In 1835, British naturalist Charles Darwin, aboard the ship HMS Beagle, arrived at the Galapagos archipelago, a cluster of islands on the equator 600 miles west of South America. During his five weeks studying the fauna in the Galapagos, Darwin found the giant tortoises there greatly differed from one another according to which island they came from. Moreover, many islands developed their own races of iguanas. These observations contributed to his theory of “natural selection,” that species evolved over thousands of millions of years.