The flora and fauna of the Galápagos Archipelago, a group of islands 650 miles west of Ecuador in the Pacific Ocean, provided the inspiration for Charles Darwin's On the Origin of Species by Means of Natural Selection, which outlined his theory of evolution. Although Darwin spent some time studying medicine and later prepared for the clergy, graduating in 1828 from Christ's College, Cambridge, he couldn't deny his interest in geology and natural history. He spent five years (1831-36) as a naturalist aboard the HMS Beagle on an exploration of South America and Australia. In September 1835 the Beagle reached the Galápagos Archipelago. This archipelago, Darwin wrote, seems to be a little world within itself, the greater number of its inhabitants, both vegetable and animal, being found nowhere else. Darwin observed 26 species of birds, only one of which was known to exist anywhere else, as well as giant tortoises and other unusual reptiles. Each species, he observed, was uniquely adapted to the particular island on which it lived. Upon his return to England, Darwin refined his notes and continued to make scientific observations, this time of his own garden and of the animals kept by his family. In 1859, after 23 years of sustained work, he published On the Origin of Species, in which he argued that traits such as size and color vary from species to species and that individual variations of these traits are passed down from parents to offspring. More progeny are produced than there is available sustenance. Variations that contribute more successfully to attracting a mate and reproducing are passed down to more offspring, eventually influencing the entire species. Through this process of natural selection, the highly complex species of today gradually evolved from earlier, simpler organisms.