The throbbing grew louder, more insistent. Presently the man-apes began to move forward, like sleepwalkers, toward the source of that compulsive sound. Sometimes they took little dancing steps, as their blood responded to rhythms that their descendants would not create for ages yet. Totally entranced, they gathered round the monolith, forgetting the hardships of the day, the perils of the approaching dusk, and the hunger in their bellies.

The drumming became louder, the night darker. And as the shadows lengthened and the light drained from the sky, the crystal began to glow.

First it lost its transparency, and became suffused with a pale, milky luminescence, Tantalizing, ill-defined phantoms moved across its surface and in its depths. They coalesced into bars of light and shadow, then formed intermeshing, spoked patterns that began slowly to rotate.

Faster and faster spun the wheels of light, and the throbbing of the drums accelerated with them. Now utterly hypnotized, the man-apes could only stare slack-jawed into this astonishing display of pyrotechnics. They had already forgotten the instincts of their forefathers and the lessons of a lifetime; not one of them, ordinarily, would have been so far from his cave, so late in the evening. For the surrounding brush was full of frozen shapes and staring eyes, as the creatures of the night suspended their business to see what would happen next.

Now the spinning wheels of light began to merge, and the spokes fused into luminous bars that slowly receded into the distance, rotating on their axes as they did so. They split into pairs and the resulting sets of lines started to oscillate across one another, slowly changing their angles of intersection. Fantastic, fleeting geometrical patterns flickered in and out of existence as the glowing grids meshed and unmeshed; and the man-apes watched, mesmerized captives of the shining crystal.

They could never guess that their minds were being probed, their bodies mapped, their reactions studied, their potentials evaluated. At first, the whole tribe remained half crouching in a motionless tableau, as if frozen into stone. Then the man-ape nearest to the slab suddenly came to life.

He did not move from his position, but his body lost its trancelike rigidity and became animated as if it were a puppet controlled by invisible strings. The head turned this way and that; the mouth silently opened and closed; the hands clenched and unclenched. Then he bent down, snapped off a long stalk of grass, and attempted to tie it into a knot with clumsy fingers.

He seemed to be a thing possessed, struggling against some spirit or demon who had taken over control of his body. He was panting for breath, and his eyes were full of terror as he tried to force his fingers to make movements more complex than any that they had ever attempted before.

Despite all his efforts, he succeeded only in breaking the stalk into pieces. As the fragments fell to the ground, the controlling influence left him, and he froze once more into immobility.

Another man-ape came to life, and went through the same routine. This was a younger, more adaptable specimen; it succeeded where the older one had failed. On the planet Earth, the first crude knot had been tied.

Others did stranger and still more pointless things. Some held their hands out at arm's length, and tried to touch their fingertips together – first with both eyes open, then with one closed. Some were made to stare at ruled patterns in the crystal, which became more and more finely divided until the lines had merged into a gray blur. And all heard single pure sounds, of varying pitch, that swiftly sank below the level of hearing.

When Moon-Watcher's turn came, he felt very little fear. His main sensation was a dull resentment, as his muscles twitched and his limbs moved at commands that were not wholly his own. Without knowing why, he bent down and picked up a small stone. When he straightened up, he saw that there was a new image in the crystal slab.

The grids and the moving, dancing patterns had gone. Instead, there was a series of concentric circles, surrounding a small black disk. Obeying the silent orders in his brain, he pitched the stone with a clumsy, overarm throw. It missed the target by several feet.

Try again, said the command. He searched around until he had found another pebble. This time it hit the slab with a ringing, bell-like tone. He was still a long way off, but his aim was improving. At the fourth attempt, he was only inches from the central bull's-eye. A feeling of indescribable pleasure, almost sexual in its intensity, flooded his mind. Then the control relaxed; he felt no impulse to do anything, except to stand and wait.

One by one, every member of the tribe was briefly possessed. Some succeeded, but most failed at the tasks they had been set, and all were appropriately rewarded by spasms of pleasure or of pain.

Now there was only a uniform featureless glow in the great slab, so that it stood like a block of light superimposed oil the surrounding darkness. As if waking from a sleep, the man-apes shook their heads, and presently began to move along the trail to their place of shelter. They did not look back, or wonder at the strange light that was guiding them to their homes – and to a future unknown, as yet, even to the stars.

Moon-Watcher and his companions had no recollection of what they had seen, after the crystal had ceased to cast its hypnotic spell over their minds and to experiment with their bodies. The next day, as they went out to forage, they passed it with scarcely a second thought; it was now part of the disregarded background of their lives. They could not eat it, and it could not eat them; therefore it was not important.

Down at the river, the Others made their usual ineffectual threats. Their leader, a one-eared man-ape of Moon-Watcher's size and age, but in poorer condition, even made a brief foray toward the tribe's territory, screaming loudly and waving his arms in an attempt to scare the opposition and to bolster his own courage.

The water of the stream was nowhere more than a foot deep, but the farther One-Ear moved out into it, the more uncertain and unhappy he became. Very soon he slowed to a halt, and then moved back, with exaggerated dignity, to join his companions.

Otherwise, there was no change in the normal routine. The tribe gathered just enough nourishment to survive for another day, and no one died.

And that night, the crystal slab was still waiting; surrounded by its pulsing aura of light and sound. The program it had contrived, however, was now subtly different.

Some, of the man-apes it ignored completely, as if it was concentrating on the most promising subjects.

One of them was Moon-Watcher; once again he felt inquisitive tendrils creeping down the unused byways of his brain. And presently, he began to see visions. They might have been within the crystal block; they might have been wholly inside his mind. In any event, to Moon-Watcher they were completely real. Yet somehow the usual automatic impulse to drive off invaders of his territory had been lulled into quiescence.

He was looking at a peaceful family group, differing in only one respect from the scenes he knew. The male, female, and two infants that had mysteriously appeared before him were gorged and replete, with sleek and glossy pelts – and this was a condition of life that Moon-Watcher had never imagined. Unconsciously, he felt his own protruding ribs; the ribs of these creatures were hidden in rolls of fat. From time to time they stirred lazily, as they lolled at ease near the entrance of a cave, apparently at peace with the world. Occasionally; the big male emitted a monumental burp of contentment.