When their sun began to wane, the Xlarnans at first retreatedunderground to hoard the heat and life-supporting energies which theirnuclear generators could supply. But as their world grew colder,century after century, they devised a means of creating a substitutefor the ionosphere—a protective layer of radioactive gases in theupper reaches of the sky which could warm them by means of its slow,controlled reaction, give them eternal light, and yet absorb its ownharder radiations.

Thus—a planetary cell of life, isolated from the universe,independent of solar heat. And the Xlarnans at last emerged from theirsubterranean cities to take up life anew in a tropical Paradise thatknew neither nightfall nor seasons. They missed the starlit nightskies of old, the sunrises and sunsets, and most of all the stupendouscelestial rainbow, the Great Ring, which some of them believed to beformed of the particles of a large satellite that had encircled theirworld back in the dim Beginning.

But the time arrived when they knew they were losing control of theirreaction sphere in the sky. The hard radiations increased inexorablyin spite of all the coolants they could generate and send aloft. Theyhad to admit that the day would come when they would be destroyed bythe very instrument that had given them an extra hundred millenniums oflife.

At the end of time—the Xlarnans, pressed against a wall, the reactionsphere, from which came hard radiations, burning them. The ethnicurge to survive in the face of swiftly approaching death. Necessitymothering invention. And then—

The Chronotron....

Electronic envelopes speeding faster than light. Three dimensionalnature rejecting the envelope. Only in Time can anything be in twoplaces—along the duration line.

The Chronotron—planting new Cause in the beginning of Effect. Andthere is alternate time.

Large numbers of Xlarnans, through the Chronotron, back to thebeginning of the reaction sphere era, an already advanced race withthe course of another hundred thousand years to run before facing thethreat from the sky once more.

The first cycle ends, and at the last extremity of alternate timeveritable super beings achieve immortality. With immortality, lessprocreation. And at last, sterility.

Still the deadly threat above them. The daily promise of sudden andcomplete devastation. Now there are rockets at last, but certaintechniques and necessary discoveries in the fields of chemistry andmetallurgy elude them. Attempted space flights end in collisions withmeteors or death due to radiations in the outer void—but escapevelocity never achieved.

Then came—THE THEORY....

Very vague and unidentifiable fossils discovered in astoundingly deepstrata. Nothing definite, but a bothersome hint of high development.Hypothesis evolved into theory; Xlarn had known a complete geologicalcycle before the Be