He released the girl from her time trap, and she imploded into infancy.

Soon another young woman stared out at him. She was dressed in a long skirt and ill-fitting, badly made blouse. She was waving a British Union Flag, and she was being embraced by a soldier in a flat tin helmet. The street behind her was crowded, men in suits and caps and overalls, the women in long coats. It was raining, a dismal autumnal day, but nobody seemed to mind.

“November 1918,” David said aloud. “The Armistice. The end of four years of bloody slaughter in Europe. Not a bad night to be conceived.” He turned. “Don’t you think, Bobby?”

The shadow, motionless against the wall, seemed to hesitate. Then it separated, moved freely, took on the outline of a human form. Hands and face appeared, hovering disembodied.

“Hello, David.”

“Sit with me,” David said.

His brother sat with a rustle of SmartShroud smart cloth. He seemed awkward, as if unused to being so close to anybody in the open. It didn’t matter; David demanded nothing of him.

The Armistice Day girl’s face smoothed, diminished, shrank to an infant, and there was another transition: a girl with some of the looks of her descendants, the blue eyes and strawberry hair, but thinner, paler, her cheeks hollow. Shedding her years, she moved through a blur of dark urban scenes — factories and terraced houses — and then a flash of childhood, another generation, another girl, the same dismal landscape.

“They seem so young,” Bobby murmured; his voice was scratchy, as if long unused.

“I think we’re going to have to get used to that,” David said grimly. “We’re already deep in the nineteenth century. The great medical advances are being lost, and hygiene awareness is rudimentary. People are dying of simple, curable diseases. And of course we’re following a line of women who at least lived long enough to reach childbearing age. We aren’t glimpsing their sisters who died in infancy, leaving no descendants.”

The generations fell away, faces deflating like balloons, one after the other, subtly changing from generation to generation, slow genetic drift working.

Here was a girl whose scarred face was marked by tears at the moment she gave birth. Her baby had been taken from her, David saw — or rather, in this time-reversed view; given to her — moments after the birth. Her pregnancy unravelled in misery and shame, until they reached the moment that defined her life: a brutal rape committed, it seemed, by a family member, a brother or uncle. Cleansed of that darkness, the girl grew younger, pretty, smiling, her face filling with hope despite the squalor of her life, as she found beauty in simplicity: a flower’s brief bloom, the shape of a cloud. The world must be full of such anguished biographies, David thought, unravelling as they sank into the past, effects preceding cause, pain and despair falling away as the blankness of childhood approached.

Suddenly the background changed again. Now, around this new grandmother’s face, some ten generations remote, there was countryside: small fields, pigs and cows scratching at the ground, a multitude of grimy children. The woman was careworn, gap-toothed, her face lined, appearing old — but David knew she could be no more than thirty-five or forty.

“Our ancestors were farmers,” Bobby said.

“Most everybody was, before the great migrations to the cities. But the Industrial Revolution is unwinding. They probably can’t even make steel.”

The seasons pulsed, summer and winter, light and dark; and the generations of women, daughter to mother, followed their slower cycle from careworn parent to bright maiden to wide-eyed child. Some of the women erupted onto the ’Screen with faces twisted in pain: they were those unfortunates, increasingly more common, who had died in childbirth.

History withdrew. The centuries were receding, the world emptying of people. Elsewhere the Europeans were drawing back from the Americas, soon to forget those great continents even existed, and the Golden Horde — great armies of Mongols and Tartars, their corpses leaping from the ground — was re-forming and drawing back into central Asia.

None of that touched these toiling English peasants, without education or books, working the same piece of ground for generation on generation: people to whom, David reflected, the local collector of tithes would be a far more formidable figure than Tamerlaine or Kublai Khan. If the WormCam had shown nothing else, he thought, it was this, with pitiless clarity: that the lives of most humans had been miserable and short, deprived of freedom and joy and comfort, their brief moments in the light reduced to sentences to be endured.

At last, around the framed face of one girl — hair matted and dark, skin sallow, expression rat-like, wary — there was an abrupt blur of scenery. They glimpsed dismal countryside, a ragged family of refugees walking endlessly — and, here and there, heaps of corpses, burning.

“A plague,” Bobby said.

“Yes. They are forced to flee. But there is nowhere to go.”

Soon the image stabilized on another anonymous scrap of land set in a huge, flat landscape; and once more the generations of toil, so calamitously interrupted, resumed.

On the horizon there was a Norman cathedral, an immense, brooding, sandstone box. If this was the fens, the great plain to the east of England, then that could be Ely. Already centuries old, the great construction looked like a giant sandstone spaceship which had descended from the sky, and it must utterly have dominated the mental landscapes of these toiling people — which was, of course, its purpose.

But even the great cathedral began to shrink, collapsing with startling swiftness into smaller, simpler forms, at last disappearing from view altogether.

And the numbers of people were still falling, the great tide of humanity drawing back all over the planet. The Norman invaders must already have dismantled their great keeps and castles and withdrawn to France. Soon the waves of invaders from Scandinavia and Europe would return home from Britain. Farther afield, as the death and birth of Muhammad approached, the Muslims were withdrawing from northern Africa. By the time Christ was brought down from the Cross, there would be only around a hundred million people left in all the world, less than half the population of the United States of David’s day.

As the faces of their ancestors pulsed by, there was another change of scene, a brief migration. Now these remote families scratched at a land of ruins — low walls, exposed cellars, the ground littered with blocks of marble and other building stone.

Then buildings grew like time-lapsed flowers, the scattered stones coalescing.

David paused. He fixed on the face of a woman, his own remote ancestor some eighty generations removed. She was perhaps forty, handsome, her strawberry hair tinged with grey, her eyes blue. Her nose was proudly prominent, Romanesque.

Behind her the dismal fields had vanished, to be replaced by an orderly townscape: a square surrounded by colonnades and statues and tall buildings, their roofs tiled red. The square was crowded with stalls, vendors frozen in the act of hawking their wares. The vendors seemed comical, so intent were they on their slivers of meaningless profit, all unaware of the desolate ages that lay in their own near future, their own imminent deaths.

“A Roman settlement,” Bobby said.

“Yes.” David pointed at the ’Screen. “I think this is the forum. That is probably the basilica, the town hall and law courts. These rows of colonnades lead to shops and offices. And the building over there might be a temple…”

“It looks so orderly,” Bobby murmured. “Even modern. Streets and buildings, offices and shops. You can see it’s all set out on a rectangular grid, like Manhattan. I feel as if I could walk into the ’Screen and go look for a bar.”