 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 . 
