Charles Darwin and Abraham Lincoln never met in person, but they sure would have had plenty to talk about. For starters, both of these visionary men were born on the exact same day: February 12, 1809. Both lost their mothers at a tragically young age. And both came to hate that “peculiar institution” called slavery.

In 1831, Darwin—then a recent college grad—took the trip of a lifetime aboard the H.M.S. Beagle. Over the next five years, he’d become the resident naturalist, gathering New World plant and animal specimens by the hundreds before shipping them back to England. During these travels, Darwin also began laying the groundwork of an idea that would forever change his life and our world: evolution by natural selection.

But fossils and tortoise shells weren’t the only sights catching Darwin’s eye. After returning home, he penned a memoir entitled The Voyage of H.M.S. Beagle. The scientist described in vivid, uncomfortable detail some of the “heart-sickening atrocities” he’d witnessed in the “slave-country” of Brazil:

Near Rio de Janeiro I lived opposite to an old lady, who kept screws to crush the fingers of her female slaves … I have seen a little boy, six or seven years old, struck thrice with a horse-whip (before I could interfere) on his naked head, for having handed me a glass of water not quite clean; I saw his father tremble at a mere glance from his master's eye.

And Darwin doesn’t stop there. “Picture to yourself the chance,” he instructed his readers, “ever hanging over you, of your wife and your little children … being torn from you and sold like beasts to the first bidder! And these deeds are done and palliated by men, who profess to love their neighbours as themselves, who believe in God, and pray that his Will be done on earth! It makes one's blood boil, yet heart tremble, to think that we Englishmen and our American descendants, with their boastful cry of liberty, have been and are so guilty.”

Given these passionate words, when America’s Civil War broke out, you can guess which side Darwin supported. Shortly after southern forces fired on Fort Sumter in 1861, he contacted his Yankee colleague, botanist Asa Gray, and wrote:

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 … Great God how I should like to see the greatest curse on Earth, slavery, abolished.

Lincoln never read this document, but his razor-sharp political instincts were second to none. Anti-slavery sentiments just like Darwin’s were firmly-rooted throughout much of Europe—a fact upon which “Honest Abe” capitalized with his famous Emancipation Proclamation in 1863.

When that brilliant decree rang out, Darwin’s reaction was a bit on the skeptical side. “Well,” he wrote Gray, “your President has issued his fiat against slavery—God grant it may have some effect.” Gray, for his part, believed that the Union would emerge victorious and that slavery’s death knell had finally begun. “You see, slavery is dead, dead,” Gray had announced that year. Darwin—who once told Gray “you are too hopeful on your side of the water”—had his doubts: