Why did Charles Darwin sail around the world on the Beagle?

Charles Darwin joined the voyage of the HMS Beagle primarily as a self-financed gentleman companion to the ship's captain, Robert Fitzroy. Fitzroy, an aristocrat who feared the isolation of command, sought a social equal to accompany him on the expedition.1

The voyage was an imperial-evangelical mission with several official objectives:
* Surveying: The primary goal was to survey the coastal regions of Patagonia to facilitate British trade.1
* Navigation: The crew aimed to obtain a complete circle of measurements of longitude around the globe.2
* Repatriation: The ship was tasked with returning three indigenous people from Tierra del Fuego who had been brought to England previously to be Christianized.12

Inspired by the travel narratives of Alexander von Humboldt, Darwin accepted the invitation to serve as the ship's naturalist. The specimens and observations he collected during this five-year circumnavigation provided the essential evidence for his later theory of natural selection.123