Two of Charles Darwin’s notebooks, which went missing more than 20 years ago, have been returned to the Cambridge University Library unscathed. 

The university said Tuesday that the nearly 200-year-old notebooks were found March 9, left in a public area of the library inside a bright pink gift bag with a note wishing the librarian a “Happy Easter.” 

The notebooks, the size of postcards, contain Mr. Darwin’s notes as he worked out his theory of evolution. One of the notebooks contains the naturalist’s “Tree of Life” sketch from 1837, which sought to map out evolution and the relationship between species. Above the sketch are two words: “I think.”

Dr. Jessica Gardner, the university’s librarian, said that a colleague spotted the pink bag and brought it in. They looked inside to find a brown envelope with a typed message:

Librarian

Happy Easter

X

“That note is quite unusual,” Dr. Gardner said. “It absolutely adds to the mystery.”

Within the envelope was a box they recognized as belonging to the library. Inside, the two notebooks were wrapped in plastic wrap, which isn’t how the library stored them. Still, Dr. Gardner recognized the notebooks and was sure it was the missing ones. She didn’t remove the plastic wrap until the police gave the OK to do so six days later. 

“It was a really thrilling moment,” Dr. Gardner said about unwrapping the notebooks and looking through the pages. 

The notebooks are believed to have gone missing in 2000, when they were taken out of the library’s archive to be photographed. The library didn’t notice they were gone until a routine check in 2001. Some at the library thought they may have just been misplaced.