The familiar was tireless. Over hours it learned the canal bed. Each thing it found it used, some in several ways. Some it used in conjunction with other pieces. Some it discarded after a while. With each use, each manipulation (and only with that manipulation, that change) it read meanings. The familiar accumulated brute erudition, forgetting nothing, and with each insight the next came easier, as its context grew. Dust had been the first and hardest thing to know.

When the familiar emerged from the water with the dawn, it was poured into a milk-bottle carapace. Its clutch of eyes poked from the bottleneck. It nibbled with a nail clipper. With precise little bullets of stone it had punctured holes in its glass sides, from which legs of waterlogged twig-wood and broken pens emerged. To stop it sinking into wet earth its feet were coins and flat stones. They looked insecurely attached. The familiar dragged the brown sack that had contained it. Though it had not found a use for it, and though it had no words for the emotion, it felt something like sentiment for the hessian.

All its limbs were permanently reconfigured. Even those it tired of and kicked off were wormed with organic ruts for its juices. Minuscule muscles and tendons the thickness of spider-silk but vastly stronger rooted through the components of its bric-a-brac body, anchoring them together. The flesh at its centre had grown.

The familiar investigated grass, and watched the birds with its inadequate eyes. It trouped industrious as a beetle on variegated legs.

Through that day and night the familiar learned. It crossed paths with small mammals. It found a nest of mice and examined their parts. Their tails it took for prehensile tentacles; their whiskers bristled it; it upgraded its eyes and learned to use ears. It compared what it found to dust, blades, water, twigs, fish ribs, and sodden rubbish: it learned mouse.

It learned its new ears, with focused fascination. Young Londoners played in the gardens, and the familiar stayed hidden and listened to their slang. It heard patterns in their sequenced barks.

There were predators in the gardens. The familiar was the size of a cat, and foxes and dogs sometimes went for it. It was now too big for the bottle-armour, had burst it, but had learned instead to fight. It raked with shards of china, nails, and screws-not with anger, but with its unchanging beatific interest. It was impossibly sure-footed on its numerous rubbish legs. If an attacker did not run fast enough, the familiar would learn it. It would be used. The familiar had brittle fingertips, made of dogs' teeth.

The familiar moved away from the gardens. It followed the canal bank to a graveyard, to an industrial sidings, to a dump. It gave itself a shape with wheels, plunging its veins and tissue into the remnants of a trolley. When later it discarded them, pulling them out, the wheels bled.

Sometimes it used its tools like their original owners, as when it took its legs from birds (scampering over burnt-out cars like a rock rabbit on four or six avian feet). It could change them. In sun, the familiar shaded its eyes with flanges of skin that had been cats' ears.

It had learned to eat. Its hunger, its feeding was a tool like dust had been: the familiar did not need to take in nourishment but doing so gave it satisfaction, and that was enough. It made itself a tongue from strips of wet towel, and made a mouth full of interlocking cogs. These teeth rotated in its jaw, chewing, driving food scraps back towards the throat.

In the small hours of morning, in a waste lot stained by chemical spill, the familiar finally made a tool of the sack that had delivered it. It found two broken umbrellas, one skeletal, the other ragged, and it busied itself with them, holding them tight with hair-grip hands, manipulating them with rat tails. It secured the sackcloth to them with its organic roots. After hours of calculated tinkering, during which it spoke English words in the mind it had built itself, the reshaped umbrellas spasmed open and shut on analogues of shoulders, and with a great gust the familiar flew.

Its umbrellas beat like scooping bat wings, and the greased hessian held it. It flew random as a butterfly, staring at the moon with cats' and dogs' eyes, its numerous limbs splayed. It hunted with urban bramble, thorned stalks that whipped and pinioned prey from the air and the ground. It scoured the scrubland of cats. It spasmed between tower blocks, each wing contraction jerking it through the air. It shouted the words it had learned, without sound.

There were only two nights that it could fly, before it was too large, and it loved them. It was aware of its pleasure. It used it as it grew. The summer became unusually hot. The familiar hid in the sudden masses of buddleia. It found passages through the city. It lived in wrecking yards and sewers, growing, changing, and using.

Though it replaced them regularly, the familiar kept its old eyes, moving them down itself so that its sight deteriorated along its back. It had learned caution. It was educated: two streets might be empty, but not identically so, it knew. It parsed the grammar of brick and neglected industry. It listened at doors, cupping the cones of card, the plastic funnels with which it extended its ears. Its vocabulary increased. It was a Londoner.

Every house it passed it marked like a dog: the familiar pissed out its territory with glands made from plastic bottles. Sniffing with a nose taken from a badger, it sprayed a liquid of rubbish-tip juices and the witch's blood in a rough circle across the flattened zones of the north city, where the tube trains emerged from underground. The familiar claimed the terraced landscape.

It seemed a ritual. But it had watched the little mammals of the landfills and understood that territory was a tool, and it used it and learned it, or thought it did until the night it was tracing its limits into suburban spaces, and it smelt another's trail.

The familiar raged. It was maddened. It thrashed in a yard that reeked of alien spoor, chewing tires and spitting out their rags. Eventually, it hunkered down to the intruder's track. It licked it. It bristled throughout its body of witch flesh and patchwork trash. The new scent was sharper than its own, admixed with different blood. The familiar hunted.

The trail ran across back gardens, separated by fences that the familiar vaulted easily, trickled across toys and drying grass, over flowerbeds and rockeries. The prey was old and tough: it told in the piss. The familiar used the smell to track, and learned it, and understood that it was the newcomer here.

In the sprawl of the outer city the stench became narcotic. The familiar stalked silently on rocks like hooves. The night was warm and overcast. Behind empty civic halls, tags, and the detritus of vandalism. It ended there. The smell was so strong, it was a fight-drug. It blistered the familiar's innards. Cavities opened in it, rudimentary lungs like bellows: it made itself breathe, so that it could pant to murder.

Corrugated iron and barbed wire surrounds. The witch's familiar was the intruder. There were no stars, no lamplight. The familiar stood without motion. It breathed out a challenge. The breath drifted across the little arena. Something enormous stood. Debris moved. Debris rose and turned and opened its mouth and caught the exhalation. It sucked it in out of all the air, filled its belly. It learned it.

Dark expanded. The familiar blinked its eyelids of rain-wet leather offcuts. It watched its enemy unfold.

This was an old thing, an old familiar, the bull, the alpha. It had escaped or been banished or lost its witch long ago. It was broken bodies, wood and plastic, stone and ribbed metal, a constellation of clutter exploding from a mass of skinless muscle the size of a horse. Beside its wet bloody eyes were embedded cameras, extending their lenses, powered by organic current. The mammoth shape clapped some of its hand-things.

The young familiar had not known until then that it had thought itself alone. Without words, it wondered what else was in the city-how many other outcasts, familiars too foul to use. But it could not think for long as the monstrous old potentate came at it.

The thing ran on table legs and gripped with pincers that were human jaws. They clenched on the little challenger and tore at its accrued limbs.

Early in its life the familiar had learned pain, and this attack gave it agony. It felt itself lessen as the attacker ingested gulps of its flesh. The familiar understood in shock that it might cease.

Its cousin taught it that with its new mass it could bruise. The familiar could not retreat. Even bleeding and with arms, legs gone, with eyes crushed and leaking and something three times its size opening mouths and shears and raising flukes that were shovels, the intoxicant reek of a competitor's musk forced it to fight.

More pain and the loss of more self. The little insurgent was diminishing. It was awash in rival stink. A notion came to it. It pissed up in its adversary's eyes, spraying all the bloody muck left in it and rolling away from the liquid's arc. The hulking thing clamoured silently. Briefly blinded, it put its mouth to the ground and followed its tongue.

Behind it, the familiar was motionless. It made tools of shadows and silence, keeping dark and quiet stitched to it as the giant tracked its false trail. The little familiar sent fibres into the ground, to pipework inches below. It connected to the plastic with tentacles quickly as thick as viscera, made the pipe a limb and organ, shoving and snapping it a foot below its crouching opponent. It drove the ragged end up out of the earth, its plastic jags spurs. It ground it into the controlling mass of the old familiar, into the dead centre of meat, and as the wounded thing tried to pull itself free, the guileful young familiar sucked through the broken tube.

It ballooned cavities in itself, gaping vacuums at the ends of its new pipe intestine. The suction pinioned its enemy, and tore chunks of bloody matter from it. The familiar drew them through the buried duct, up into its own body. Like a glutton it swigged them.

The trapped old one tried to raise itself but its wood and metal limbs had no purchase. It could not pull itself free, and the pipe was too braced in earth to tear away. It tried to thread its own veins into the tubing and vie for it, to make its own oesophagus and drink down its attacker, but the vessels of the young familiar riddled the plastic, and the dying thing could not push them aside, and with all the tissue it had lost to the usurper, they were now equal in mass, and now the newcomer was bigger, and now bigger still.

Tissue passed in fat pellets into the swelling young familiar sitting anchored by impromptu guts. Venting grave little breaths, the ancient one shrivelled and broke apart, sucked into a plughole. The cobweb of its veins dried up from all its borrowed limbs and members, and they disaggregated, nothing but hubcaps again, and butcher's remnants, a dead television, tools, mechanical debris, all brittled and sucked clean of life. The limbs were arranged around clean ground, from which jagged a shard of piping.

All the next day, the familiar lay still. When it moved, after dark, it limped though it replaced its broken limbs: it was damaged internally, it ached with every step it took, or if it oozed or crawled. All but a few of its eyes were gone, and for nights it was too weak to catch and use any animals to fix that. It took none of its opponent's tools, except one of the human jaws that had been pincers. It was not a trophy, but something to consider.

It metabolised much of the flesh-matter it had ingested, burnt it away (and the older familiar's memories, of self-constitution on Victorian slag-heaps, troubled it like indigestion). But it was still severely bloated. It pierced its distended body with broken glass to let out pressure, but all that oozed out of it was its new self.

The familiar still grew. It had been enlarging ever since it emerged from the canal. With its painful victory came a sudden increase in its size, but it knew it would have reached that mass anyway.

Its enemy's trails were drying up. The familiar felt interest at that, rather than triumph. It lay for days in a car-wrecking yard, using new tools, building itself a new shape, listening to the men and the clatter of machines, feeling its energy and attention grow, but slowly. That was where it was when the witch found it.

An old lady came before it. In the noon heat the familiar sat loose as a doll. Over the warehouse and office roofs, it could hear church bells. The old lady stepped into its view and it looked up at her.

She was glowing, with more, it seemed, than the light behind her. Her skin was burning. She looked incomplete. She was at the edge of something. The familiar did not recognise her but it remembered her. She caught its eye and nodded forcefully, moved out of sight. The familiar was tired.

"There you are."

Wearily the familiar raised its head again. The witch stood before it.

"Wondered where you got to. Buggering off like that."

In the long silence the familiar looked the man up and down. It remembered him, too.

"Need you to get back to things. Job to finish."

The familiar's interest wandered. It picked at a stone, looked down at it, sent out veins and made it a nail. It forgot the man was there, until his voice surprised it.

"Could feel you all the time, you know." The witch laughed without pleasure. "How we found you, isn't it?" Glanced back at the woman out of the familiar's sight. "Like following me nose. Me gut."

Sun baked them all.

"Looking well."

The familiar watched him. It was inquisitive. It felt things. The witch moved back. There was a purr of summer insects. The woman was at the edge of the clearing of cars.

"Looking well," the witch said again.

The familiar had made itself the shape of a man. Its flesh centre was several stone of spread-out muscle. Its feet were boulders again, its hands bones on bricks. It would stand eight feet tall. There was too much stuff in it and on it to itemise. On its head were books, grafted in spine-first, their pages constantly riffling as if in wind. Blood vessels saturated their pages, and engorged to let out heat. The books sweated. The familiar's dog eyes focused on the witch, then the gently cooking wrecks.

"Oh Jesus."

The witch was staring at the bottom of the familiar's face, half pointing.

"Oh Jesus what you do?"

The familiar opened and closed the man-jaw it had taken from its opponent and made its own mouth. It grinned with third-hand teeth.

"What you fucking do Jesus Christ. Oh shit man. Oh no."

The familiar cooled itself with its page-hair.

"You got to come back. We need you again." Pointing vaguely at the woman, who was motionless and still shining. "Ain't done. She ain't finished. You got to come back.

"I can't do it on my own. Ain't got it. She ain't paying me no more. She's fucking ruining me." That last he screamed in anger directed backwards, but the woman did not flinch. She reached out her hand to the familiar, waved a clutch of mouldering dead snakes. "Come back," said the witch.

The familiar noticed the man again and remembered him. It smiled.

The man waited. "Come back," he said. "Got to come back, fucking back." He was crying. The familiar was fascinated. "Come back." The witch tore off his shirt. "You been growing. You been fucking growing you won't stop, and I can't do nothing without you now and you're killing me."

The woman with the snakes glowed. The familiar could see her through the witch's chest. The man's body was faded away in random holes. There was no blood. Two handspans of sternum, inches of belly, slivers of arm-meat all faded to nothing, as if the flesh had given up existing. Entropic wounds. The familiar looked in interest at the gaps. He saw into the witch's stomach, where hoops of gut ended where they met the hole, where the spine became hard to notice and did not exist for a space of several vertebrae. The man took off his trousers. His thighs were punctuated by the voids, his scrotum gone.

"You got to come back," he whispered. "I can't do nothing without you, and you're killing me. Bring me back."

The familiar touched itself. It pointed at the man with a chicken-bone finger, and smiled again.

"Come back," the witch said. "She wants you; I need you. You fucking have to come back. Have to help me." He stood cruciform. The sun shone through the cavities in him, breaking up his shadow with light.

The familiar looked down at black ants labouring by a cigarette end, up at the man's creased face, at the impassive old woman holding her dead snakes like a bouquet. It smiled without cruelty.

"Then finish," the witch screamed at it. "If you ain't going to come back then fucking finish." He stamped and spat at the familiar, too afraid to touch but raging. "You fucker. I can't stand this. Finish it for me you fucker." The witch beat his fists against his naked holed sides. He reached into a space below his heart. He wailed with pain and his face spasmed, but he fingered the inside of his body. His wound did not bleed, but when he drew out his shaking hand it was wet and red where it had touched his innards. He cried out again and shook blood into the familiar's face. "That what you want? That do you? You fucker. Come back or make it stop. Do something to finish."

From the familiar's neck darted a web of threads, which fanned out and into the corona of insects that surrounded it. Each fibre snaked into a tiny body and retracted. Flies and wasps and fat bees, a crawling handful of chitin was reeled in to the base of the familiar's throat, below its human jaw. The hair-thin tendrils scored through the tumour of living insects and took them over, used them, made them a tool. They hummed their wings loudly in time, clamped to the familiar's skin.

The vibrations resonated through its boccal cavity. It moved its mouth as it had seen others do. The insectile voice box echoed through it and made sound, which it shaped with lips.

"Sun," it said. Its droning speech intrigued it. It pointed into the sky, over the nude and fading witch's shoulder, up way beyond the old woman. It closed its eyes. It moved its mouth again and listened closely to its own quiet words. Rays bounced from car to battered car, and the familiar used them as tools to warm its skin.

Replacements.

Lisa Tuttle.

Walking through gray north London to the tube station, feeling guilty that he hadn't let Jenny drive him to work and yet relieved to have escaped another pointless argument, Stuart Holder glanced down at a pavement covered in a leaf-fall of fast-food cartons and white paper bags and saw, amid the dog turds, beer cans, and dead cigarettes, something horrible.

It was about the size of a cat, naked-looking, with leathery, hairless skin and thin, spiky limbs that seemed too frail to support the bulbous, ill-proportioned body. The face, with tiny bright eyes and a wet slit of a mouth, was like an evil monkey's. It saw him and moved in a crippled, spasmodic way. Reaching up, it made a clotted, strangled noise. The sound touched a nerve, like metal between the teeth, and the sight of it, mewling and choking and scrabbling, scaly claws flexing and wriggling, made him feel sick and terrified. He had no phobias, he found insects fascinating, not frightening, and regularly removed, unharmed, the spiders, wasps, and mayflies which made Jenny squeal or shudder helplessly.

But this was different. This wasn't some rare species of wingless bat escaped from a zoo, it wasn't something he would find pictured in any reference book. It was something that should not exist, a mistake, something alien. It did not belong in his world.

A little snarl escaped him and he took a step forward and brought his foot down hard.

The small, shrill scream lanced through him as he crushed it beneath his shoe and ground it into the road.

Afterward, as he scraped the sole of his shoe against the curb to clean it, nausea overwhelmed him. He leaned over and vomited helplessly into a red-and-white-striped box of chicken bones and crumpled paper.

He straightened up, shaking, and wiped his mouth again and again with his pocket handkerchief. He wondered if anyone had seen, and had a furtive look around. Cars passed at a steady crawl. Across the road a cluster of schoolgirls dawdled near a man smoking in front of a newsagent's, but on this side of the road the fried chicken franchise and bathroom suppliers had yet to open for the day and the nearest pedestrians were more than a hundred yards away.

Until that moment, Stuart had never killed anything in his life. Mosquitoes and flies of course, other insects probably, a nest of hornets once, that was all. He had never liked the idea of hunting, never lived in the country. He remembered his father putting out poisoned bait for rats, and he remembered shying bricks at those same vermin on a bit of waste ground where he had played as a boy. But rats weren't like other animals; they elicited no sympathy. Some things had to be killed if they would not be driven away.

He made himself look to make sure the thing was not still alive. Nothing should be left to suffer. But his heel had crushed the thing's face out of recognition, and it was unmistakably dead. He felt a cool tide of relief and satisfaction, followed at once, as he walked away, by a nagging uncertainty, the imminence of guilt. Was he right to have killed it, to have acted on violent, irrational impulse? He didn't even know what it was. It might have been somebody's pet.

He went hot and cold with shame and self-disgust. At the corner he stopped with five or six others waiting to cross the road and because he didn't want to look at them he looked down.

And there it was, alive again.

He stifled a scream. No, of course it was not the same one, but another. His leg twitched; he felt frantic with the desire to kill it, and the terror of his desire. The thin wet mouth was moving as if it wanted to speak.

As the crossing-signal began its nagging blare he tore his eyes away from the creature squirming at his feet. Everyone else had started to cross the street, their eyes, like their thoughts, directed ahead. All except one. A woman in a smart business suit was standing still on the pavement, looking down, a sick fascination on her face.

As he looked at her looking at it, the idea crossed his mind that he should kill it for her, as a chivalric, protective act. But she wouldn't see it that way. She would be repulsed by his violence. He didn't want her to think he was a monster. He didn't want to be the monster who had exulted in the crunch of fragile bones, the flesh and viscera merging pulpily beneath his shoe.

He forced himself to look away, to cross the road, to spare the alien life. But he wondered, as he did so, if he had been right to spare it.

Stuart Holder worked as an editor for a publishing company with offices an easy walk from St. Paul's. Jenny had worked there, too, as a secretary, when they met five years ago. Now, though, she had quite a senior position with another publishing house, south of the river, and recently they had given her a car. He had been supportive of her ambitions, supportive of her learning to drive, and proud of her on all fronts when she succeeded, yet he was aware, although he never spoke of it, that something about her success made him uneasy. One small, niggling, insecure part of himself was afraid that one day she would realize she didn't need him anymore. That was why he picked at her, and second-guessed her decisions when she was behind the wheel and he was in the passenger seat. He recognized this as he walked briskly through more crowded streets toward his office, and he told himself he would do better. He would have to. If anything drove them apart it was more likely to be his behavior than her career. He wished he had accepted her offer of a ride today. Better any amount of petty irritation between husband and wife than to be haunted by the memory of that tiny face, distorted in the death he had inflicted. Entering the building, he surreptitiously scraped the sole of his shoe against the carpet.

Upstairs two editors and one of the publicity girls were in a huddle around his secretary's desk; they turned on him the guilty-defensive faces of women who have been discussing secrets men aren't supposed to know.

He felt his own defensiveness rising to meet theirs as he smiled. "Can I get any of you chaps a cup of coffee?"

"I'm sorry, Stuart, did you want . . . ?" As the others faded away, his secretary removed a stiff white paper bag with the NEXT logo printed on it from her desktop.

"Joke, Frankie, joke." He always got his own coffee because he liked the excuse to wander, and he was always having to reassure her that she was not failing in her secretarial duties. He wondered if Next sold sexy underwear, decided it would be unkind to tease her further.

He felt a strong urge to call Jenny and tell her what had happened, although he knew he wouldn't be able to explain, especially not over the phone. Just hearing her voice, the sound of sanity, would be a comfort, but he restrained himself until just after noon, when he made the call he made every day.

Her secretary told him she was in a meeting. "Tell her Stuart rang," he said, knowing she would call him back as always.

But that day she didn't. Finally, at five minutes to five, Stuart rang his wife's office and was told she had left for the day.

It was unthinkable for Jenny to leave work early, as unthinkable as for her not to return his call. He wondered if she was ill. Although he usually stayed in the office until well after six, now he shoved a manuscript in his briefcase and went out to brave the rush hour.

He wondered if she was mad at him. But Jenny didn't sulk. If she was angry she said so. They didn't lie or play those sorts of games with each other, pretending not to be in, "forgetting" to return calls.

As he emerged from his local underground station Stuart felt apprehensive. His eyes scanned the pavement and the gutters, and once or twice the flutter of paper made him jump, but of the creatures he had seen that morning there were no signs. The body of the one he had killed was gone, perhaps eaten by a passing dog, perhaps returned to whatever strange dimension had spawned it. He noticed, before he turned off the high street, that other pedestrians were also taking a keener than usual interest in the pavement and the edge of the road, and that made him feel vindicated somehow.