It is enough that the Galapagos finches provide another bit of evidence for bioevolution. It is not necessary that they weren’t a major factor influencing Darwin himself. And the more recent study described in the book provides specific quantitative evidence for the operation of natural selection. (Creationists make the usual lame responses: (1) They are still finches. (2) The trend reverses when conditions change.)

In any case, the subject was a modern book where the evolution of the finches really was the subject, but I took the opportunity counter some myths.

Indeed, and notice also that neither of those claims were made, and that was very intentional. I’ve written about this previously. The birds (mockingbirds and finches) on the Galapagos did contribute, about as much as the Rheas as well as various animals on the Falklands, and fossils, and much of that was observed prior to arriving at the Galapagos.

His thinking about evolution clearly has roots during his voyage, but his most explicit evolution, as it were, of the evolution ideas may have been in mid to late 1837, the year after his voyage. By 1838 he was well under way in formulating his multiple theories about evolution. I would not count that as “long after” but it was certainly after. He was still probably more interested in, and thinking about, geological questions prior to that.

But, as I noted above, I think his thinking was developing coeval with observations he was making before entering the Pacific.

See for example http://darwin-online.org.uk/content/frameset?itemID=A932&viewtype=text&pageseq=1 (other in-depth articles can also be found)