The standard assumption about Darwin is that he first had insight into the fact that the different species were not immutable from studying the differences in the size of the beaks of the finches on the different islands of the Galapagos. The truth is that Darwin failed to even keep track of which islands the finches he collected came from, and even misclassified them as other birds. He had to go back to Captain Fitz Roy for specimens because Fitz Roy had kept tract of which islands his own specimers were from.
In reality, on the voyage home as Darwin was examining the Mockingbirds he had collected and on which he did keep track of the islands of origin, he noted differences in the length and curvature of the beak and the coloration of the feathers. This suggested to him that species could change according to the unique environments of each island.
The Galapagos Mockingbird is of moderate size with a moderate size beak. It does not have a distinct black malar (neck) stripe. The Hood Mockingbird, unique to Espanola, is larger and has a longer more down curved beak and a prominent black malar stripe. As is typical of many of the wildlife of the Galapagos it is not afraid of humans as shown by the figure of a Hood Mockingbird almost climbing on a visitors lap.