterns of birth and death point to other patterns of coincidence in bigger things. Lincoln and Darwin can be seen as symbols of the two pillars of the society we live in: one representing liberal democracy and a faith in armed republicanism and government of the people, the other the human sciences, a belief that objective knowledge about human history and the human condition, who we are and how we got here, exists. This makes them, plausibly, "heroes." But they are also amazing men, something more than heroes, defined by their private struggles as much as by their public acts.

Both men are our contemporaries still, because they were among the first big men in history who belonged to what is sometimes called "the bourgeois ascendancy." They were family men. They loved their wives uxo­riously, lived for their children and were proud of their houses. Darwin was born to money, and though he kept some gentry tastes and snobberies, like the royal family of Albert and Victoria, who superintended most of his life, he chose to live not in imitation of the old aristocracy but in the manner of the new bourgeoisie—involving his children in every element of his life, having them help with his experiments, writing an autobiography for them and very nearly sacrificing his chance at history for the love of his religious wife. Lincoln's rise in history was to the presidency—but his first and perhaps even harder rise was to the big middle-class house and expensive wife he adored. What we wonder at is that a simple Springfield lawyer could become president; from his point of view, what probably was really amazing was that a cabin-born bumpkin had become a Springfield lawyer.

Both men were shaped in crucial ways by the worst of still-present 19th-century woes, the death of children at the height of their charm and wisdom. They both even had what one might call the symptomatic diseases of middle-class modernity, the kind that we pick out among the great roll call of human ills to name and obsess over. Lincoln was a depressive; Darwin subject to anxiety so severe that he wrote down one of the most formidable definitions of a panic attack that exists. Though the source of these ailments—in nature or genes, bugs or traumas—remains mysterious, their presence, the way they manifested themselves, is part of the familiarity the two men have for all the distance between us. They had the same domestic pleasures, and the same domestic demons, as we do.

We must be realistic about what they were like; not saints nor heroes nor Gods but people. Darwin and Lincoln are admirable and in their way even lovable men. But Lincoln, we have always to remember, was a war commander, who had men shot and boy deserters hanged. We would, I think, be taken aback at a meeting. Lincoln summed up in one word was shrewd, a backwoods lawyer with a keen sense of human weakness and a knack for clever argument, colder than we would think, and more of a pol and more of a wiseguy than we would like him to be: someone more concerned with winning—elections, cases and arguments—than with looking noble. Lincoln was smart, shrewd and ambitious before he was, as he became, wise, far-seeing and self-sacrificing. If we were around to watch him walk across a room, instead of stride through history, what we would see is the normal feet that left the noble prints.

Darwin we would likely find far more frumpy and tedious than we would like our heroes to be—one of those naturalists who run on and on narrowly on their pet subjects. He would have frowned and furrowed his brow and made helpless discomfited harrumphs if any of today's fervent admirers arrived and asked him what he thought of man's innate tendencies to relish Tchaikovsky. One can easily imagine him brought back to earth and forced onto a television studio platform with eager admirers (like this one) pressing him for his views on sexual equality or the origins of the love of melody in the ancient savanna, and his becoming more and more unhappy and inarticulate, and at last swallowed up in a vast, sad, melancholy, embarrassed English moan.

Not that Lincoln didn't care about morality; but he cared more about winning wars and arguments than about appearing to be a paragon. Not that Darwin wasn't interested in speculative consequences of his theory—he was—but the habit of pontification was completely alien to him, unless it was reassuringly tied with a bow of inductive observation.

Fifty years ago, not many would have chosen Darwin and Lincoln as central figures of the modern imagination. Freud and Marx would perhaps have been the minds that we saw as the princes of our disorder. But with the moral (and lesser intellectual) failure of Marxism, and the intellectual (and lesser moral) failure of Freud, their ideas have retreated back into the history of modernity, of the vast systematic ideas that proposed to explain it all to you. Lincoln and Darwin, by contrast, have never been more present: Lincoln is the subject of what seems to be the largest biographical literature outside those of Jesus and Napoleon, while Darwin continues not only to cause daily fights but to inspire whole new sciences—or is it pseudosciences? For the irony is that the most radical thing around, at the birth of the new millennium, turned out to be liberal civilization—both the parliamentary, "procedural" liberalism of which Lincoln, for all his inspirational gifts, was an adherent, and the scientific liberalism, the tradition of cautious pragmatic free thought, that engaged Darwin, who was skeptical of grand systems even as he created one. Science and democracy still look like the hope of the world (even as we recognize that their intersection gave us the means to burn alive every living thing on the planet at will).

The deepest common stuff the two men share, though, is in what they said and wrote—their mastery of a new kind of liberal language. They matter most because they wrote so well. Lincoln got to be president essentially because he made a couple of terrific speeches, and we remember him most of all because he gave a few more as president. Darwin was a writer who published his big ideas in popular books. A commercial publishing house published The Origin of Species in the same year that it published novels and memoirs, and Darwin's work remains probably the only book that changed science that an amateur can still sit down now and read right through. It's so well written that we don't think of it as well written, just as Lincoln's speeches are so well made that they seem to us as obvious and natural as smooth stones on the beach. (We don't think, "Well said!" we just think, "That's right!")

Darwin and Lincoln helped remake our language and forge a new kind of rhetoric that we still respond to in politics and popular science alike. They particularized in everything, and their general vision rises from the details and the nuance, their big ideas from small sightings. They shared logic as a form of eloquence, argument as a style of virtue, close reasoning as a form of uplift. Each, using a kind of technical language—the fine, detailed language of naturalist science for Darwin; the tedious language of legal reasoning for the American—arrived at a new ideal of liberal speech. The way that Darwin uses insanely detailed technical arguments about the stamen of an orchid to pay off, many pages later, in a vast cosmic point about the nature of survival and change on a planetary time scale, and the way that Lincoln uses lawyerly arguments about who signed what and when among the Founders to make the case for war, if necessary, to end slavery—these things have in common their hope, their faith, in plain English, that people's minds and hearts can be altered by the slow crawl of fact as much as by the long reach of revelation. Their phrases still ring because they were struck on bells cast of solid bronze, not chimes set blowing in the breeze.

In all these ways—their love of family, their shrewdness and sensitivity, their invention of a new kind of plain speaking—these two men are worth looking at together precisely because they aren't particularly remarkable. The things that they loved and pursued, the things that intrigued and worried them, were the same things that most other intelligent people in their day worried about and that worry and intrigue us still. Even mountains are made of pebbles, built up over time, and an entire mountain range of minds has risen slowly between them and us. Most of the rest have been submerged by time, but Darwin and Lincoln remain high peaks within those mountains of modernity, and they look out toward each other. From the top of one you can see the other, and what you see is what we are.

Copyright © 2009 by Adam Gopnik. Adapted by the author from Angels and Ages, by Adam Gopnik, published by Alfred A. Knopf in January.

Adam Gopnik is a staff writer at the New Yorker.

Joe Ciardiello's artwork has appeared regularly in the New York Times Book Review.