The Amphibians

From the back of the front cover of the Galaxy Novel printing: "A masterpiece by a master of science fiction, this is the story of Earth's inconceivably remote future... a world as strangely different from ours as ours is from the world of dinosaurs! <p>Into this world goes the Time Traveler, a man of our era - a barbaric alien in comparison with the possessors of a science vastly superior to ours - accompanied by the enigmatically fascinating Amphibian Woman. <p>In the Time Traveler's challenge to the Dwellers, giants both in body and mind, and his adventures in this alien life of ages yet to come, the reader will find an invitation to one of the greatest quests in all science fiction!"

Online version available at S. Fowler Wright website.
Originally intended to be a trilogy, the third installment was never written. Publications can be confusing as both the first and second installment have been published as The World Below. They are typically paired as The Amphibians (part 1) and The World Below (part 2), or as The World Below (part 1) and The Dwellers (part 2).