Cumberland Island, the southernmost of the Georgia barrier islands, is one of the world’s most secluded, pristine natural habitats. Occupying an area larger than Manhattan, the island boasts three ecosystems, including salt marshes, forest and miles of beach, where wild horses can often be observed running along the shores. Thomas M. Carnegie first purchased land on the island in the 1880s, and many descendants of the Carnegie family still live there today, including jewelry designer Gogo Fergus...