Eternal Frontier — James H. Schmitz

Ape Man, Space Man

Earth's colonists have spread throughout the cosmos, and have almost divided into two separate species. One is the Swimmers, who have adapted to living in zero-gravity, and regard themselves as the next step in evolution, and those who prefer to live on the surface of a planet as little better than apes. The latter group, the Walkers, are not about to say farewell to the planets they grew up on, and think the Swimmers are not so much advanced as deranged.

Crowell, born a Swimmer but now a Walker by choice, is caught in the middle as the two sides seem headed for war. Then he discovers the true cause of the altercation: a hidden alien race moving behind the scenes to provoke a war so that they can pick up the pieces after the two sides have obliterated each other. And if Crowell cannot head off the war and convince both sides of the existence of the real enemy, both branches of the human race may be headed for untimely extinction.

This full-length novel and much more, fill a huge volume from the master of science fiction adventure