In a letter to FitzRoy accepting the post Darwin explained that he expected the voyage to be a "second birth." There is no doubt that the years he spent exploring the South American continent and the offshore islands of the Galapagos honed his skills as a collector, observer, and theorist.
Often seasick, Darwin rested horizontally in a hammock during the worst motion and spent long periods of time ashore whenever the opportunity arose. He delighted in the exotica of the tropics.
Adventures
Adventurous, he braved his way through armed political rebellions, rode with the gauchos in Argentina, and on collecting and shooting expeditions justified his earlier devotion to sport.
He joined the crew in towing the ship's boats upstream and once rescued the expedition by running to save a boat from a tidal wave. He seemed to relish danger and was sustained in the considerable discomfort by a lively curiosity.
"We have in truth the world before us. Think of the Andes; the luxuriant forest of the Guayquil, the islands of the South Sea & New South Wales. How many magnificent & characteristic views, how many and curious tribes of men we shall see. What fine opportunities for geology and studying the infinite host of living beings: Is this not a prospect to keep up the most flagging spirit?" Darwin had brought his own books to augment the ship's extensive library. The most important scientific work was the first volume of Charles Lyell's Principles of Geology, which Henslow had urged him to read though not to believe.
Lyell argued that the face of the Earth had changed gradually over long periods of time through the continuing, cumulative effects of local disturbances, such as eruptions, earthquakes, erosion, and deposition. Such disturbances had existed in the distant past and could be observed in the present. This view differed dramatically from that held by most contemporary geologists, who hewed to the belief that changes in the face of the Earth resulted from short- lived events of great violence that could raise mountains or flood the entire planet. During the first months of his journey Darwin was converted to Lyell's views by his own observations.
Visited São Tiago
About 1,800 miles southwest of the Canary Islands the Beagle visited São Tiago, a volcanic island in the Cape Verde Islands. From the harbour Darwin saw a band of white rock extending horizontally at a height of about 45 feet above the base of the sea cliffs. The formation was calcareous and contained numerous shells, almost all of which could be found on the coast.
Darwin reasoned that a stream of lava from the ancient volcanoes had flowed over what had been ancient seabed, baking it to form the hard white rock. The whole island had subsequently been heaved up to make the sea cliff from the white band downward. Darwin also realized that the island's surface had been formed by a succession of volcanic events, not a single dramatic one. He discerned an initial subsidence, the settling of the surface around the original craters, its building up from new lava spills from different craters, and further subsidence and building up over a long period of time.
Later, in Chile, Darwin witnessed his first earthquake. He saw the land rise before his eyes. Then, after crossing the Andes in 1835, he wrote to his sister that he could understand "to a certain extent the description & manner of the force, which has elevated this great line of mountains." He had found fossil shells at an elevation of 12,000 feet, and he theorized that a chain of suboceanic volcanoes had poured forth enormous quantities of lava that formed the Andes through a further process of upheaval and fracturing.
Marveled at the whole South American continent
Darwin marveled at the whole South American continent, which he read as a vast testing ground for Lyell's ideas. To his cousin William Fox he wrote, "Everything in America is on such a grand scale. The same formations extend for 5 or 600 miles without the slightest change--for such geology one requires 6 league boots."
The data Darwin collected on the Beagle provided him with material for three books on South American geology. Although his theories of continental change have been superseded by the theory of plate tectonics, his descriptions in letters to Henslow, which Henslow excerpted and read before the Cambridge Philosophical Society and the Geological Society of London, brought him celebrity in scientific circles even before his return. While still on the voyage he challenged Lyell's view of the formation of coral reefs by volcanic action.
Darwin contended that the reefs were part of a process of gradual changes in the Earth's crust resulting from the subsidence of some landmasses and corresponding elevations elsewhere. He explained that coral, which only grows in shallow waters, forms a reef by building up on the seafloor as the floor subsides.
Prediction
He predicted that if a whole island sank below the ocean's surface, and the coral continued to grow, a reef would turn into an atoll around a lagoon. Lyell was convinced and supported Darwin's reinterpretation, which deep-sea borings in the 20th century have confirmed. Darwin's geoloic ponderings were important for geology and to his scientific development.
Many of the rocks he examined contained fossils, and his constant exposure to the evidence of extinct species and the similarity of many of them to living species kept one problem at the fore: By what mechanism did new species replace extinct ones? During the voyage Darwin developed confidence in his own observations as well as the ability to grasp a problem and work at it steadily. The isolation of the voyage, combined with the exposure to new phenomena, taught him to think for himself within the familiar scientific culture of his time.
He developed a rare combination of strengths: a dedication to careful fact gathering and a propensity to theorize about the facts. His geologic pondering on a continental scale encouraged him to search for universal laws. The voyage of the Beagle transformed Darwin into an independent and adventurous scientist who had the courage to embrace the heretical idea of the transmutation of species.