Because Darwin and Lincoln are forever paired, thanks to their shared birthdate 200 years ago and the profound and lasting (but separate) influence of their ideas and actions, as Adam Gopnik explains, a question arises: What did they think of each other?

In today's hyper-mediated, celebrity-saturated global village the world's leading biologist and the leader of the free world might be expected to meet at, say, the World Economic Forum in Davos, Switzerland (though we're not aware that Lincoln or Darwin skied), at a climate-policy summit or over pints at Bono's.

But Darwin and Lincoln did not cross paths. And though a perusal of reliable sources suggests that the two did not mention each other by name in writing, there's evidence they were at least aware of one another's efforts.

Darwin, a staunch abolitionist, as our Times of London, whose correspondent in the States was not sufficiently against slavery, Darwin wrote, and covered the war "detestably."

Asa Gray between 1862 and 1865 referring to the Civil War, slavery or the "president." Darwin was not forthcoming about Lincoln and appeared to grow more pessimistic about the war as the years went on.

On June 5, 1861, Darwin wrote to Gray:

I never knew the newspapers so profoundly interesting. N. America does not do England justice: I have not seen or heard of a soul who is not with the North. Some few, & I am one, even wish to God, though at the loss of millions of lives, that the North would proclaim a crusade against Slavery. In the long run, a million horrid deaths would be amply repaid in the cause of humanity. What wonderful times we live in.... Great God how I shd like to see that greatest curse on Earth Slavery abolished.

Lincoln issued the final Emancipation Proclamation on January 1, 1863. Eighteen days later, Darwin wrote to Gray, an abolitionist evidently more optimistic about the course of the war than Darwin:

Well, your President has issued his fiat against Slavery— God grant it may have some effect.— ... I sometimes cannot help taking most gloomy view about your future. I look to your money depreciating so much that there will be mutiny with your soldiers & quarrels between the different states which are to pay In short anarchy & then the South & Slavery will be triumphant. But I hope my dismal prophecies will be as utterly wrong as most of my other prophecies have been. But everyone's prophecies have been wrong; those of your Government as wrong as any.— It is a cruel evil to the whole world; I hope that you may prove right & good come out of it.

It cannot be said that Lincoln, for his part, gave Darwin that much thought. The one passage we turned up about Lincoln and evolution focuses on his interest in a book that preceded Darwin's On the Origin of Species by some 15 years.

That was Vestiges of the Natural History of Creation, first published anonymously in 1844 by the Scottish journalist Robert Chambers. It presented a cosmic theory of evolution that lacked Darwin's key insight (the mechanism of natural selection), posited a biased view of human progress, was roundly criticized by scientists as mistaken about geology and other subjects, and in subsequent editions took pains to say it was perfectly compatible with Christian theology. Still, it did advance the idea to a wide audience that species we see today were not fixed but had descended from other forms, and the controversy it stirred gave Darwin pause.

The following passage comes from the groundbreaking 1889 biography Springfield, Illinois. Herndon writes:

For many years I subscribed for and kept on our office table the Westminster and Edinburgh Review and a number of other English periodicals. Besides them I purchased the works of Spencer, Darwin, and the utterances of other English scientists, all of which I devoured with great relish. I endeavored, but had little success in inducing Lincoln to read them. Occasionally he would snatch one up and peruse it for a little while, but he soon threw it down with the suggestion that it was entirely too heavy for an ordinary mind to digest. A gentleman in Springfield gave him a book called, I believe,"Vestiges of Creation," which interested him so much that he read it through. The volume was published in Edinburgh, and undertook to demonstrate the doctrine of development or evolution. The treatise interested him greatly, and he was deeply impressed with the notion of the so-called "universal law" evolution; he did not extend greatly his researches, but by continued thinking in a single channel seemed to grow into a warm advocate of the new doctrine. Beyond what I have stated he made no further investigation into the realm of philosophy. "There are no accidents," he said one day, "in my philosophy. Every effect must have its cause. The past is the cause of the present, and the present will be the cause of the future. All these are links in the endless chain stretching from the finite to the infinite.

That's the extent of what's known about Lincoln's thoughts on evolution, says Michael Lind, a senior fellow at the New America Foundation and author of the 2004 book What Lincoln Believed. "Herndon's testimony suggests that Lincoln was not only familiar with the idea of evolution," Lind says in an email, "but convinced by it."