Megan Specia writes: “Twenty years ago, two historic notebooks belonging to the renowned naturalist Charles Darwin were found to be missing from the archives at Cambridge University Library. The tiny books recorded Darwin’s thoughts after he returned to England from his famed voyage aboard H. M. S. Beagle, as he grasped toward ideas that would form the foundations of modern evolutionary biology. Now, the library has said it believes the notebooks were most likely stolen, and it launched a public appeal for any information about their whereabouts.”