Hidden in a patch of shadow, his scent blowing downwind, Solo saw the notharctus troop scurrying to safety.

And he saw Noth's mother with her infant.

She was a fertile, healthy female: that was what the presence of the infant told him about her. But there was a mate with her, and since she already had a pup, she was unlikely to come into heat again this season. Neither of those factors were an obstacle to Solo. He waited until Noth's family had settled on a branch, calming down, out of immediate danger.

Solo, at three years old, was a mature, powerful male notharctus. And he was something of a freak.

Most males roamed the forests in small bands, seeking out the larger, more sedentary troops of females where they might find a chance to mate. Not Solo. Solo preferred to travel alone. He was larger and more powerful than almost all the females he had encountered in his travels in this polar forest. Again, in this Solo was unusual; the average adult male was smaller than the average female.

And he had learned to use his strength to get what he wanted.

With a lithe swing Solo dropped down to the branch and stood upright before Noth's mother. He looked unbalanced, for his hind legs were comparatively massive, his forearms short and slender, and he held his long tail up in the air so that it hooked over his head. But he was tall, and very still, and very intimidating.

Noth's mother could smell this huge stranger: not kin. not kin. She immediately panicked. She hissed and pushed Left behind her. She immediately panicked. She hissed and pushed Left behind her.

Noth's father came forward. He raised himself up on his hind legs and faced the intruder. Moving with fast jerky gestures he rubbed his genital glands against the foliage around him, and swept his tail over his forearms so that the horny spurs above his wrist glands combed through his tail fur and impregnated it with his scent. Then he waved the lushly stinking tail above his head at the intruder. In the scent-dominated world of the notharctus, it was an awesome display. Get away. This is my place. This is my troop, my young. Get away. Get away. This is my place. This is my troop, my young. Get away.

There was nothing sentimental in the father's behavior. Producing healthy offspring that survived to breeding age was the only purpose of this father's life; he was preparing to take on the intruder solely through a selfish drive to see his own heritage preserved.

Usually this game of malodorous bluff would have continued until one or the other of the males backed down, without physical contact. But again Solo was unusual. He did not respond with any form of display, save for a cold stare at the other's feverish posturing.

Noth's father was unnerved by the newcomer's eerie stillness. He faltered, his scent glands drying, his tail drooping.

Then Solo struck.

With teeth bared he lunged at Noth's father, slamming into his chest. Noth's father fell back, squealing. Solo dropped to all fours and fell on him, biting into his chest through a layer of fur. Noth's father screamed and scurried out of sight. He was only slightly injured, but his spirit was broken.

Now Solo turned on the females. The aunts could easily have resisted Solo, if they had combined their efforts. But they scrambled out of Solo's way. Solo's assault had disturbed them as much as its victim. They had never seen anything like it. All of them were mothers; all thought immediately of the infants they had left parked in the high branches.

Solo ignored them too. With a carnivore's steely movements he advanced on Noth's mother, his principal target.

She hissed, she showed her teeth, she even kicked at him with her powerful hind legs. But he resisted her blows easily, walked through her kicking- and took the unresisting, baffled infant from her grasp. He bit quickly into the pup's throat, opening up the flesh, and rummaged there until he had ripped open the infant's trachea. It was over in heartbeats. He dropped the quivering scrap into the forest below, where mesonychids, alerted by the scent of fresh blood, ran forward with their eerie uncanine-like barking. His mouth and hands bloodied, Solo turned to Noth's mother. Of course she would not be fertile yet, perhaps not for some weeks, but he could mark her with his scent, make her his own, and repel the attentions of other males.

There was nothing truly cruel in Solo. If her pups were killed, it was possible Noth's mother would come into heat again before the end of the summer- and if Solo covered her then, he could generate more offspring through her. So, for Solo, infanticide was a good tactic.

Solo's brutal strategy wouldn't have been sustainable for everybody. Notharctus males were not equipped to fight. They lacked the canine teeth that later primate species would use to inflict damage on rivals. And this polar forest was a marginal environment where true fights were literally a waste of energy, a squandering of scarce resources, which was why the ritual stink fights had evolved. But for Solo, the exception, it was a strategy that worked, over and over, and which had won him many mates- and which had generated many offspring, scattered through the forest, whose veins ran with Solo's blood.

But it wasn't going to work this time.

Noth's mother, marked by the killer's scent, gazed down into the green void below. She had lost her baby- just as Purga, her remote grandmother, had once endured. But, considerably more intelligent than Purga had been, she was much more acutely aware of her pain.

Blackness filled her. She lunged at Solo, her small limbs flailing, mouth gaping. Startled, he darted back.

She lunged past him. And she fell.

Noth saw his mother fall into the pit where his infant sister had fallen before. Immediately her twisting form was lost under the slick, writhing bodies of the mesos.

Noth had been weaned a few weeks after he had been born. Soon would have come a time when he would have wandered from the troop. His link to his mother was tenuous. And yet he felt a loss as powerful as if his mother's breast had been ripped from his mouth.

And still the rain fell, harder all the time.

Noth, shivering, crawled through the branches. With the wind low, the rain fell in massive drops that pounded exposed flesh and hammered against the trees' broad leaves.

Following lingering traces of his mother's scent, he found his baby sister. She still clung motionless to the tree trunk where her mother had parked her- where she would have clung, probably, until she starved. Noth sniffed her damp fur. He huddled up close and wrapped his arms around her. She was a tiny shivering mass against his belly fur, but he was sheltering her from the rain.

He was drawn to stay with her. She smelled of family; she shared much of his genetic inheritance, and therefore he had a stake in any offspring she might one day have.

But the rain fell through a night and a day, as the sun continued its purposeless dance around the sky. The forest floor became sodden, and glimmering pools, laden with floating leaf debris, began to cover the ground, hiding gnawed and scattered bones.

And the continuing rain washed away the last traces of the scent markers of Noth's troop from the trees. Noth and his sister were lost.

II.

As the endless day wore on, as the sun wheeled through its meaningless cycles, Noth and Right stumbled through the forest's branches.

They had already been lost for a week. They had found none of their own kind. But here in the forest canopy there were many adapids, cousins of the notharctus. Many of them were smaller than Noth. He would glimpse their glowing eyes, like eerie yellow pits, peering out of a shadowed nook. These miniature insect hunters looked more like mice. Some of them scuttled along branches, racing from shaded cover to cover. But one made a spectacular upright leap from tree to tree, its powerful hind legs dangling, its paws reaching. Its membranous ears swiveling like a bat's, it caught an insect, plucking it out of the air in its jaws in midjump.

One solitary little creature clung to the rotten bark of an ancient tree. It had a scruffy black coat, batlike ears, and prominent front teeth, and it tapped patiently at the wood with a claw-tipped finger, its large ears swiveling. When it heard a larva burrowing under the bark, it ripped off the bark with its teeth and plunged in a peculiarly long middle finger to hook the larva and deliver it to its gaping, greedy mouth. This was a primate that had learned to live like a bird, like a woodpecker.

Once Noth blundered into a giant, slothlike creature hanging upside down from a thick branch, its primate's hands locked around the wood. This monster's head swiveled to inspect Noth and Right, its eyes blank. Its mouth chewed slowly, crammed full of the fat deciduous leaves that were its principal diet. Its kind had been driven to larger sizes by the need to accommodate a gut big enough to break down the cellulose in the leaves' cell walls. The sloth-thing's face was oddly immobile, static, limited in its expressiveness. The social life of this gloomy hanging creature was unexciting; its slow metabolism, and lack of spare energy to devote to social activities, saw to that.

The world had warmed steadily since the terrible impact. Waves of vegetation had migrated away from the equator, until tropical rain forest eventually covered all of Africa and South America, North America to what would become the Canadian border, China, Europe as far north as France, and much of Australia. Even at the poles there were jungles.

North America was still joined by mighty land bridges to Europe and Asia, while the southern continents lay in a great band below the equator, like scattered islands. India and Africa were both migrating north, but for now the Tethys Sea still girdled the equator, a mighty current that spread warmth around the belly of the planet. The Tethys was like a river through Eden.

In response to the great warming, the children of Plesi and other mammals had at last thrown off their past. It was as if the Earth's inheritors had finally realized that the empty planet offered them a lot more than just another kind of grub to chew. While the reptilian survivors, the lizards, crocodiles, and turtles, clung on largely unchanged, soon the foundations of the successful mammalian lineages of the future would be laid down.

Plesi, like Purga, had been a low-slung crawler, with the typical mammalian four-footed head-down body stance. But her primate descendants grew larger, with more powerful hind limbs to support upright bodies and heads. Meanwhile the primates' eyes had moved forward to the front of their faces. This would give them three-dimensional vision, enabling them to judge their increasingly long leaps, and to triangulate on the prey insects and small reptiles that still formed part of their diet. And as they explored different ways to make a living, the primates would fan out into many different forms.

There was no design in this: no sense of improvement, of purpose. All that was happening was that each organism was struggling to preserve itself, its offspring, and its kin. But as the environment slowly changed, so through relentless selection did the species that inhabited it. It was not a process fueled by life, but by death: the elimination of the less well adapted, the endless culling of inappropriate possibilities. But the potential of an unseen future was no consolation to those who lived through the relentless culling.

Many of the adapids had become too specialized. This comfortable planet-swaddling warmth would not last forever. In cooler times in the future, as the forests became sparse and seasonal differences became more pronounced, it wouldn't seem so smart to be a fussy eater. Extinctions would follow, as they always had.

Meanwhile, amid this clutter of exotic primates, the siblings found no notharctus.

Exploring the forest floor, Noth found a plant with podded fruit, a kind of pea. He broke open a few pods and let his sister feed.

A kind of anteater, a meter long, approached a pillarlike ants' nest. It fell on the nest, wielding its powerful arm and shoulder muscles. As though it wielded a pickaxe, all its force was concentrated on a single point, the tip of its strongly flexed middle finger. The ants swarmed- they were huge, each some ten centimeters long- and the anteater quickly ingested them with its long, sticky tongue before the soldiers could unite in defense. The anteater was a descendant of South American stock, which had wandered here over temporary land bridges many generations before.

Noth and Right watched, wide-eyed. But as he kept an eye on the anteater, concern gnawed at Noth's unconscious.

He had tried to keep them both feeding, to fatten up their tails with the winter storage that would see them through the long months of hibernation to come. That was just as his innate programming instructed. But they weren't eating enough. Isolated from the support of the troop, he was having to spend much too much of his time watching for predators.

He could have gone back. Like all his species- the mobile males more than the sedentary females- he kept track of his position by dead reckoning, integrating time, space, and the angle of the slanting sunlight. It was an ability that helped him find scattered sources of food and water. If he needed to Noth could find his way back "home," to the stand of trees that had been the center of his troop's range. But he never heard the distinctive warbling song of his troop; his rudimentary decision-making machinery pressed him to keep searching for a troop that would accept him and his sister.

Meanwhile, though the sun still circled endlessly above the horizon, much of the daylight was tinged with the red of sunset, and here on the forest floor brown spores clung to the fern fronds. Autumn was coming. And then there would be winter. They were underfed, and time was running out.

Right became distressed, as she so often did. She dropped the pea pods and folded over on herself, rocking, keening softly, her hands over her small face. Noth took her in his arms and carried her to the crook of a branch, where he began to groom her. He worked carefully through the sparse fur on her back, neck, head, and belly, removing dirt, bits of leaf, and dried feces, untangling knots, picking out parasites that were attempting to feast on her young skin.

Right quickly calmed. The grooming's mixture of pleasure, attention, and mild pain flooded her system with endorphins, her body's natural opiates. Before she grew much older she would be addicted, literally, to this pleasurable scratching- as her brother already was. Noth badly missed the strong, nipping caress of adult fingers on his back.

But Noth was worried about her, on deep levels he could not understand.

Right's bewildering grief served a purpose. It was a signal to her that she had suffered a loss, that there was a hole in her world that she must fix. And though Noth was not capable of true empathy- if you didn't really understand that other people had minds and thoughts and feelings like yours, you couldn't possibly be empathetic- still the signs of grief in his sister triggered a kind of protectiveness in him. He wanted to put the world right for his sister: The instinct to help the orphaned went very deep.

But in the end obsessive grief was maladaptive. If Right was unable to recover, in the end there would be nothing he could do for her. He would have to abandon her, and then she would surely die.

As day followed day, the sun, at the lowest point of its arc in the sky, began to slip beneath the southern horizon. At first the brief nights were like twilight, and on clear nights purple-red curtains of light climbed into the tall sky. But quickly the sun's excursions into invisibility became longer, and there were increasing intervals when stars shone in a deepening blue. Soon true darkness would return to the polar forest.

The weather quickly became colder and drier. Rainfall was scarce now, and on some days the warmth of the sun barely seemed to penetrate the lingering mist. Already many of the birds of the forest canopy had departed, skein after skein of them flitting over the sky to the warmer lands to the south, watched by uncomprehending primate eyes.

Noth became exhausted, ragged, and his dreams were full of flashing teeth and biting claws, visions of his scrap of a sister taken by gigantic mouths.

Now their biggest problem was thirst. It had been so long since the last rain that the treetops were becoming parched. And already the trees were starting to shed; the last leaves were withered and brown. Soon Noth was reduced to licking the bark each morning for the cold dew.

At length, driven by their thirst, the siblings went in search of ground water. Near the closest large lake they scurried down a tree trunk, eyes wide.

Approaching the water, the primates crept past a pair of what looked like miniature deer. The size of small dogs with long, trailing tails, these fast, solitary runners, browsing on leaves and fallen fruit, were ancestors of the mighty artiodactyl family, which would one day include pigs, sheep, cattle, reindeer, antelope, giraffes, and camels. Right disturbed a frog, which hopped away, croaking in protest. She cowered back, eyes wide at its strangeness. Soon they saw more amphibians, frogs and toads and salamanders. Birds crowded the bushes, raising shrill cries that filled the dank air.

Noth was uneasy. The shore was too crowded: Noth and Right were not the only thirsty creatures in this shivering jungle.

A meter-long creature like a long-tailed kangaroo ran past; this was a leptictidium, a hunter of small animals and insects. Exploring the ground with its mobile nose, it disturbed a pholidocercus, a spiky-haired ancestor of the hedgehogs, that indignantly hopped away like a rabbit. Here was a close-packed herd of horses. They were tiny: no larger than terriers, with perfectly formed equine heads. Shyly these exquisite little creatures picked their way through the undergrowth. They walked on pads, like cats, and on each foot they had several hoofed toes. Their genus had emerged in Africa only a few million years earlier. A rough growl from an impatient carnivore startled the little horses, making them stir into sudden flight.

Through this exotic crowd the two primates proceeded cautiously, moving in scurries, in fits and starts.

The water itself was a languid sheet, dense with matted vegetation, dead reeds, and algal blooms. In places ice had already formed in thin gray slices. But on the open water birds waded, ancestors of flamingos and avocets, and huge water lilies rested languidly on the surface.

Over the open water a spider was suspended on a thread of silk, and huge ants flew, each as large as a human hand, on their way to found new nests. Through this crowd of insects flapped a family of delicate bats. Recently evolved, as huge and fragile as paper kites, the new flying mammals snapped at the insects. Primitive bony fish broke the surface and gulped at the aerial fodder, as did a twisting eel.

The primates found a place far enough from any of the predators to be able to drink unhindered. They bent and plunged their muzzles into the chill water, sucking it up gratefully.

The largest animals of all wallowed at the muddy fringes of the lake.

A pair of uintatheres stood side by side. These great animals looked like gargantuan rhinos, each with a set of six bony horns on its head and long upper canine teeth like a saber-toothed cat's. Their thick hides were coated with mud, which helped keep them cool and kept off insects. They cropped placidly on the soft vegetation of the lake bottom, sucking at water stained green by algae, while a fat youngster, more agile and lively, played around his parents' legs, barging their tree-trunk knees with a head ladened with stubby, unformed tusks. Noth watched their huge feet fearfully. Closer to the shore there walked a family of moeritherium. No more than a meter tall, the adults moved through the water with a stately calm, rumbling reassurance to each other, while their round-bodied infants splashed at their feet. They worked the lake bottom vegetation efficiently with their long noses. These were among the first proboscideans, the ancestors of elephants and mammoths. They were still more piglike than elephantine, but they were already clever and social animals.

Around the herbivorous herds circled carnivores. These were mostly creodonts; they looked like foxes and wolverines. And there was one pack of hoofed predators- like carnivorous horses- bizarre, terrifying creatures with no analogies in human times.

Many of these creatures looked slow and lumbering, oddly ill-formed, the results of nature's first experiments in producing large herbivores and predators from the mammalian stock that had survived the dinosaur extinction. Open grasslands still lay millions of years in the future, along with the fleet, long-legged, graceful herbivorous forms that would adapt to their open lush spaces, and the cleverer, faster carnivores that would arise to prey on them. When that happened most of the species around Noth would succumb to extinction. But the orders that would be familiar to humans- the true primates, the hoofed animals, the rodents and bats, the deer and the horses- had already made their entrance on the stage.

And there was no more complex and crowded an ecology anywhere on Earth right now than here on Ellesmere Island. This place was a pivot on the great migratory routes up through the Americas and over the roof of the world to Europe, Asia, and Africa. Here, pangolins from Asia, carnivores from North America, hoofed creatures from Africa, European insectivores like ancestral hedgehogs, and even anteaters from South America mingled and competed.

Suddenly Noth pulled back his head.

From inside the water two primates were looking out at him, a burly male and a small female. He could not smell the male, could not tell if he was kin or stranger. He screeched, baring his teeth. The male primate bared his teeth in response.

Enraged, Noth got to his feet and displayed his musk glands to the stranger in the water- who displayed back, angering him further- and then he stamped at the water until the reflected notharctus was gone.

Noth could recognize others of his species, could distinguish them as male or female, and as kin or not kin. But he could not recognize himself, for his mind did not contain the ability to look inward. All his life he would feel threatened by any such chance reflection.

A sleek form burst from the water itself and came lurching up on clumsy flippered limbs onto the rocky platform. Noth and Right stumbled back. Over a snout like a crocodile's the newcomer gazed at the two baffled primates.

This ambulocetus was a relation of the hyena-like mesonychids. Like an otter, it was covered in sleek black fur, and it had large, powerful back legs equipped with toes ten centimeters long. Ages ago this animal's ancestors had returned to the water, seeking a better living, and selection had begun its relentless molding. Already the ambulocetus looked more aquatic than terrestrial.

Soon its kind would take permanently to the oceans. Its skull and neck would become shorter, and the nose migrate backward, while its ears would close so that sound would have to pass through a layer of fat. Its legs would morph at last into fins, with more bones added, the fingers and toes becoming shrunken and useless, at last disappearing. When it reached the vast spaces of the Pacific and Atlantic, it would begin to grow- ultimately becoming as large compared to its present form as a human was to a mouse- but those mighty seagoing descendants would still retain within their bodies, like fossils of bone and molecular traces, vestiges of the creatures they had once been.

The walking whale stared uncomprehendingly at the two timid primates. Deciding this crowded shore wasn't such a good place to bask after all, it flexed its spine and swam gracefully away.

As the light faded, Noth and Right retreated to the shelter of the trees. But the branches were now all but bare, and cover was hard to find. They huddled in a branch's crook.

The herbivores splashed out of the water, family groups calling to each other. And the predators began to call, harsh doglike barks and leonine growls echoing through the sparse forest.

As the chill settled deeper Noth felt torpor steal over him. But he felt cold, cold, stuck here like this with only his baby sister, cold away from the huddle of his troop. stuck here like this with only his baby sister, cold away from the huddle of his troop.

And then, to his surprise, he was startled awake by a powerful musk scent.

Suddenly there were notharctus all around. They were on the branches above and below him, huddled shapes with their legs drawn up beneath them and their long, fat tails dangling. Their scent told him this was his kind, but not his kin. He had not detected their scent markings before; in fact the markings were sealed in by layers of frost. But the strange notharctus had noticed him.

Two powerful females gathered closely, drawn by the scent of an infant. One, who he thought of as Biggest, pushed aside the other- who was merely Big- to get a closer look at Right.

Noth's mind churned. He knew that it was vital that they be accepted by this new group. So he reached for the female closest to him, Big, and began, tentatively, to dig his fingers into the fur at the back of her legs. Big responded to his grooming, stretching out her legs with pleasure.

But when Biggest saw what was going on she hooted and slapped them both. Noth cowered, trembling.

Noth was bright enough to understand his own place on the social ladder- in this case, down on the bottom rung. But his social mentality had its limits. Just as he could not detect the beliefs and desires of others, so he was not smart enough to form judgments about the relative ranking of others in a group. He had got it wrong: Biggest outranked Big, and she expected this new male to pay her attention first.

So Noth waited as Biggest played with the drowsy Right. But at least she did not drive him away. And at length Biggest let Noth approach her and groom her own dense, rank-smelling fur.

III.

Every day was shorter than the last, every night longer. Soon there were just a few hours of bright daylight, and the intervals between the darknesses were lit only by a pink-gray twilight.

The forest was all but silent now. Most of the birds and the large herbivore herds had long gone, migrated south to warmer, easier climes, taking their dinning cries with them. The buzzing insect swarms of high summer were a memory, leaving only larvae or deep-buried eggs, sleeping dreamlessly. The big deciduous trees had already dropped their broad leaves, which lay in a thick litter on the ground, welded together by the persistent frost. The bare trunks and leafless branches would show no signs of life until the sun returned in a few months' time. Beneath them, plants like the ground fern had died back to their roots and rhizomes, soon to be sealed into the earth under a lid of frost and snow.

The species here- derived from ancestral stock adapted to the balmy conditions of the tropics- had had to make ferocious adjustments to survive the extreme conditions at the pole. Every plant, wherever it lived, depended on sunlight for energy and growth, and during the endless days of summer the vegetation had lapped up the light with broad, angled leaves. But now there approached a season when for months there would be no light but that of the Moon and stars, useless for growth: If the plants had kept on growing and respiring they would have burned up their energy store. So the flora were heading for a vegetable hibernation, each according to its own strategy.

Even the plants were sleeping.

The notharctus troop was thirty strong, and they had huddled in the branches of a big conifer. They looked like big furry fruit, their hands and feet clinging to the branches as they slept, their faces buried in their chests, their backs exposed to the cold. Frost sparkled on their new winter coats, and where a muzzle showed breath steamed, glowing blue white.

Noth slept away the lengthening nights, his fur bristling, immersed in the body heat of the others of the troop. Sometimes he dreamed. He saw his mother fall into the jaws of the mesos. Or he was alone in an open space surrounded by hard-eyed predators. Or he was like a pup again, pushed out of a troop by adults bigger and stronger than he was, excluded by rules of which he had no innate understanding. But sometimes the dreams faded, and he fell into a kind of torpor, a blankness that prefigured the long months of hibernation to come.