wouldn't believe the truth, they wouldn't dare believe, and God alone knows what they'd do."

Looking around, Radek saw a large, plastic-lined room, filled with cages. As the lights went on, white rats and guinea pigs stirred sleepily. One of the rats came up to nibble at the wires and regard the humans from beady pink eyes.

Lang bent over and studied the label. "This fellow is, um, 66 years old. Still fat and sassy, in perfect condition, as you can see. Our oldest mammalian inmate is a guinea pig: a hundred and forty-five years. This one here."

Lang stared at the immortal beast for a while. It didn't look unusual ... only healthy. "How about monkeys?" he asked.

"We tried them. Finally gave it up. A monkey is an active animal—it was too cruel to keep them penned up forever. They even went insane, some of them."

Footfalls were hollow as Lang led the way toward the inner door. "Do you get the idea?"

"Yes ... I think I do. If heavy radiation speeds up aging—then natural radioactivity is responsible for normal aging."

"Quite. A matter of cells being slowly deranged, through decades in the case of man—the genes which govern them being mutilated, chromosomes ripped up, nucleoplasm and cytoplasm irreversibly damaged. And, of course, a mutated cell often puts out the wrong combination of enzymes, and if it regenerates at all it replaces itself by one of the same kind. The effect is cumulative, more and more defective cells every hour. A steady bombardment, all your life ... here on Earth, seven cosmic rays per second ripping through you, and you yourself are radioactive, you include radiocarbon and radiopotassium and radiophosphorus ... Earth and the planets, the atmosphere, everything radiates. Is it any wonder that at last our organic mechanism starts breaking down? The marvel is that we live as long as we do."

The dry voice was somehow steadying. Radek asked: "And this place is insulated?"

"Yes. The original plant and animal life in here was grown exogenetically from single-cell zygotes, supplied with air and nourishment built from pure stable isotopes. The Institute had to start with low forms, naturally; at that time, it wasn't possible to synthesize proteins to order. But soon our workers had enough of an ecology to introduce higher species, eventually mammals. Even the first generation was only negligibly radioactive. 

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