According to the Introduction: 'THE three books gathered under the title 'In Search of the Castaways' occupied much of Verne's attention during the three years following 1865. The characters used in these books were afterwards reintroduced in 'The Mysterious Island,' which was in its turn a sequel to 'Twenty Thousand Leagues Under the Sea.' Thus this entire set of books form a united series upon which Verne worked intermittently during ten years. 'In Search of the Castaways,' which has also bee