What specific feature of the Galapagos Finches made Darwin question their variations?

Darwin's observations of the Galapagos Finches highlighted significant variations in the size and shape of their beaks, which were closely linked to the types of food available on the different islands. He noted that each finch species had evolved distinct beak adaptations that suited their feeding habits, such as larger, stronger beaks for cracking seeds or narrower beaks for probing flowers. This variation in beak size and shape became a key piece of evidence for Darwin's theory of natural selection, as it illustrated how environmental factors could influence physical traits and contribute to the survival and reproductive success of species. Thus, the beak size of the Galapagos Finches serves as a critical example of adaptive evolution, prompting Darwin to further explore the implications of these variations in the context of species adaptation and evolution.