fourth morning they found out why. They saw, far, far away, a truly monstrous mountain-mass at the end of the long-way expanse of the plateau. It was like the prow of a ship. It lay, so Huyghens observed, directly in line with the prevailing winds, and divided them as a ship's prow divides the waters. The moisture-bearing air-currents flowed beside the plateau, not over it, and its interior was pure sere desert in the unscreened sunshine of high altitudes.

It took them a full day to get halfway up the slope. And here, twice as they climbed, Semper flew screaming over aggregations of sphexes to one side of them or the other. These were much larger groups than Huyghens had ever seen before—fifty to a hundred monstrosities together, where a dozen was a large hunting-pack elsewhere. He looked in the screen which showed him what Semper saw, four to five miles away. The sphexes padded uphill toward the Sere Plateau in a long line. Fifty—sixty—seventy tan-and-azure beasts out of hell.

"I'd hate to have that bunch jump us," he said candidly to Roane. "I don't think we'd stand a chance."

"Here's where a robot tank would be useful," Roane observed.

"Anything armored," conceded Huyghens. "One man in an armored station like mine would be safe. But if he killed a sphex he'd be besieged. He'd have to stay holed up, breathing the smell of dead sphex, until the odor had gone away. And he mustn't kill any others or he'd be besieged until winter came."

Roane did not suggest the advantages of robots in other directions. At that moment, for example, they were working their way up a slope which averaged fifty degrees. The bears climbed without effort despite their burdens. For the men it was infinite toil. Semper, the eagle, manifested impatience with bears and men alike, who crawled so slowly up an incline over which he soared.

He went ahead up the mountainside and teetered in the air-currents at the plateau's edge. Huyghens looked in the vision-plate by which he reported.

"How the devil," panted Roane—they had stopped for a breather, and the bears waited patiently for them—"do you train bears like these? I can understand Semper."

"I don't train them," said Huyghens, staring into the plate. "They're mutations. In heredity the sex-linkage of physical characteristics is standard stuff. But there's been some sound work done on the gene-linkage of psychological factors. There was need, on my home planet, for an animal who 

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