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. "His thoughts are very frank, very fresh.” Above: Darwin's Library at Home View of Darwin’s Library on the shelves of Down House, circa 1876. Image: Cambridge University Library.

Reading Lyell and Henslow Two of Darwin’s most-respected fellow scientists and mentors, John Henslow and Charles Lyell, had published books when Darwin returned. He immediately immersed himself. “These men are his teachers, literally in the case of Henslow,” said Kohn, “and you can see him really willing to challenge his old professor. He takes on their ideas seriously, because they really don’t look at the world the way he does anymore.” He begins to write comments in the margins, some critical such as: “Argument is in a circle.” “These are things he would never say to them directly!” Kohn said. *Image: Charles Lyell's *Principles of Geology/Darwin Manuscripts Project.

Adios Theory Henslow, one of Darwin’s professors, directed him to Charles Lyell’s Principles of Geology. The book, which he smothered in notes, became central to Darwin’s theory. In Principles Lyell writes that the earth was formed by slow-moving processes, still in operation today, and argues against Jean-Baptiste Lamarck’s view of evolution. Lamark thought creatures could alter their characteristics in a single lifetime, then pass those changes to offspring. A giraffe, for example, could lengthen its neck and have calves with slightly longer necks. Lyell said species could adapt, but only within narrow bounds. This posed a problem for Darwin's ideas. On the final page of Lyell’s Principles of Geology Darwin writes, “If this were true adios theory.” *Image: Charles Lyell's *Principles of Geology/Darwin Manuscripts Project.

Tree of Life Not all notes were challenges. In a page from Descriptive and Physiological Botany, Darwin sees broader applicability in Henslow’s writings about physical trees. In the margin he writes “Tree of Life,” sketching in July 1837 just such a tree in one of his notebooks. Image: John Henslow's Descriptive and Physiological Botany/Darwin Manuscripts Project.

Reading Malthus In the image above, on the back cover of Henslow’s Descriptive and Physiological Botany, Darwin has written, “People constantly speak about every organism being being [sic] perfectly adapted to circumstances if so how can there be a rare species breeding power being efficient — food not sufficiently abundant is the answer.” This slightly garbled sentence shows a break with Henslow and Lyell and reveals a key insight. The concept of adaptation being perfect is pure Henslow and Lyell, said Kohn. They believed that God set organisms in their own ideal environment, like round pegs in round holes. Darwin didn’t believe adaptation guided organisms to perfection. He thought it only caused them to become as good as, or slightly better than, organisms they competed against. The second part of the passage identifies a mechanism to drive natural selection. It seems inspired by Thomas Malthus, an economist who argued that populations will always be under stress, since natural resources are limited. Populations have the potential to grow exponentially (“species breeding power”) but they don’t because food is not sufficiently abundant. “This is the central Malthusian insight: that it’s this pressure that drives adaptation, according to Darwin," said Kohn. "Here he is saying, this is the answer to people talking about perfect adaptation.” Image: John Henslow's Descriptive and Physiological Botany/Darwin Manuscripts Project.

Material Matters One page, from John Abercrombie’s Inquiries Concerning the Intellectual Powers and the Investigation of Truth, provides a window into Darwin’s struggles with the implications of his thoughts. It’s a philosophical book about human intelligence. In it, Abercrombie gives a damning critique of Materialism — the conviction that only physical things are real. “Will my theory apply here?” Darwin asks himself in the margin. "If I pursue my theory, am I plunging into badness?” Image: John Abercrombie's Inquiries Concerning the Intellectual Powers and the Investigation of Truth /Darwin Manuscripts Project.

Oh You Materialist! In a notebook from the same era, Darwin says he believes thought is a “secretion of the brain,” not a divine gift. On the same page he writes, “Oh you Materialist!” “This is a fascinating passage,” Kohn said. “Is the entry in the notebook tongue-in-cheek? Is he shaking in his boots? Is he recognizing that he’s possibly an atheist?” It’s worth remembering that Darwin was living at a time when people had been tried for heresy in the recent past. His marginalia show him encountering the scientific and philosophical implications of his theory, struggling with them and considering the opposition people would have. “When Henslow or Lyell expresses a creationist idea in print,” Kohn said, “you can see Darwin with his new theory thinking, 'That’s a strong argument against my theory' or ‘I have the answer to that!’” Darwin knew how controversial people would find his theory, and sat on his research for years. He could have published Origins at least 15 years before he did. “He saw the consequences, that people would really object to his scientific theory because of its religious and metaphysical implications,” Kohn said. “For a while it seems he came to the conclusion that he simply couldn’t talk about it.” Image: Notebook C Syndics of Cambridge University Library/Darwin Manuscripts Project.