Darwin left England aboard HMS Beagle on December 27, 1831, under the command of Captain Robert Fitzroy. The Beagle voyaged across the Atlantic ocean, while the crew carried out hydrographic surveys around the coasts of southern South America, then sailed to Tahiti and Australia, returning nearly five years later on October 2, 1836. Throughout this voyage, the second for the Beagle, Darwin spent most of his time, 39 months out of the 57-month-long voyage, exploring on land. This time was dedicated to his studies of geology, collecting fossils, and making detailed observations of plants and animals that would later form the basis for his theory of evolution by natural selection.
Several times during the voyage of the Beagle Darwin sent his notes and samples back to Cambridge, along with letters to his family. These materials focused on his areas of expertise in geology, beetle collecting, and marine invertebrates, but also increasingly on zoology and related biological subjects in which he was a novice. In the seacliffs of Patagonia he discovered extinct fossil species including skeletons of giant mammals, which he named Megatherium (now known to be a type of giant ground sloth that lived from 2 million to 8,000 years ago) and shipped back to England. Also in Patagonia Darwin noted that some areas showed stepped terraces along the coast, and each had seashells along the flat surfaces. He correctly interpreted the terraces to reflect that the land was rising relative to the sea, and these shelves represented former shorelines. While in Chile Darwin experienced a major earthquake and noticed that many mussel beds were stranded above the high-water mark, reminiscent of the raised beaches he saw in Tierra del Fuego. While high in the Andes Darwin found beds of marine shelly fossils and beach deposits; he inferred that these ranges had been uplifted from the sea.
The Beagle visited the Galápagos Islands in September and October 1835, then traveled to other islands including atolls in the Pacific. Darwin investigated the coral reefs around these islands and hypothesized that the volcanic islands gradually sank below sea level, and as they did the corals grew upward, eventually forming a ring of coral reefs surrounding a sunken island. While in the Galápagos, Darwin found that different types of mockingbirds were on different islands, and from this he inferred, nearly a quarter century later, that the different species evolved separately on each island, from a common ancestor. While the Beagle was at the Galápagos, Darwin also collected many samples of finches. At the time, however, he did not appreciate the differences between them, and only after he returned to England and shared the finches with the ornithologist John Gould on January 4, 1837, did he recognize that the finches represented 12 separate species. After more work Darwin appreciated that the finches, mockingbirds, and also tortoises had similar ancestors on the south American mainland and had all evolved into distinct species on each of the separate islands of the Galápagos. The term Darwin's finches was coined by English ornithologist Percy Lowe for these birds in 1936 and popularized by British ornithologist David Lack (1910-73) in 1947.
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