When evolution theorist Charles Darwin finished reading Charles Lyell’s “Principles of Geology,” he scribbled on the last page, “if this were true[,] adios theory.” Such comments appear in the margins of about half of the famed naturalist’s 1,480 books, revealing an avid, thoughtful reader constantly evaluating his ideas and those of other authors. Now anyone can peer into those pages to see how Darwin’s thinking changed as he developed his theory of evolution. The 330 most heavily annotated titles are now digitized and online at the Biodiversity Heritage Library (www.biodiversitylibrary.org), a project to put natural history information on the Web. The image of each page comes with a window describing the locations of Darwin’s notes and a transcription of his scrawl. One can also search his scribblings by subject.