In the regions where we all lived, there was never snow or ice.

We would tell our children: 'If you travel as far as you can that way, as far as you can that way, you will come to places that lie more distant from our sun than we do. You will find thick water, not light and quick-moving as it is with us. The water is slow with cold, and on its surface it wrinkles as it moves, or even, sometimes, makes plates or flakes that are solid. This is ice.'

When, rarely, storms brought lumps of ice from the sky, a great thing was made of it; we called our children; we said: 'Look, this is ice! At the poles of our world the cold slow water sometimes makes this substance, you might walk half a day and see no water that was not in this form: white, solid, glistening.'

And, when they were older: 'On some other planets as much of their surface is ice as on our planet is vegetation and fruitfulness.'

We would say to them: 'On our planet, in those regions lying back from the sun, sometimes from the sky fall small white flakes so light and so delicate you can blow them about and around with a breath. This is snow, this is how the water that is always in the air, though invisible to us, changes in those parts when it is frozen by the cold.'

And the children would of course marvel and wonder and wish they might see snow, and the gelid wrinkling waters, and the ice that sometimes made crusts or even plates and sheets.

And then, snow fell.

Across light blue sunlit skies drove thick grey that came swarming down around us in a white fall, and everywhere we stood about, gazing up, gazing down, holding out our hands where the faint white flakes of the tales we told our children lay for an instant before they sank into blobs and smears of water.

It was not a prolonged fall, but it was heavy. One instant our world was as always, green and brown, and coloured with the shine and glisten of moving water, and the easy movement of light clouds. And the next it was a white world. Everywhere, white, and the black jut of the wall rising from it, and on the top of the black, a white crest.

Very often, looking back, we say that we did not understand clearly what was happening, the importance of an event. But I can say that this fall of white from our capacious and mild skies was something that struck into us, our minds and our understandings. Oh yes, we knew, we understood. And, looking into each other's faces for confirmation of what we felt, it was there - the future.

That scene is as clear in my memory as any. We were all out of our dwellings, we had run together everywhere and were in groups and little crowds, and we were gazing into more than this cold white that had so suddenly enveloped us.

We were a tall lithe people, lightly but strongly built, and our colour was brown, and our eyes were black, and we had long straight black hair. We loved strong and vibrant colours in clothes and in the decoration of our houses: these were what we saw when we looked out at our world - the many blues of the sky, the infinite greens of the foliage, the reds and browns of our earth, mountains shining with pyrites and quartz, the dazzle of water and of sun.

We had not thought, ever, to wonder about our congruity with our surroundings, but on that day we did. We had never seemed to ourselves anything but comely, but against the white glisten that now covered everything we seemed to ourselves dingy and shrunken. Our skins were yellow, and our eyes puckered and strained because of the cold glare we could not escape except by shutting them. The strong colours of our clothes were harsh. We stood there shivering with the suddenness of the drop in temperature, and everywhere could be seen the same involuntary movement: of people looking at each other, finding what they saw ugly, and then, as they remembered that this was how they must be striking others, their eyes turning away, while they hugged themselves in their own arms not only because of the cold, but in a way that suggested a need for comfort, consolation.

Canopus arrived while the snow still lay, unmelted.

There were five of them, not the usual one, or two; and this alone was enough to impress us. They were among us while the snow melted so that our world returned to its warmth and the comfortable colours of growth - and while the snow again fell, and this time stayed for longer. Nor did they leave when this second affliction of white shrank and went. It was never the way of Canopus to demand, announce, threaten - or even to stand high on the crest of our wall, as we sometimes did on civic occasions, to address large crowds. No, they moved quietly among us, staying for a while in one dwelling, and then moving on to another, and while nothing dramatic or painful was ever said, before long we had all gathered from them what was needed.

The snow would come again, and more often; slowly the balance of warmth and cold on our planet would change, and there would be more snow and ice for us than there would be green and growth. And this and this and this was what we must do to prepare ourselves...

We were learning how those on harsher planets matched themselves against cold. We were hearing of houses built thick and strong to withstand weights of snow and the pressures of winds we had never known. We were told of clothing, and footwear, and how to wrap a head in thick cloth so that only the eyes would be exposed - this last impressed us fearfully, for the falls of snow we had seen had not done more than make us shiver and pull our light clothes more tightly around us.

While we were deciding how to make sure those settlements and towns nearest the poles would be protected first, we were told by Canopus that they should be abandoned altogether. All day and night, along that great black wall of ours, pressed crowds of people. We stood on it, we massed beside it. We laid our hands on the cold hard shine of it. We looked at the vast weight and thickness of it. We crowded close under it and looked up at how it towered and we felt it as a safety and guarantee. The wall - our wall - our great black useless monument, that had swallowed all our wealth and our labour and our thoughts and our capacities... it was going to save us all.

We were all now to live on one side of it, leaving the smaller part of our globe empty, for it would soon be uninhabitable. We travelled, many of us, all over those mild and agreeable lands where the crops were still in the fields, the vegetation many-coloured and warm. We were moving there, we knew, because of our need to comprehend. For we did not. One may be told something, act on it, trust in it - but that is not the same as feeling it, as a truth. We - those of us entrusted with the task of moving the populations out of their threatened homes - were always at work, in our imaginations, on the task of really knowing that shortly ice and snow would rule here. And those who had to submit to the move were not taking it in either.

Soon there were new towns and manufactories everywhere on the side of the wall that we believed would remain more or less as it had been... with perhaps snow and even storms, but not so very different from what we had known.

And now, when we stood gathered on the summit of that barrier wall that was going to have to hold the pressures of massing and thrusting ice, and gazed over a still fertile landscape where the future was not visible, except in the skies that had a pallid and pinched look, we felt grief, we were struck and slowed with grief, for at last we had become enabled to feel, really feel, in our substance, in our deepest selves, that our world, our way of living, everything we had been - was done, was over. Finished.

How dark it was, in our minds and our hopes, during that time of preparation, while we busied ourselves with resettling so many people in their new homes, while we took in what we could from Johor and the other emissaries they sent us.