I just spent a couple of highly enjoyable hours (I know, I really should get out more) surfing Darwin Online. It really is a stonking resource. Someone should give John van Wyhe and his colleagues a medal.
For some bizarre reason, it had never occurred to me before to see if Darwin's original Red Notebook—the one from which this weblog takes its name—is available on the site. It is, of course.
Editor Sandra Herbert's excellent introduction puts Darwin's Red Notebook in context, explaining how he began it towards the end of the Beagle voyage—before he was a believer in evolution—where he used it as a field notebook. On returning to England, however, he turned the book through 90 degrees (for ease of writing), and began making his first notes on transmutation.
Herbert makes a convincing case for supporting the chronology recorded retrospectively in Darwin's personal 'Journal', which states:
In July  opened first note Book on "transmutation of Species". — Had been greatly struck from about month of previous March on character of S. American fossils—& species on Galapagos Archipelago. — These facts origin (especially latter) of all my views.
The notebook referred to by Darwin was not his Red Notebook, but one of the two books which immediately followed it—his first notebook dedicated to transmutation. But Darwin's first notes on transmutation begin on pages 129—130 of the Red Notebook (my emphasis added):
Should urge that extinct Llama [Macrauchenia patachonica, collected by Darwin during the Beagle voyage] owed its death not to change of circumstances; reversed argument. knowing it to be a desert. — Tempted to believe animals created for a definite time: — not extinguished by change of circumstances:
The same kind of relation that common ostrich bears to (Petisse. & diff kinds of Fourmillier): extinct Guanaco to recent: in former case position, in latter time. (or changes consequent on lapse) being the relation. — As in first cases distinct species inosculate, so must we believe ancient ones: [therefore] not gradual change or degeneration. from circumstances: if one species does change into another it must be per saltum — or species may perish. = This inosculation representation of species important, each its own limit & represented. — Chiloe creeper: Furnarius. Caracara Calandria; inosculation alone shows not gradation; —
Darwin had a long way to go. At the time he made these preliminary notes—in all likelihood, 170 years ago this month—he had only just begun to believe in evolution. He was yet to have his eureka moment and work out the mechanism for evolution (Natural Selection), believing that transmutation must occur per saltum—through leaps—an idea he was later to reject. Furthermore, he did not yet believe that species could become extinct through change of circumstances. But this Red Notebook entry does mark a key moment in Darwin's life: his first tentative step on the road towards On the Origin of Species.