What observations did Darwin make on the Galápagos Islands that helped convince him that evolution occurs?a. the existence of animals that did not fit into Linnaeus's classification system;b. the similarities and differences among cacti and tortoises on the different islands;c. the presence of species he had seen on other tropical islands far from the Galápagos;d. the radioactive age of the rocks of the islands;e. fossils of human ancestors