Once they passed a handful of buildings, the biggest of which seemed to be a chemical plant. Several times they watched boxy vehicles pass below them. They were seen only once. A box stopped suddenly, and humanoid shapes spilled out, ran in circles, then produced sticks which they pointed at the lander. A moment later they were out of sight.

There were great pale shapes in the wet jungle. They couldn’t be glacier-scoured boulders; not here. Louis wondered if they might be tremendous fungi. He stopped wondering when he saw one move. He tried to point it out to Chmeee. The kzin ignored him.

The road curved away to antispinward as it approached a range of craggy mountains; it jogged through a notch in the range, rather than carving its own path, then jogged right to run alongside the swamp again.

But Chmeee swerved left and accelerated. The lander streaked along the portward side of the range, trailing a plume of fire. Abruptly the kzin spun the lander around, braked, and set down at the foot of a granite cliff.

He said, “Let us step outside.”

The scrith shell of the mountain would block the Hindmost’s microphones, but they’d feel still safer outside the lander. Louis followed the kzin.

The day was bright and sunny—too bright, as this arc of the Ringworld approached its nearest point to the sun. A stiff warm wind was blowing. The kzin asked, “Louis, were you about to tell the Hindmost of the Ringworld engineers?”

“Probably. Why not?”

“I assume we’ve come to the same conclusion.”

“Doubtful. What would a kzin know of Pak protectors?”

“I know everything in the records of the Smithsonian Institute, what little there is. I have studied the testimony of the asteroid belt miner, Jack Brennan, and holos of the mummified remains of the alien Phssthpok and of the cargo pod from his ship.”

“Chmeee, how did you get hold of that stuff?”

“Does it matter? I was a diplomat. The existence of the Pak has been a Patriarch’s Secret for generations, but any kzin who must deal with humans is required to study the records. We learn to know our enemy. I may know more of your ancestry than you do. And I surmise that the Ringworld was built by Pak.”

Six hundred years before Louis Wu’s birth, a Pak protector arrived in Sol system on a mission of mercy. It was through this Phssthpok, via the Belter Jack Brennan, that historians learned the rest of the story.

The Pak were native to a world in the galactic core. They lived their lives in three stages: child, breeder, protector. The adults or breeders were just intelligent enough to swing a club or throw a stone.

In middle age, if they lived long enough, Pak breeders developed a compulsion to gorge on the plant called tree-of-life. A symbiotic virus in the plant triggered the change. The breeder lost its gonads and teeth. Its skull and brain expanded. Lips and gums fused into a hard, blunt beak. Its skin wrinkled and thickened and hardened. Its joints became enlarged, offering a larger moment arm to the muscles, increasing their strength. A two-chambered heart developed in the groin.

Phssthpok came tracking a Pak colony ship that had reached Earth more than two million years earlier.

The Pak were in a constant state of war. Previous colonies to nearby worlds in the galactic core had always been overrun by subsequent waves of ships. Perhaps that was the reason this ship had come so far.

The colony was large and well-equipped, and guided by beings tougher and smarter than humans. It had failed nonetheless. Tree-of-life grew in Earth’s soil, but the virus didn’t. The protectors had died out, leaving a lost population of Pak breeders to fend for themselves … and leaving records of a cry for help that had crossed thirty thousand light-years to the Pak home world.

Phssthpok found those records in an ancient Pak library. And Phssthpok crossed thirty thousand light-years, all alone in a slower-than-light craft, seeking Sol system. The resources that built that craft, in knowledge and minds and materials, were resources Phssthpok had conquered and held by war. His cargo pod was jammed with tree-of-life roots and seeds, and bags of thallium oxide. His own research had discovered the need for that unusual soil additive.

It might have occurred to him that the breeders would mutate.

Among the Pak a mutant stood no chance. If the children smelled wrong to their protector forefathers, they were killed. On Earth—perhaps Phssthpok counted on a lower mutation rate, this far from the savage cosmic-ray density among the core suns. Perhaps he took his chances.

The breeders had mutated. By Phssthpok’s time they showed little resemblance to the Pak breeder—barring certain changes at middle age, when the production of eggs stopped in females, and when both sexes showed wrinkling of skin, lost teeth, swelling of joints, and a restlessness and dissatisfaction that was all that remained of the hunger for tree-of-life. Later in life, heart attacks would result from the lack of the second heart.

Phssthpok learned none of this. The rescuer died almost painlessly, with no more than a suspicion that those he intended to rescue had become monsters, and had no need of him at all.

Such was the tale that Jack Brennan told to United Nations representatives before his disappearance. But Phssthpok was dead by then, and Jack Brennan’s testimony was doubtful. He had eaten tree-of-life. He had become a monster; his braincase in particular was expanded and distorted. Perhaps he had become mad too.

It was as if a load of spinach noodles had been spilled all over this rocky area. Strips of greenery, fuzzy to the touch, hugged the ground in places where dirt had packed itself between the boulders. Clouds of insects buzzed around their ankles, staying within inches of the ground.

“Pak protectors,” said Louis. “That’s what I thought, but I’ve been having trouble making myself believe it.”

Chmeee said, “The vacuum suits and the Grass Giant’s armor show their shape: humanoid, but with enlarged joints and a face pushed forward. There is more proof. We’ve met so many hominids, all different. They had to be derived from a common ancestor: your own ancestor, the Pak breeder.”

“Sure. It’d also tell us how Prill died.”

“Does it?”

“Boosterspice was tailored for the metabolism of Homo sapiens. Halrloprillalar couldn’t use it. She had her own longevity drug, and it could be used by a number of species. It struck me that Prill’s people might have made it from tree-of-life.”

“Why?”

“Well, the protectors lived thousands of years. Some factor of tree-of-life, or a subcritical dose of it, might trigger just enough of the change to do that for a hominid. And the Hindmost says Prill’s supply was stolen.”

Chmeee was nodding. “I remember. One of your asteroid mining craft boarded the abandoned Pak spacecraft. The oldest man in the crew smelled tree-of-life and went mad. He ate beyond the capacity of his belly, and died. His crewmates could not restrain him.”

“Yah. Now, is it too much to expect that the same thing happened to some UN lab assistant? Prill walks into the UN building carrying a flask of Ringworld longevity drug. The UN wants a sample. A kid barely too young for his first dose of boosterspice—forty, forty-five—opens the flask. He’s got the eyedropper all ready. Then he gets a whiff. He drinks it all.”

Chmeee’s tail lashed air. “I would not go so far as to say that I liked Halrloprillalar. Still, she was an ally.”

“I liked her.”

The hot wind blew around them, filled with dust. Louis felt harried. They wouldn’t get another chance to talk in privacy. The probe that relayed signals to and from Needle would soon be too high up the Arch for this kind of trick to work.

“Can you think like a Pak for me, Chmeee?”