Search the Sky (Sci-Fi Classic)

Halsey's Planet is in decline, and when a generation ship arrives, having failed to contact six other planets, Ross is sent to discover the state of the interstellar colonies. He is given a ship which can make the trip from colony to colony almost instantaneously. The technology used in the ship has been kept secret because it could give rise to interstellar war if one colony decided to conquer others. However, the isolated populations are also affected by genetic drift resulting in a decline in their societies.