A tortoise is a reptile of the order Testudines which lives on land. Like their aquatic cousins, the sea turtles, tortoises are shielded from predators by a shell. Most are quite small, but island tortoises have several times evolved to a large size: those on the Galapagos are famous. Darwin made one of his rare mistakes when he did not note which island each of them came from. Later it became clear that they were slightly different on different islands.