LONDON (AP) — Cambridge University launched an appeal Tuesday to find two valuable notebooks written by Charles Darwin after they were reported as stolen from the university’s library.

The notebooks, estimated to be worth millions of pounds, include the 19th-century scientist’s famous “Tree of Life” sketch. They haven’t been seen since 2000, and for years staff at the library believed that the manuscripts had probably been misplaced in the vast archives.

But after doing a thorough search, library staff now conclude it’s likely that the notebooks were stolen. Police are now investigating and Interpol has also been notified.