stages, pausing now and then while Gibson gathered a small packet of bone fragments from the mudflats and underbrush.

"Some of these are older than others," he explained when Stryker remarked on his selection. "But none are recent. It should help to know their exact age."

An hour later, they were bathed and dressed, sealed off comfortably in the ship against the humid heat and stink of the swamp. Farrell lay on a chart room acceleration couch, resting, while Stryker taped his swollen ankle. Gibson and Xavier, the one disdaining rest and the other needing none, used the time to run a test analysis on the bones brought in from the lakeside.

The results of that analysis were more astonishing than illuminating.

A majority of the fragments had been exposed to climatic action for some ten years. A smaller lot averaged twenty years; and a few odd chips, preserved by long burial under alluvial silt, thirty.

"The older natives died at ten-year intervals, then," Stryker said. "And in considerable numbers; the tribe must have been cut to half strength each time. But why?" He frowned unhappily, fishing for opinion. "Gib, can it really be a perversion of religious custom dreamed up by the Hymenops to keep their slaves under control? A sort of festival of sacrifice every decade, climaxing in tribal decimation?"

"Maybe they combine godliness with gluttony," Farrell put in, unasked. "Maybe their orgy runs more to long pig than to piety."

He stood up, wincing at the pain, and was hobbling toward his sleeping cubicle when Gibson's answer to Stryker's question stopped him with a cold prickle along his spine.

"We'll know within twenty-four hours," Gibson said. "Since both the decimations and the winter darkness periods seem to follow the same cycle, I'd say there's a definite relationship."

For once Farrell's cubicle, soundproofed and comfortable, brought him only a fitful imitation of sleep, an intermittent dozing that wavered endlessly between nightmare and wakefulness. When he crawled out again, hours later, he found Xavier waiting for him alone with a thermo-bulb of hot coffee. Stryker and Gibson, the mechanical said blandly, had seen no need of waking him, and had gone out alone on a more extensive tour of investigation.

The hours dragged interminably. Farrell uncased his beloved accordion, but could not bear the sound of it; he tried his sketch-book, and could summon to mind no better subjects than drab miasmic bogs and steaming mudflats. He discarded the idea of chess with Xavier without even weighing it—he would not have lasted past the fourth move, and both he and the mechanical knew it.

He was reduced finally to limping about the ship on his bandaged foot, searching for some routine task left undone and finding nothing. He even went so far as to make a below-decks check on the ship's matter-synthesizer, an indispensable unit designed for the conversion of waste to any chemical compound, and gave it up in annoyance when he found that all such operational details were filed with infallible exactness in Xavier's plastoid head.

The return of Stryker and Gibson only aggravated his impatience. He had expected them to discover concealed approaches to the maze of bridging overhead, tunnelings in the cliff-face to hidden caverns complete with bloodstained altars and caches of sacrificial weapons, or at least some ominous sign of preparation among the natives. But there was nothing.

"No more than yesterday," Stryker said. Failure had cost him a share of his congenital good-humor, leaving him restless and uneasy. "There's nothing to find, Arthur. We've seen it all."

Surprisingly, Gibson disagreed.

"We'll know what we're after when darkness falls," he said. "But that's a good twelve hours away. In the meantime, there's a possibility that our missing key is outside the crater, rather than here inside it."

They turned on him together, both baffled and apprehensive.

"What do you mean,