 The first systematic survey of giant tortoises was by Albert Günther of the British Museum, in 1875. Günther identified at least five distinct populations from the Galápagos, and three from the Indian Ocean islands. He expanded the list in 1877 to six from the Galápagos, four from the Seychelles, and four from the Mascarenes. Günther hypothesized that all the giant tortoises descended from a single ancestral population which spread by sunken land bridges. This hypothesis was later disproven by the understanding that the Galápagos, Seychelles, and Mascarene islands are all of recent volcanic origin and have never been linked to a continent by land bridges. Galápagos tortoises are now thought to have descended from a South American ancestor, while the Indian Ocean tortoises derived from ancestral populations on Madagascar.
