THE ETERNAL FRONTIERS

Schmitz postulates a galaxy populated by three uneasily coexisting groups: Swimmers -- humans who have mutated to live in a no-gravity environment; Walkers -- who insist man was meant to live on the surface of planets; and Galestrals -- superscientists from a mysterious, dangerous planet. There's complicated action involving political tensions, several ghosts and a host of alien monsters, but the characters are flimsy and the chief virtue here remains the intriguing sociological premise.