Top: A blue-footed booby. Second: A male frigatebird uses its red inflatable pouch to attract a mate. Third: Cactus trees differ by island. Fourth: Marine iguanas of the Galapagos are the only lizards in the world that can swim. Bottom: Seals make yearly visits from the mainland to the islands.
In 1835, after leaving South America, the Beagle sailed to the Galapagos Islands, about thirteen small islands six hundred miles from South America, near the Equator. There he saw many unique creatures found only on those islands, but similar to those on the mainland. Among these were marine iguanas and giant tortoises, which Darwin and the Beagle crew rode like horses. Not only were the animals different from those anywhere else on earth, many species had differences depending on which island they came from. For example, the Galapagos natives could tell from which island a tortoise came by the shape of its shell.
On the Galapagos Islands, Darwin also saw several different types of finch, a different species on each island. He noticed that each finch species had a different type of beak, depending on the food available on its island. The finches that ate large nuts had strong beaks for breaking the nuts open. Finches that ate small nuts and seeds had beaks for cracking nuts and seeds. Darwin noticed that fruit-eating finches had parrot-like beaks, and that finches that ate insects had narrow, prying beaks. He wrote: "One might really fancy that from an original paucity [scarcity] of birds ... one species had been taken and modified for different ends."
Later, Darwin concluded that several birds from one species of finch had probably been blown by storm or otherwise separated to each of the islands from one island or from the mainland. The finches had to adapt to their new environments and food sources. They gradually evolved into different species.