Here’s a nice coincidence: Charles Darwin and Abraham Lincoln were born on the same day: February 12, 1809. Both were great emancipators: Lincoln, as every school-child knows, freed the slaves; Darwin, as every creationist resists, freed our minds. Lincoln made it respectable to be an American, in an era when slavery was finally becoming indefensible. Darwin made it respectable to be an atheist in an era when the complexity of life seemed to make non-religious explanations indefensible.

Each man was genuinely conflicted about assuming the role of liberator: Lincoln because he feared that doing so would unleash resistance and resentment on the part of Southern slaveholders (and their Northern allies), making it more difficult to hold the Union together; Darwin because he feared that doing so would unleash resistance and resentment of those slavishly devoted to a literalist, fundamentalist, biblical view of creation, making it difficult for him to hold onto his own cherished peace and quiet. And so, each delayed making his particular emancipation proclamation.

Lincoln was eventually swayed, not so much by a burning desire to liberate those whose bodies were enslaved, as by his hope of prodding at least parts of the rebellious confederacy into abandoning their course. Thus, the Emancipation Proclamation appears to have been issued as an act of war and propaganda more than of conscience. It freed enslaved people within those states still in rebellion, leaving in bondage, at least for a time, those not part of the confederacy.

Darwin’s “proclamation,” titled On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life had been gestating even longer before its publication, and whereas Lincoln was prodded, in part, by the earlier Union victory at Antietam, Darwin was moved by fear that he was about to be “defeated” (actually, scooped) by Alfred Russell Wallace, who had intuited the process of natural selection and hurriedly expressed his insights to Darwin. Darwin had been working on The Origin for decades previously, but prior to Wallace’s letter, couldn’t bring himself to “go public.”

Lincoln’s reluctance to “go public” with the Emancipation Proclamation has been seen by some as a blot on his legacy, by others as a concession to the practical necessities of governing. Thus, it is unclear whether Lincoln had also been harboring plans to end slavery, comparable to Darwin’s decades-long hesitation about proclaiming his evolutionary insight.

In any event, it is generally agreed that the Civil War was undertaken to preserve the Union rather than to end human bondage, just as Darwin’s publication was spurred by a desire to preserve his own intellectual priority.

Here is a coincidence of admittedly lesser moment: Lincoln appears on the omnipresent – and increasingly endangered - U.S. copper penny, while Darwin has recently replaced Dickens on the British ten pound note, coinage that would also be endangered if Great Britain ever joined the Eurozone. (Also, both were notably bearded, but in an era when facial hair was common, such a convergence is about as significant as observing that the legs of both men were precisely long enough to reach the ground.)

Historians have spilled of ink – more recently, they have disoriented gazillions of electrons – trying to illuminate the lives and motivations, not to mention the impact, of both men. I would bet that more has been written about Charles Darwin than about any other scientist, and, similarly, more about Abraham Lincoln than about any other American politician.

Yet, despite the glare of biographers and intellectual historians, both Darwin and Lincoln remain enigmas. Why did Darwin delay publication so long, and what was the exact nature of the physical infirmity that kept him a convalescent and virtual recluse after his globe-girdling trip on H. M. S. Beagle? Was Lincoln clinically depressed? Was he gay? A brilliant idealist? A cynical politician?

The two great liberators pointed to the same phenomenon: reconciliation and unity beneath conflict and diversity. Thus, the underlying logic that defeated slavery is not that a country cannot survive half slave and half free, but that human beings are not qualitatively discontinuous, divisible into distinct “species,” namely the slave-holders and the enslaved. This was and remains a fundamental truth, part of our biology no less than of our politics.

The underlying logic of evolution is similarly one of unity: living things are not qualitatively discontinuous (although for the most part they are divisible into meaningful species). More important, both for our purposes , there is an unbroken thread connecting all, including ourselves. And evolution, too, is gradually becoming part of our politics, just as – since Darwin – it has been the cornerstone of all that we know of biology.

Perhaps the best way to honor both birthday boys, however, is to let them speak in their own words, first Lincoln at the end of his Second Inaugural Address, and then Darwin at the end of The Origin, and to reflect upon how these two towering figures – one a politician and one a scientist – born on the same day and yet functioning in seemingly different realms, arrived, independently, at strikingly similar realizations.

“With malice toward none, with charity for all, with firmness in the right as God gives us to see the right, let us strive on to finish the work we are in, to bind up the nation's wounds, to care for him who shall have borne the battle and for his widow and his orphan, to do all which may achieve and cherish a just and lasting peace among ourselves and with all nations.”

“Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.”

David P. Barash, an evolutionary biologist, is professor of psychology at the University of Washington. His most recent book is Homo mysterious: evolutionary mysteries of human nature.

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