He went to the mouth of the cavern.

He saw only darkness within. Neither sulidoror nor nildoror were evident at the entrance. He took a cautious step inward. The cavern was cool, but it was a dry coolness far more agreeable than the mist-sodden chill outside. Drawing his fusion torch, he risked a quick flash of light and discovered that he stood in the center of an immense chamber, the lofty ceiling of which was lost in the shadows overhead. The walls of the chamber were a baroque fantasy of folds and billows and buttresses and fringes and towers, all of stone, polished and translucent, gleaming like convoluted glass during the instant that the light was upon them. Straight ahead, flanked by two rippling wings of stone that were parted like frozen curtains, lay a passageway, wide enough for Gundersen but probably something of a trial for the bulky nildoror who had earlier come this way.

He went toward it.

Two more brief flashes from the torch got him to it. Then he proceeded by touch, gripping one side of the opening and feeling his way into its depths. The corridor bent sharply to the left and, about twenty paces farther on, angled just as sharply the other way. As Gundersen came around the second bend a dim light greeted him. Here a pale green fungoid growth lining the ceiling afforded a minimal sort of illumination. He felt relieved and yet suddenly vulnerable, for, while he now could see, he could also be seen.

The corridor was about twice a nildor’s width and three times a nildor’s height, rising to the peaked vault in which the fungoids dwelled. It stretched for what seemed an infinite distance into the mountain. Branching off it on both sides, Gundersen saw, were secondary chambers and passages.

He advanced and peered into the nearest of these chambers.

It held something that was large and strange and apparently alive. On the floor of a bare stone cell lay a mass of pink flesh, shapeless and still. Gundersen made out short thick limbs and a tail curled tightly over broad flanks; he could not see its head, nor any distinguishing marks by which he could associate it with a species he knew. It might have been a nildor, but it did not seem quite large enough. As he watched, it swelled with the intake of a breath, and slowly subsided. Many minutes passed before it took another breath. Gundersen moved on.

In the next cell he found a similar sleeping mound of unidentifiable flesh. In the third cell lay another. The fourth cell, on the opposite side of the corridor, contained a nildor of the western species, also in deep slumber. The cell beside it was occupied by a sulidor lying oddly on its back with its limbs poking rigidly upward. The next cell held a sulidor in the same position, but otherwise quite startlingly different, for it had shed its whole thick coat of fur and lay naked, revealing awesome muscles beneath a gray, slick-looking skin. Continuing, Gundersen came to a chamber that housed something even more bizarre: a figure that had a nildor’s spines and tusks and trunk but a sulidor’s powerful arms and legs and a sulidor’s frame. What nightmare composite was this? Gundersen stood awed before it for a long while, trying to comprehend how the head of a nildor might have been joined to the body of a sulidor. He realized that no such joining could have occurred; the sleeper here simply partook of the characteristics of both races in a single body. A hybrid? A genetic mingling?

He did not know. But he knew now that this was no mere way-station on the road toward rebirth. This was the place of rebirth itself.

Far ahead, figures emerged from one of the subsidiary corridors and crossed the main chamber: two sulidoror and a nildor. Gundersen pressed himself against the wall and remained motionless until they were out of sight, disappearing into some distant room. Then he continued inward.

He saw nothing but miracles. He was in a garden of fantasies where no natural barriers held.

Here was a round spongy mass of soft pink flesh with just one recognizable feature sprouting from it: a sulidor’s huge tail.

Here was a sulidor, bereft of fur, whose arms were foreshortened and pillar-like, like the limbs of a nildor, and whose body had grown round and heavy and thick.

Here was a sulidor in full fur with a nildor’s trunk and ears.

Here was raw meat that was neither nildor nor sulidor, but alive and passive, a mere thing awaiting a sculptor’s shaping hand.

Here was another thing that resembled a sulidor whose bones had melted.

Here was still another thing that resembled a nildor who had never had bones.

Here were trunks, spines, tusks, fangs, claws, tails, paws. Here was fur, and here was smooth hide. Here was flesh flowing at will and seeking new shapes. Here were dark chambers, lit only by flickering fungoid-glow, in which no firm distinction of species existed.

Biology’s laws seemed suspended here. This was no trifling gene-tickling that he saw, Gundersen knew. On Earth, any skilled helix-parlor technician could redesign an organism’s gene-plasm with some cunning thrusts of a needle and a few short spurts of drugs; he could make a camel bring forth a hippopotamus, a cat bring forth a chipmunk, or, for that matter, a woman bring forth a sulidor. One merely enhanced the desired characteristics within sperm and ovum, and suppressed other characteristics, until one had a reasonable facsimile of the creature to be reproduced. The basic genetic building-blocks were the same for every life-form; by rearranging them, one could create any kind of strange and monstrous progeny. But that was not what was being done here.

On Earth, Gundersen knew, it was also possible to persuade any living cell to play the part of a fertilized egg, and divide, and grow, and yield a full organism. The venom from Belzagor was one catalyst for that process; there were others. And so one could induce the stump of a man’s arm to regrow that arm; one could scrape a bit of skin from a frog and generate an army of frogs with it; one could even rebuild an entire human being from the shards of his own ruined body. But that was not what was being done here.

What was being done here, Gundersen realized, was a transmutation of species, a change worked not upon ova but upon adult organisms. Now he understood Na-sinisul’s remark, when asked if sulidoror also underwent rebirth: “If there were no day, could there be night?” Yes. Nildor into sulidor. Sulidor into nildor. Gundersen shivered in shock. He reeled, clutching at a wall. He was plunged into a universe without fixed points. What was real? What was enduring?

He comprehended now what had happened to Kurtz in this mountain.

Gundersen stumbled into a cell in which a creature lay midway in its metamorphosis. Smaller than a nildor, larger than a sulidor; fangs, not tusks; trunk, not snout; fur, not hide; flat footpads, not claws; body shaped for walking upright.

“Who are you?” Gundersen whispered. “What are you? What were you? Which way are you heading?”

Rebirth. Cycle upon cycle upon cycle. Nildoror bound upon a northward pilgrimage, entering these caves, becoming … sulidoror? Was it possible?

If this is true, Gundersen thought, then we have never really known anything about this planet. And this is true.

He ran wildly from cell to cell, no longer caring whether he might be discovered. Each cell confirmed his guess. He saw nildoror and sulidoror in every stage of metamorphosis, some almost wholly nildoror, some unmistakably sulidoror, but most of them occupying intermediate positions along that journey from pole to pole; more than half were so deep in transformation that it was impossible for him to tell which way they were heading. All slept. Before his eyes flesh flowed, but nothing moved. In these cool shadowy chambers change came as a dream.

Gundersen reached the end of the corridor. He pressed his palms against cold, unyielding stone. Breathless, sweat-drenched, he turned toward that last chamber in the series and plunged into it.