It’s a positive note. Two of Charles Darwin’s notebooks from 1837, stolen over 20 years ago from a library in England, were anonymously returned to the library last month. The notebooks, which include an early sketch of Darwin’s famous “Tree of Life” diagram, were no worse for wear despite disappearing for two decades, Cambridge University Library said in a press release. On March 9, library staff discovered the cling-wrapped books and a note that read simply: “Librarian, Happy Easter, X.” “My sense of relief at the notebooks’ safe return is profound and almost impossible to adequately express…