The treadmill was six meters across and four wide, a fragile wheel of branchwood sticks. Water running down the trunk helped to spin it, but runners were still needed.

It was getting easier; the treadmill was spinning faster. The cages must be almost passing each other. "Out!" Mark panted. "Runners, out!" Seven laughing children jumped from both sides of the treadmill, until only Mark and Karilly were left.

Above was a sudden glare as the sun passed into view. Karilly's dark skin shone with sweat; she breathed deeply as she bounded uphill alongside him. He knew she could understand him. "Karilly. When the up cage is at the top it ... doesn't weight anything. It takes all of us ... to lift the down cage. Right now ... the cages are next to each other. I can run by myself. In a little while . . .the down cage will be falling. I'll have to get out. Use the brake. Slow it down." She watched him as if she were listening. "So you jump out now."

Then he saw that she was afraid.

"Okay." He let the cage carry him around. Inverted, he scrambled down the other side. "I'm slowing it. Can you get out now?"

Karilly scrambled out.

Twenty klomters over his head, Lawri and her student flyers must be wondering what had gone wrong. Mark started the cage spinning again, letting his body do its accustomed work while his mind drifted.

Long ago and far away, there had been civilization.

London Tree had had stationary bicycles to run the elevators to the tree midpoint, and copsiks to run the bicycles. Citizens Tree was primitive. They had London Tree's CARM, of course: a thing of science dating from the day men came from the stars. Otherwise they must build everything.

Mark had shown the refugees how to build a lift. Mark had wanted to make bicycles, but the Scientists had built the treadmill instead. They kept the silver suit next to the treadmill with its helmet open. Citizens at the CARM could call for the lift through the radio in the suit.

Below him he could see the hollow space of the commons, and two children bounding east. The tall, dark girl was far ahead of the smaller boy, who moved in slower, shorter steps, as if tide were heavier for him.

His son. His size proved it. Mark would not have wished that on him; yet Rather would be the next Silver Man. Mark wondered if the citizens would appreciate their fortune. In the short lifetime of Citizens Tree there had been no need for an invulnerable fighter, and the silver suit had become a mere communications device.

Had it not been for one stupid, stubborn act, Mark would still be a citizen of London Tree. But he would never have seen the stars, and he would never have seen his son.