What did Charles Darwin observe in finches and tortoises in the Galapagos what theory is this?

In the finches, Charles Darwin observed the different beak shapes according to where they lived in the Galapagos. He later linked their beak shapes to the food they ate. Their beaks were shaped to make it more convenient to get their food. In tortoises, he observed the shapes of their shells. The shell shapes got either progressively curvier or progressively less curvy as you travel up or down the island. And I'm not 100% sure about the theory thing, but I think it's the Theory of Evolution