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been lucky; it had disabled its victim with a single blow. All it had to do now was wait until the titan was too weak to do the raptor any harm. It could even take its food while its prey was still alive.
    The raptor would not trouble with such small fry as Purga while such a giant meal awaited it. Moving cautiously, watchfully, Purga left the shelter of the fern, and scurried across the scrubby floodplain, through the devastated track left by the anatotitan herd, until she reached the security of the trees.

    • • •
    For the first time in four billion years, heat had touched the Devil’s Tail. Fragile ice sculptures older than Earth were quickly lost.
    Gases boiled through fissures in the crust. Soon a shining cloud of dust and gas the size of the Moon had gathered around the comet. The wind from the sun, of light and sleeting particles, made the gas and dust stream behind the falling comet nucleus in tails millions of kilometers long. The twin tails were extremely tenuous, but they caught the light and began to shine.
    For the first time, uncomprehending eyes on Earth made out the approaching comet.
    Spitting, rotating, its dark nucleus founting gases with ever greater vigor, the Devil’s Tail swam on.

    III
    Another long, hot Cretaceous day wore away.
    Purga slept through the day, her new family curled around her. She slept even when her pups suckled. The snug burrow floor was littered with the primates’ soft fur— and it smelled, indubitably, of Purga, of her new mate, and of the three pups who were half of herself.
    Purga’s mate gave himself no name, and nor did Purga name him, any more than she named herself. But if she had— in recognition that he could never be the first in her life— she might have called him Second.
    As Purga slept, she dreamed. Primates already had brains large and complex enough to require self-referential cleansing. So she dreamed of warmth and darkness, of flashing claws and teeth, and of her own mother, huge in her memory.
    Purga, like all mammals, was hot-blooded.
    All animal metabolisms were based on the slow cellular burning of food in oxygen. The first animals to colonize the land— gasping fish, driven from drying rivulets, using swim bladders as crude lungs— had had to rely on metabolic engines designed for swimming. In those first land-walkers the metabolic fires had glowed dimly. Still, their decisive move onto the land had been successful; and now and into the future every animal— mammals, dinosaurs, crocodiles, and birds, even snakes and whales— would use a variant of the same ancient tetrapod body plan of four legs, a backbone, ribs, fingers, and toes.
    But some two hundred million years before Purga’s birth, certain animals had begun to develop a new kind of metabolism. They had been predators, driven by selection to burn food more briskly in order to improve their luck in the chase.
    It had meant a complete redesign. These ambitious predators needed more food, a higher rate of digestion, a more efficient system of waste elimination. All this had raised their metabolic rate, even when resting, and they had had to increase the size of heat-producing organs like the heart, kidneys, liver, and brain. Even the working of their cells had speeded up. In the end a new and stable high body temperature had been set.
    The new hot-blooded bodies had had an unplanned advantage. Cold-bloods relied on drawing heat from the environment. But the hot-bloods did not. They could operate at peak efficiency in the cool of night, when the cold-bloods had to rest, or in extreme heat, when cold-bloods would have to hide. They could even prey on cold-bloods— frogs, small reptiles, insects— at times like dawn and dusk, when those slow movers were vulnerable.
    But they could not topple the dinosaurs from their thrones; the dinosaurs’ supreme energy efficiency saw to that.
    Purga’s dreams were disturbed by the immense stomping of the dinosaurs as they went about their