Hood mockingbird, endemic to Española island, Galapagos. Ref: Wikimedia Commons.
Darwin's Galapagos finches and their differing beaks (see image here) are often credited as the original inspiration for Darwin's theory of evolution through natural selection. It's true that he brought many specimens of finches back from the Galapagos, and that they subsequently received a lot of attention in his scientific work and in "On the Origin of Species", but what many people don't know is that it was in fact a different group of birds that inspired the original thought: Mockingbirds, or Tencas in Spanish. Thanks to the wonders of the Internet, all of Darwin's writings are available online and we can go looking in his notebooks for this original thought in his own handwriting.
Amongst pages of notes about the geology of the Galapagos islands, geology being his primary focus on the voyage, he made a small note about the mockingbirds on Galapagos ("Galapagos Otaheite Lima" notebook, page 68):
The Thenca very tame & curious in these Islds. I certainly recognise S. America in ornithology, would a botanist?
Floreana mockingbird (left) and San Cristóbal mockingbird (right) from "The zoology of the voyage of H.M.S. Beagle... Part 3. Birds" by John Gould. Darwin sought expert knowledge for the description of the specimens he brought back, and edited this bird volume between 1938-41. Click on the image to see it larger.
He'd observed the mockingbirds in the west-coast of South America and could now recognize them among the birds of the Galapagos islands. The relationship between the species of the Galapagos islands and the South American mainland would form one of the pillars in his line of evidence for evolution through natural selection, but here at the very brink of uncovering the evidence it's the differences between the mockingbirds on the different islands that sparks the original thought (ornithological notes, pages 120-221).
Thenca: male: Charles Isd —
do: do: Chatham Isd. —
These birds are closely allied in appearance to the Thenca of Chile (2169) or Callandra of la Plata (1216). In their habits I cannot point out a single difference; <...> I have specimens from four of the larger islands; the two above enumerated, and (3349: female. Albermarle Isd.) & (3350: male: James Isd).—The specimens from Chatham & Albermarle Isd appear to be the same; but the other two are different. In each Isld. each kind is exclusively found: habits of all are indistinguishable. When I recollect, the fact that the form of the body, shape of scales & general size, the Spaniards can at once pronounce, from which Island any Tortoise may have been brought. When I see these Islands in sight of each other, & possessed of but a scanty stock of animals, tenanted by these birds, but slightly differing in structure & filling the same place in Nature, I must suspect they are only varieties. The only fact of a similar kind of which I am aware, is the constant asserted difference—between the wolf-like Fox of East & West Falkland Islds. — If there is the slightest foundation for these remarks the zoology of Archipelagoes—will be well worth examining; for such facts [would] undermine the stability of Species.
Here we have it, for ever recorded on paper: Darwin's original idea: ... such facts [would] undermine the stability of species! Written down in his ornithological notes on board the H.M.S. Beagle after leaving Galapagos in October of 1835, 24 years before the publication of "On the Origin of Species". If you go to the original page, you can see that in typical style, he's later added a careful "would" before "undermine the stability of Species". It says a lot about the man.