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Genetic relatedness was estimated among seven of the extant races of Giant tortoises (Geochelone elephantopus), of the Galapagos Archipelago and between these forms and mainland South American taxa, by starch gel electrophoresis. Genetic similarity among the island races is unrelated to carapace shape, suggesting that the latter was independently derived several times on separate islands. The Galapagos species shares no greater similarity with any mainland species than the latter do among themselves. Thus, the most direct common ancestor of the Galapagos forms is undoubtedly extinct.