Today marks the 155th anniversary of the publication of Charles Darwin’s On the Origin of Species. Since 2009, the American Museum of Natural History—working with Cambridge University, Eton College, the Natural History Museum in London, the American Philosophical Society, and other institutions—has been digitizing Darwin's documents pertaining to evolution. “Once Darwin came back from the Beagle, 99.9 percent of his life-defining work is on evolution, one way or another,” says David Kohn, director of the Darwin Manuscript Project, which, as of today, has digitized 12,000 of 30,000 documents—and provided transcriptions of them. (Darwin's handwriting was messy!)

The process of writing The Origin of Species took Darwin 25 years, and the project website includes the scientist’s manuscripts, notebooks, evidence, writings from the Beagle expedition, and even what he read, which shows what he was like as a working scientist. Digitizing the documents, Kohn says, shows “the growth and development of this great theory, this tremendously articulated process, and a window into Darwin’s creativity.” Here are a few fun things you’ll find digging through the archive.    

1. THE FIRST USE OF THE TERM "NATURAL SELECTION."

When Darwin first discovered natural selection in 1838, Kohn says, he didn't give it a name; in fact, he didn’t start using "natural selection" until 1842. "We find it at the chapter heading of this first essay on evolution that he ever wrote called the 1842 Pencil Sketch," Kohn says. “It’s a mere 35 pages, but [it spans] the whole tree of Darwin’s thinking." You can find the first use of "natural selection" on page 5.

2. …AND THE PHRASE HE WAS CONSIDERING BEFORE "NATURAL SELECTION."

On the back of the page that bears the first use of "natural selection," you’ll see a first draft of the phrase: "A natural means of selection," which is contained in an entirely crossed out paragraph that Darwin presumably didn’t like. "It’s interlined into that sentence, and he found [it] not satisfactory for the head of a section," Kohn says. "So the act of actually making the head gets him away from this pretty awkward ‘natural means of selection,’ and he just condenses it as natural selection. You can see that intellectual process." Many people had looked for what came before "natural selection" in the sketch, first published in 1909, but couldn't find it, Kohn says; the museum’s new transcription of the sketch picked up the detail.

3. THE FAMOUS BRANCHING TREE.

Turn to Page 36 of Notebook B and you’ll see the famous branching tree—what Darwin would ultimately call the principle of divergence—and the words “I think.” In the papers, Kohn says, you can see the scientist struggle with the tree. “He is left trying to understand how the tree, why it’s irregular, and how does branching occur—which is nothing less of a problem [than] what is the origin of species?” Kohn says. “He has no answer for that, even after, two years later, he has discovered natural selection. He doesn’t see how natural selection produces the tree. So, in other words, this is a big problem ... but he denies it’s a problem.”

As he’s trying to work it out, Kohn says, Darwin hits a wall, “three or four times, from 1838 to 1855. The wall is progress. He can’t see a way of explaining the tree without relying on a principle that there is progress in nature, that nature is inherently progressive, getting better and better. That, he feels, is metaphysically wrong. He doesn’t want to make that kind of statement.”

For a five year period, from 1854 until he published in 1859, Darwin performed a number of experiments to see how divergence might occur, some of which took place in the fields around his house; analyzed scientific literature for patterns and biogeographic information; and read widely, including the works of Adam Smith and Thomas Malthus. “You can see five or six pieces of note taking,” Kohn says, “where it finally crystallizes that if there is sufficiently strong natural selection that could actually create new species.”

4. THE MYSTERIOUS EDITOR OF ORIGIN’S LAST LINE.

In 1844, Darwin wrote "1844 Essay," the draft of which was 189 pages long. "He gives the draft to someone who copies it out for him, in clear handwriting," Kohn says. "That’s one of the signs that he was thinking of publishing, by the way, because no one could have read his draft. He’s definitely going to show it, and he does show it to a few people." 

These close and trusted associates included his wife, Emma; Josiah Wedgwood, a potter who was family by marriage (“He was the person who told Darwin’s father to let his son go on the Beagle voyage,” Kohn says); and botanist Joseph Hooker, the only scientist Darwin trusted. All of the readers made notes; “the vast majority are by Hooker,” who, Kohn says, had “25 or 30 sometimes quite extensive comments.”

But one as-yet unidentified reader made a significant change to the last line of the essay. When Darwin sent it around, the last line read “From so simple a beginning, endless forms most beautiful and most wonderful have been evolved.” (This is the first time, Kohn notes, that the scientist used the word “evolved” in the book.) The mysterious editor added “& are being,” so that the line read “From so simple a beginning, endless forms most beautiful and most wonderful have been, and are being, evolved.” Darwin, Kohn says, “was smart enough to keep it, because it really does help. The rhythm, or poetry of that sentence gains a certain amount of power by this little piece just before the last word. It gives you the sense of drama and of a dynamic, developing process.”

There are a few candidates for who might have inserted the phrase; Kohn believes the writer is a woman, possibly Georgina Tollet, who “[Darwin] regarded as an intellectual equal." He plans to visit Cambridge to study correspondence between the two, which hasn’t yet been digitized, to compare her handwriting to the writing on the essay and hopefully solve the mystery.

5. HIS KIDS’ DRAWINGS ON PAGES OF THE ORIGIN.

Darwin didn’t seem to care much about preserving the drafts of On the Origin of Species, which totaled more than 500 pages. We have his children to thank for the 41 pages that did survive, some of which have their drawings on the back. “He didn’t seem to care that the kids would take the sheets out of the desk drawer or the cubby where he kept these things,” Kohn says. “There’s a report from his daughter—who was, by that time, grown up—that he threw out a whole bunch of Origin sheets, and she picked up a few to have them. So between the ones that they kept just because the kids drew on them, and the few that Henrietta salvaged, that’s the basic core of what survives, these 41 pages.” On the back, Kohn says, are heavily modified passages of Origin: “You can see the revision process, so it’s something that we would have really loved to have that final piece. This is the last piece of the creation of the Origin, really.”

6. A LETTER NAMING HIS WIFE HIS LITERARY EXECUTOR.

After he finished the 1844 draft, Darwin took that document and what he had written in 1842 "and stuck them in a cupboard under the stairs once he decided not to publish,” Kohn says. “It was just left there, and he didn't take it out again, apparently, until a decade later. But at that time, when he decided not to publish, he wrote a letter to his wife, to Emma Darwin, and made her his literary executor, because he was contemplating—‘I’ve written my book, or most of my book, and I might die.’ You have this description of how he wants her to find a scientific editor who will take care of it if he dies, and he gives this whole list of people and his opinion of them. It's lovely.” Among the candidates were geologist Charles Lyell ("Mr Lyell would be the best if he would undertake it: I believe he wd find the work pleasant & he wd learn some facts new to him"), naturalist Edward Forbes ("The next best Editor would be Professor Forbes of London"), and Hooker ("Dr Hooker would perhaps correct the Botanical Part probably—he would do as Editor—Dr Hooker would be very good").