LONDON — Two of naturalist Charles Darwin’s notebooks that were reported stolen from Cambridge University’s library have been returned, two decades after they disappeared.

The university said Tuesday that the manuscripts were left in the library inside a pink gift bag, along with a note wishing the librarian a Happy Easter.

The notebooks, which include the 19th-century scientist’s famous 1837 “Tree of Life” sketch, went missing in 2001 after being removed for photographing, though at the time staff believed they might have been misplaced. After searches of the library’s collection of 10 million books, maps and manuscripts failed to find them, they were reported stolen to police in October 2020.

Local detectives notified the global police organization Interpol and launched an international hunt for the notebooks, valued at millions of pounds (dollars).

On March 9 the books reappeared, left in a public area of the building, outside the librarian’s office, which is not covered by security cameras. The two notebooks were wrapped in clingfilm inside their archive box, and appeared undamaged. The accompanying note said: “Librarian Happy Easter X.”

Darwin filled the notebooks with ideas shortly after returning from his voyage around the world on HMS Beagle, developing ideas that would bloom into his landmark work on evolution, “On the Origin of Species.”

The university’s director of library services Jessica Gardner said her feeling of relief at the books’ reappearance was “profound and almost impossible to adequately express.”

“The notebooks can now retake their rightful place alongside the rest of the Darwin Archive at Cambridge, at the heart of the nation’s cultural and scientific heritage, alongside the archives of Sir Isaac Newton and Professor Stephen Hawking,” she said.

The notebooks are set to go on public display from July as part of a Darwin exhibition at the library.

Cambridgeshire Police said its investigation was continuing, “and we are following up some lines of inquiry.”

“We also renew our appeal for anyone with information about the case to contact us,” the force said.