From the website:
Whenever science historian John van Wyhe passed through the old library of Christ's College at Cambridge University, he took a moment to glance at a photo of Charles Darwin hanging on the wall. "I've always been curious about it, because in the frame there's a signature on a bit of paper, and it looks like there's something written on the back."
When van Wyhe finally got around to investigating the matter, he was met with a pleasant surprise. The signature is actually on the back of a cheque, folded and embedded into the frame back in 1909. The find makes for a very rare discovery — most of Darwin's papers were tracked down and catalogued long ago. "New things don't turn up anymore," says van Wyhe, who is director of a project hosting Darwin's complete works online at the University of Cambridge, UK.