During his historic Galápagos visit in 1835, Darwin spent nine days making scientific observations and collecting specimens on Santiago (James Island). In the course of this visit, Darwin ascended twice to the Santiago highlands. There, near springs located close to the island's summit, he conducted his most detailed observations of Galapagos tortoises. The precise location of these springs, which has not previously been established, is here identified using Darwin's own writings, satellite maps, and GPS technology. Photographic evidence from excursions to (...) the areas where Darwin climbed, including repeat photography over a period of four decades, offers striking evidence of the deleterious impact of feral mammals introduced after Darwin's visit. Exploring the impact that Darwin's Santiago visit had on his thinking - especially focusing on his activities in the highlands - raises intriguing questions about the depth of his understanding of the evolutionary evidence he encountered while in the Galápagos. These questions and related insights provide further evidence concerning the timing of Darwin's conversion to the theory of evolution, which, despite recent claims to the contrary, occurred only after his return to England. (shrink)