According to Darwin, what evolutionary purpose did the finches' beaks serve?

The finches of the Galapagos islands are a good example of evolution by natural selection in action. According to the principle of evolution by natural selection, 1. if there is variation in the populations and 2. that variation is heritable and 3. differences in fitness (number of surviving offspring) are related to the trait for which there is variation, then the population will become adapted for the historical environment. This has been shown to have occurred with the finches.

There is variation in beak size of Galopagos finches. The beaks range from large, thick beaks to small, more fragile beaks. The beak corresponds with the food that can be consumed by the finch. Only birds with large, thick beaks can consume large, hard seeds. Finches with the smallest beaks can only eat small, thin-shelled seeds.  

During periods of drought, only large, hard seeds are produced due to the lack of water. The lack of smaller seeds during these periods of time has led to the death of most birds that lack the large, thick beaks that are capable of consuming the available seeds. Only surviving birds are able to produce offspring and it has been shown that the beak size of offspring is similar to that of the parents (the trait is heritable). Therefore, within the population the frequency of birds with large, thick beaks increased during times of drought while the frequency of birds with small, thin beaks decreased to due lack of available food.