For more details on this topic,
see Second voyage of HMS Beagle.
The voyage of the Beagle
The voyage lasted almost five years and, as FitzRoy had intended, Darwin spent most of that time on land investigating geology and making natural history collections, while the Beagle surveyed and charted coasts. He kept careful notes of his observations and theoretical speculations, and at intervals during the voyage his specimens were sent to Cambridge together with letters including a copy of his journal for his family. He had some expertise in geology, beetle collecting and dissecting marine invertebrates, but in all other areas was a novice and ably collected specimens for expert appraisal. Despite repeatedly suffering badly from seasickness while at sea, most of his zoology notes are about marine invertebrates, starting with plankton collected in a calm spell.
On their first stop ashore at St Jago, Darwin found that a white band high in the volcanic rock cliffs included seashells. FitzRoy had given him the first volume of Charles Lyell’s Principles of Geology which set out uniformitarian concepts of land slowly rising or falling over immense periods,[II] and Darwin saw things Lyell's way, theorising and thinking of writing a book on geology. In Brazil, Darwin was delighted by the tropical forest, but detested the sight of slavery.
At Punta Alta in Patagonia he made a major find of fossils of huge extinct mammals in cliffs beside modern seashells, indicating recent extinction with no signs of change in climate or catastrophe. He identified the little known Megatherium, with bony armour which at first seemed to him like a giant version of the armour on local armadillos. The finds brought great interest when they reached England. On rides with gauchos into the interior to explore geology and collect more fossils he gained social, political and anthropological insights into both native and colonial people at a time of revolution, and learnt that two types of rhea had separate but overlapping territories. Further south he saw stepped plains of shingle and seashells as raised beaches showing a series of elevations. He read Lyell’s second volume and accepted its view of “centres of creation” of species, but his discoveries and theorising challenged Lyell's ideas of smooth continuity and of extinction of species.