When Charles Darwin finished reading Charles Lyell's Principles of Geology, a book suggesting that there are clear limits to the variation of species, he wrote in the margins: "If this were true adios theory." It's a great piece of marginalia. And it's just one of many comments that adorn books in Darwin's personal library, and help illuminate his intellectual path to writing On The Origin Of Species (1859).
Thanks to Cambridge University library and several partners, 330 of Darwin's most heavily annotated texts have now been digitized and made available online at the website of the Biodiversity Heritage Library. More will come in due time. You can begin your tour here.