[Taken from Arthur C. Clarke: The Collected Stories.]
And now the sun ahead was so close that the hurricane of radiation was forcing the Swarm back into the dark night of space. Soon it would be able to come no closer; the gales of light on which it rode from star to star could not be faced so near their source. Unless it encountered a planet very soon, and could fall down into the peace and safety of its shadow, this sun must be abandoned as had so many before.
Six cold outer worlds had already been searched and discarded. Either they were frozen beyond all hope of organic life, or else they harboured entities of types that were useless to the Swarm. If it was to survive, it must find hosts not too unlike those it had left on its doomed and distant home. Millions of years ago the Swarm had begun its journey, swept starward by the fires of its own exploding sun. Yet even now the memory of its lost birthplace was still sharp and clear, an ache that would never die.
There was a planet ahead, swinging its cone of shadow through the flame-swept night. The senses that the Swarm had developed upon its long journey reached out toward the approaching world, reached out and found it good.
The merciless buffeting of radiation ceased as the black disc of the planet eclipsed the sun. Falling freely under gravity, the Swarm dropped swiftly until it hit the outer fringe of the atmosphere. The first time it had made planetfall it had almost met its doom, but now it contracted its tenuous substance with the unthinking skill of long practice, until it formed a tiny, close-knit sphere. Slowly its velocity slackened, until at last it was floating motionless between earth and sky.
For many years it rode the winds of the stratosphere from Pole to Pole, or let the soundless fusillades of dawn blast it westward from the rising sun. Everywhere it found life, but nowhere intelligence. There were things that crawled and flew and leaped, but there were not things that talked or built. Ten million years hence there might be creatures here with minds that the Swarm could possess and guide for its own purposes; there was no sign of them now. It could not guess which of the countless life-forms on this planet would be the heir to the future, and without such a host it was helpless — a mere pattern of electric charges, a matrix of order and a self-awareness in a universe of chaos. By it own resources the Swarm had no control over matter, yet once it had lodged in the mind of a sentient race there was nothing that lay beyond its powers.
It was not the first time, and it would not be the last, that the planet had been surveyed by a visitant from space — though never by one in such peculiar and urgent need. The Swarm was faced with a tormenting dilemma. It could begin its weary travels once more, hoping that ultimately it might find the conditions it sought, or it could wait here on this world, biding its time until a race had arisen which would fit its purpose.
It moved like mist through the shadows, letting the vagrant winds take it where they willed. The clumsy, ill-formed reptiles of this young world never saw its passing, but it observed them, recording, analysing, trying to extrapolate into the future. There was so little to choose between all these creatures; not one showed even the first faint glimmering of conscious mind. Yet if it left this world in search of another, it might roam the universe in vain until the end of time.
At last it made its decision. By its very nature, it could choose both alternatives. The greater part of the Swarm would continue its travels among the stars, but a portion of it would remain on this world, like a seed planted in the hope of future harvest.
It began to spin upon its axis, its tenuous body flattening into a disc. Now it was wavering at the frontiers of visibility — it was a pale ghost, a faint will-of-the-wisp that suddenly fissured into two unequal fragments. The spinning slowly died away: the Swarm had become two, each an entity with all the memories of the original, and all its desires and needs.
There was a last exchange of thoughts between parent and child who were also identical twins. If all went well with them both, they would meet again in the far future here at this valley in the mountains. The one who was staying would return to this point at regular intervals down the ages; the one who continued the search would send back an emissary if ever a better world was found. And then they would be united again, no longer homeless exiles vainly wandering among the indifferent stars.
The light of dawn was spilling over the raw, new mountains when the parent swarm rose up to meet the sun. At the edge of the atmosphere the gales of radiation caught it and swept it unresisting out beyond the planets, to start again upon the endless search.
The one that was left began its almost equally hopeless task. It needed an animal that was not so rare that disease or accident could make it extinct, not so tiny that it could never acquire any power over the physical world. And it must breed rapidly, so that its evolution could be directed and controlled as swiftly as possible.
The search was long and the choice difficult, but at last the Swarm selected its host. Like rain sinking into thirsty soil, it entered the bodies of certain small lizards and began to direct their destiny.
It was an immense task, even for a being which could never know death. Generation after generation of the lizards was swept into the past before there came the slightest improvement in the race. And always, at the appointed time, the Swarm returned to its rendezvous among the mountains. Always it returned in vain: there was no messenger from the stars bringing news of better fortune elsewhere.
The centuries lengthened into millennia, the millennia into eons. By the standards of geological time, the lizards were now changing rapidly. Presently they were lizards no more, but warm-blooded, fur-covered creatures that brought forth their young alive. They were still small and feeble, and their minds were rudimentary, but they contained the seeds of future greatness.
Yet not only the living creatures were altering as the ages slowly passed. Continents were being rent asunder, mountains being worn down by the weight of the unwearying rain. Through all these changes, the Swarm kept to its purpose; and always, at the appointed times, it went to the meeting place that had been chosen so long ago, waited patiently for a while, and came away. Perhaps the parent swarm was still searching or perhaps — it was a hard and terrible thought to grasp — some unknown fate had overtaken it and it had gone the way of the race it had once ruled. There was nothing to do but to wait and see if the stubborn life-stuff of this planet could be forced along the path to intelligence.
And so the eons passed....
Somewhere in the labyrinth of evolution the Swarm made its fatal mistake and took the wrong turning. A hundred million years had gone since it came to Earth, and it was very weary. It could not die, but it could degenerate. The memories of its ancient home and of its destiny were fading: its intelligence was waning even while its hosts climbed the long slope that would lead to self-awareness.
By a cosmic irony, in giving the impetus which would one day bring intelligence to this world, the Swarm had exhausted itself. It had reached the last stage of parasitism; no longer could it exist apart from its hosts. Never again could it ride free above the world, driven by wind and sun. To make the pilgrimage to the ancient rendezvous, it must travel slowly and painfully in a thousand little bodies. Yet it continued the immemorial custom, driven on by the desire for reunion which burned all the more fiercely now that it knew the bitterness of failure. Only if the parent swarm returned and reabsorbed it could it ever know new life and vigour.
The glaciers came and went; by a miracle the little beasts that now housed the waning intelligence escaped the clutching fingers of the ice. The oceans overwhelmed the land, and still the race survived. It even multiplied, but it could do no more. This world would never be its heritage, for far away in the heart of another continent a certain monkey had come down from the trees and was looking at the stars with the first glimmerings of curiosity.
The mind of the Swarm was dispersing, scattering among a million tiny bodies, no longer able to unite and assert its will. It had lost all cohesion; its memories were fading. In a million years, at most, they would all be gone.
Only one thing remained — the blind urge which still, at intervals which by some strange aberration were becoming ever shorter, drove it to seek its consummation in a valley that long ago had ceased to exist.
Quietly riding the lane of moonlight, the pleasure steamer passed the island with its winking beacon and entered the fjord. It was a calm and lovely night, with Venus sinking in the west out beyond the Faroes, and the lights of the harbour reflected with scarcely a tremor in the still waters far ahead.
Nils and Christina were utterly content. Standing side by side against the boat rail, their fingers locked together, they watched the wooded slopes drift silently by. The tall trees were motionless in the moonlight, their leaves unruffled by even the merest breath of wind, their slender trunks rising whitely from pools of shadow. The whole world was asleep; only the moving ship dared to break the spell that had bewitched the night.
The suddenly, Christina gave a little gasp and Nils felt her fingers tighten convulsively on his. He follows her gaze: she was staring out across the water, looking toward the silent sentinels of the forest.
'What is it, darling?' he asked anxiously.
'Look!' she replied, in a whisper Nils could scarcely hear. 'There — under the pines!'
Nils stared, and as he did so the beauty of the night ebbed slowly away and ancestral terrors came crawling back from exile. For beneath the trees the land was alive: a dappled brown tide was moving down the slopes of the hill and merging into the dark waters. Here was an open patch on which the moonlight fell unbroken by shadow. It was changing even as he watched: the surface of the land seemed to be rippling downward like a slow waterfall seeking union with the sea.
And then Nils laughed and the world was sane once more. Christina looked at him, puzzled but reassured.
'Don't you remember?' he chuckled. 'We read all about it in the paper this morning. They do this every few years, and always at night. It's been going on for days.'
He was teasing her, sweeping away the tension of the last few minutes. Christina looked back at him, and a slow smile lit up her face.
'Of course!' she said. 'How stupid of me!' Then she turned once more toward the land and her expression became sad, for she was very tender-hearted.
'Poor little things!' she sighed. I wonder why they do it?'
Nils shrugged his shoulders indifferently.
'No one knows,' he answered. 'It's just one of those mysteries. I shouldn't think about it if it worries you. Look — we'll soon be in harbour!'
They turned toward the beckoning lights where their future lay, and Christina glanced back only once toward the tragic, mindless tide that was still flowing beneath the moon.
Obeying an urge whose meaning they had never known, the doomed legions of the lemmings were finding oblivion beneath the waves.