that was his sole master then—instinct.

And it was instinct that drove Victoria Custer deeper into the jungle with her savage lover as she sensed the nearer approach of her brother—one of the two master instincts that have dominated and preserved life upon the face of the earth. Yet it was not without a struggle. She hesitated, half turning backward. Nu cast a questioning look upon her.

"They are coming, Nat-ul," he said. "Nu cannot fight these strange men who hurl lead with the thunders they have stolen from the skies. Come! We must hurry back to the cave of Oo, and on the morrow we shall go forth and search for the tribe of Nu, my father, that dwells beyond the Barren Cliffs beside the Restless Sea. There, in our own world, we shall be happy."

And yet the girl held back, afraid. Then the man gathered her in his mighty arms and ran on in the direction of the cave of Oo, the saber-toothed tiger. The girl did not even struggle to escape, instead she lay quietly, as over her fell a sensation of peace and happiness, as though, after a long absence, she was being borne home. And at their heels trotted Terkoz, the wolfhound.

Sometimes Nu took to the lower branches of the trees, for in her own age his race had been semiarboreal. Here he traveled with the ease and agility of a squirrel, though oftentimes the modern woman that still lived in the breast of Victoria Custer quailed at the dizzy leaps, and the swaying, perilous trail. Yet, as they fled, her fears were greatest now that they might be overtaken, and herself snatched back into the world of civilization where her Nu could never follow.

It was dusk of the third evening when they came again to the cave of Oo. Up the steep cliff side they clambered, hand in hand. Together they entered the dark and forbidding hole.

"Tomorrow," said Nu, "we will search for the caves of our people, and we shall find them."

Darkness settled upon the jungle, the plain and the mountains. Nu and Nat-ul slept, for both were exhausted from the long days of flight.

And then there came, out of the bowels of the earth, a deep and ominous rumbling. The earth shook. The cliff rocked. Great masses of shattered rock shaken from its summit roared and tumbled down its face.

Nu sprang to his feet, only to be hurled immediately to the floor of the cave stunned and senseless. Within all was darkness. No light filtered 

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