A trove of books from Charles Darwin’s personal library is now digitized, online and free for all to view. The collection, displaying Darwin’s scrawled-in-pencil marginalia, tantalizingly reveals his thought process as he developed the theory of evolution.

While many of his papers and notebooks are already online, Darwin didn't keep a notebook for several months after returning from his 5-year-long voyage aboard the HMS Beagle. Instead, Darwin — then just 27 years old, with his theory of evolution half-formulated — made notes in the margins of books he read.

The notes in these books represent another kind of voyage. Darwin develops arguments and considers challenges to his hypotheses. “This fills a gap in his notebook writings, and gives us a new lens to look at Darwin,” said David Kohn, director of the American Museum of Natural History's Darwin Manuscripts Project.

View of Darwin’s Library on the shelves of Down House, circa 1876.