It was a window like the one in Sunderland Avenue in Oxford. She could only see it because of the light: with the sun any higher it probably wouldn’t show up at all.

She approached the little patch of air with passionate curiosity, because she hadn’t had time to look at the first one: she’d had to get away as quickly as possible. But she examined this one in detail, touching the edge, moving around to see how it became invisible from the other side, noting the absolute difference between this and that, and found her mind almost bursting with excitement that such things could be.

The knife bearer who had made it, at about the time of the American Revolution, had been too careless to close it, but at least he’d cut through at a point very similar to the world on this side: next to a rock face. But the rock on the other side was different, not limestone but granite, and as Mary stepped through into the new world she found herself not at the foot of a towering cliff but almost at the top of a low outcrop overlooking a vast plain.

It was evening here, too, and she sat down to breathe the air and rest her limbs and taste the wonder without rushing.

Wide golden light, and an endless prairie or savanna, like nothing she had ever seen in her own world. To begin with, although most of it was covered in short grass in an infinite variety of buff-brown-green-ocher-yellow-golden shades, and undulating very gently in a way that the long evening light showed up clearly, the prairie seemed to be laced through and through with what looked like rivers of rock with a light gray surface.

And secondly, here and there on the plain were stands of the tallest trees Mary had ever seen. Attending a high-energy physics conference once in California, she had taken time out to look at the great redwood trees, and marveled; but whatever these trees were, they would have overtopped the redwoods by half again, at least. Their foliage was dense and dark green, their vast trunks gold-red in the heavy evening light.

And finally, herds of creatures, too far off to see distinctly, grazed on the prairie. There was a strangeness about their movement that she couldn’t quite work out.

She was desperately tired, and thirsty and hungry besides. Somewhere nearby, though, she heard the welcome trickle of a spring, and only a minute later she found it: just a seepage of clear water from a mossy fissure, and a tiny stream that led away down the slope. She drank long and gratefully, and filled her bottles, and then set about making herself comfortable, for night was falling rapidly.

Propped against the rock, wrapped in her sleeping bag, she ate some of the rough bread and the goat’s cheese, and then fell deeply asleep.

She awoke with the early sun full in her face. The air was cool, and the dew had settled in tiny beads on her hair and on the sleeping bag. She lay for a few minutes lapped in freshness, feeling as if she were the first human being who had ever lived.

She sat up, yawned, stretched, shivered, and washed in the chilly spring before eating a couple of dried figs and taking stock of the place.

Behind the little rise she had found herself on, the land sloped gradually down and then up again; the fullest view lay in front, across that immense prairie. The long shadows of the trees lay toward her now, and she could see flocks of birds wheeling in front of them, so small against the towering green canopy that they looked like motes of dust.

Loading her rucksack again, she made her way down onto the coarse, rich grass of the prairie, aiming for the nearest stand of trees, four or five miles away.

The grass was knee-high, and growing among it were low-lying bushes, no higher than her ankles, of something like juniper; and there were flowers like poppies, like buttercups, like cornflowers, giving a haze of different tints to the landscape; and then she saw a large bee, the size of the top segment of her thumb, visiting a blue flower head and making it bend and sway. But as it backed out of the petals and took to the air again, she saw that it was no insect, for a moment later it made for her hand and perched on her finger, dipping a long needle-like beak against her skin with the utmost delicacy and then taking flight again when it found no nectar. It was a minute hummingbird, its bronze-feathered wings moving too fast for her to see.

How every biologist on earth would envy her if they could see what she was seeing!

She moved on and found herself getting closer to a herd of those grazing creatures she had seen the previous evening, whose movement had puzzled her without her knowing why. They were about the size of deer or antelopes, and similarly colored, but what made her stop still and rub her eyes was the arrangement of their legs. They grew in a diamond formation: two in the center, one at the front, and one under the tail, so that the animals moved with a curious rocking motion. Mary longed to examine a skeleton and see how the structure worked.

For their part, the grazing creatures regarded her with mild, incurious eyes, showing no alarm. She would have loved to go closer and take time to look at them, but it was getting hot, and the shade of the great trees looked inviting; and there was plenty of time, after all.

Before long she found herself stepping out of the grass onto one of those rivers of stone she’d seen from the hill: something else to wonder at.

It might once have been some kind of lava-flow. The underlying color was dark, almost black, but the surface was paler, as if it had been ground down or worn by crushing. It was as smooth as a stretch of well-laid road in Mary’s own world, and certainly easier to walk on than the grass.

She followed the one she was on, which flowed in a wide curve toward the trees. The closer she got, the more astounded she was by the enormous size of the trunks—as wide, she estimated, as the house she lived in, and as tall—as tall as . . . She couldn’t even make a guess.

When she came to the first trunk, she rested her hands on the deeply ridged red-gold bark. The ground was covered ankle-deep in brown leaf skeletons as long as her hand, soft and fragrant to walk on. She was soon surrounded by a cloud of midgelike flying things, as well as a little flock of the tiny hummingbirds, a yellow butterfly with a wingspread as broad as her hand, and too many crawling things for comfort. The air was full of humming and buzzing and scraping.

She walked along the floor of the grove feeling much as if she were in a cathedral: there was the same stillness, the same sense of upwardness in the structures, the same awe within herself.

It had taken her longer than she thought it would to walk here. It was getting on toward midday, for the shafts of light coming down through the canopy were almost vertical. Drowsily Mary wondered why the grazing creatures didn’t move under the shade of the trees during this hottest part of the day.

She soon found out.

Feeling too hot to move any farther, she lay down to rest between the roots of one of the giant trees, with her head on her rucksack, and fell into a doze.

Her eyes were closed for twenty minutes or so, and she was not quite asleep, when suddenly, from very close by, there came a resounding crash that shook the ground.

Then came another. Alarmed, Mary sat up and gathered her wits, and saw a movement that resolved itself into a round object, about three feet across, rolling along the ground, coming to a halt, and falling on its side.

And then another fell, farther off; she saw the massive thing descend, and watched it crash into the buttress-like root of the nearest trunk and roll away.

The thought of one of those things falling on her was enough to make her take her rucksack and run out of the grove altogether. What were they? Seedpods?

Watching carefully upward, she ventured under the canopy again to look at the nearest of the fallen objects. She pulled it upright and rolled it out of the grove, and then laid it on the grass to look at it more closely.

It was perfectly circular and as thick as the width of her palm. There was a depression in the center, where it had been attached to the tree. It wasn’t heavy, but it was immensely hard and covered in fibrous hairs, which lay along the circumference so that she could run her hand around it easily one way but not the other. She tried her knife on the surface; it made no impression at all.

Her fingers seemed smoother. She smelled them; there was a faint fragrance there, under the smell of dust. She looked at the seedpod again. In the center there was a slight glistening, and as she touched it again, she felt it slide easily under her fingers. It was exuding a kind of oil.

Mary laid the thing down and thought about the way this world had evolved.

If her guess about these universes was right, and they were the multiple worlds predicted by quantum theory, then some of them would have split off from her own much earlier than others. And clearly in this world evolution had favored enormous trees and large creatures with a diamond-framed skeleton.