Notebooks belonging to Charles Darwin – the English naturalist known for proposing the theory of evolution – were mysteriously returned to Cambridge University after they were likely stolen more than 20 years ago, the British institution said. The notebooks were left in a pink gift bag placed outside of librarian Jessica Gardner's office on March 9, according to a post on the university's website. It was the first time any Cambridge staff members had seen the two items since they were alerted to their disappearance during a routine check in January 2001. "They may be tiny, just the size of postcards, but the notebooks' impact on the history of science, and their importance to our world-class collections here, cannot be overstated," Gardner said in a statement. Gardner said the notebooks appeared to have no obvious signs of damage and were wrapped in plastic. In a 2020 BBC article about the notebooks, the outlet estimated the rare items could be "worth millions." The university still does not know where the notebooks have been since their disappearance, and the person who returned them did not reveal their identity. But they did leave a message for Gardner, which simply read: "Librarian, Happy Easter, X."