Last year was the bicentennial of the births of both Charles Darwin and Abraham Lincoln. But they shared far more than just a birthday...
As well as attaining distinguished success in their chosen paths, each man dared to think for himself and played leading roles in the struggle for human equality.
Just as Lincoln's political and military influence resulted in the abolition of slavery, Darwin's published works established that all humans are members of one species with a common ancestry.
What's more, Darwin came from one of Great Britain's foremost abolitionist families.
His maternal grandfather, Erasmus Darwin, and paternal grandfather, Josiah Wedgewood, were prominent figures in the British Society for the Suppression of the Slave Trade.
"Great God how I should like to see that greatest curse on Earth Slavery abolished," wrote Charles Darwin in a letter to Asa Grey, the Harvard Botanist.
Darwin's scientific theories played a key role in undermining the pseudo-scientific arguments adopted by pro-slavery forces to support the idea of humans being created separately, which had been so integral to colonial policies.
Though they certainly never met nor corresponded directly, it was by happy coincidence that each man's destiny complemented the other's so directly.
And they have another thing in common: lasting legacies which endure to this day, evidenced in the collectible autographs, memorabilia and rare books markets.
In the last couple of days, historic artefacts related to both Lincoln and Darwin have sold well at auction...
Yesterday, (February 11) a first edition of Darwin's masterwork, On The Origin of Species, brought $131,450 at Heritage Auction Galleries.
And last year - the 200th anniversary of Darwin's birth and also the 150th anniversary of On The Origin of Species' first publication - various first editions of the work achieved good prices at auction.
In 2009, two first editions of Darwin's most famous work sold for six-figure sums.
On the very same day as the Heritage sale, a seated carte-de-visite portrait of Abraham Lincoln from August 9, 1863, sold at Swann Auction Galleries.
Below the image, taken during Lincoln's US presidency, is his autograph, bold and clear, signed "A. Lincoln".
One of four photographs taken during the session, it sold for $48,000.
There are 119 known photographs of Abraham Lincoln in existence. Last month, one of only 24 which show the President standing sold at Freeman's auctioneers for $103,000.
Elsewhere on the current autographs market, a one-of-its-kind rare military commission signed by President Lincoln is available to collectors priced £15,000 ($24,750).
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Images: Heritage and Swann