The finches beaks because each finch had to have a different size beak for gathering food like you would need a larger beak for woodpecking and a smaller beak for gathering bugs. Darwin realized that all the finches though the same species had different beak sizes which had been slowly modified by natural selection because some types of finches couldn't find enough food with their beak size so over time they evolved to adapt to their habitat and they would be able to get more food because they could use a different type of beak.
