During a trip to the galapagos islands, which observation led charles darwin to suspect that organisms change over time?

During the trip to Galapagos island, Charles Darwin suspected that the mainland finches resembles the island finches but their beaks were not same. They were not the same species. This made Darwin suspect that the organisms change over time. The change in size of the beak was due to the adaptation. This made Charles Darwin give the theory of Natural Selection.

i am going to guess that it is co2.