More than two decades after they were discovered to be missing, two of Charles Darwin’s early notebooks in which he outlined his first concept of natural selection were mysteriously returned to the Cambridge University library last month, the Washington Post reports.

Known as Notebooks B and C, the postcard-sized books from 1837 were found in a pink gift bag in a hallway in the library offices with a note attached to it:  “Librarian, Happy Easter, X.”  After examination by police, librarians unwrapped the notebooks from their plastic wrap and authenticated the items.

“It quickly became apparent to us that we were indeed looking at what we’d been hoping would be returned to us one day — the two precious Charles Darwin notebooks,” Cambridge University librarian Jessica Gardner said in an interview.

The notebooks provide extensive insight into the formulation of the theory of evolution, for which Darwin was an early advocate.  Page 36 of Notebook B holds the first illustration of a common figure in evolutionary biology:  Darwin’s Tree of Life.  “I was shaking — to see that page,” Gardner said.

The notebooks weren’t definitively thought to be stolen when they went missing in January 2001, with some hoping that they were just misplaced in the vast library collection.  However, their return makes it apparent that the books were taken.

“These two notebooks are perhaps the most important evidence of scientific discovery — the theory of evolution,” Gardner said. “And however playfully they came back, in terms of packaging and that little short note, these are the real objects, and they’re in great condition.”