FIVE-YEAR-OLD ANDY GILMAN, towheaded and gap-toothed, was kneeling on a chair by the kitchen window. Half a dozen plastic Warbirds were scattered on the floor beneath him. With the tip of one finger, he was writing his name in the large smudge his nose had left on the glass. Suddenly he stiffened and pointed beyond the pane. "Look, Daddy!" he cried. "See the bird! By the feeder! A big one!"

Nick Gilman grinned and crossed the room in a stride. He looked, and the kid was right. A Chickadee, the size of an old-fashioned Piper Cub, was on the lawn beside the back porch. It wasn't wearing its two-seater passenger or engine pods. As Nick watched, it cocked its head to one side, inserted its beak between the shelf and the overhanging roof of the feeder, and seized a mouthful of seeds. Then, shaking its head as if the treat had been more effort than it was worth, it stepped back a pace.

As it did so, nongengineered birds of more normal size approached to try to reach the seeds remaining in the feeder. Few succeeded, for as they fluttered past the Chickadee, they fell prey instead to its darting beak. Nick shuddered, remembering when all chickadees had been vegetarians. "C'mon, Andy. We're in a rush. Gotta go get Mommy."

"But, Daddy! I wanna watch!"

Nick had no time for nonsense. Emily's jet would be late, of course, but it was due in an hour, and he had to be there just in case she was on time or -- God forbid! -- early. He should have left ten minutes before, but the casserole had needed its finishing touches and he had had to adjust the oven and he had had to run a comb through his hair and he had had to straighten the throw rug that had slid beneath his feet and... It wasn't easy being a househusband.

The radio began to mutter that, on this hot and muggy Tuesday in July of 2044, terrorist attacks were becoming more frequent, but he had no time to listen. Nor did he care to think of what such a thing might mean for Emily, or him, or their towheaded son. He turned it off and grabbed his jacket. Then he picked the boy up in his arms, wiped the snot from the boy's nose with a handkerchief, and rushed from the room.

Emily was a high-bracket gengineer, she would be back soon from her trip -- she had flown to Washington on Sunday to testify before a patent board on Monday -- he loved her dearly, and he didn't want to leave her waiting. Sometimes he wished their roles were reversed, with him the one wandering the world on high adventures and she the one at home in their small, old-fashioned brick house. But his doctorate had been in Romantic Poets, there were fewer new college students than ever, few colleges were hiring young faculty, and his attempts at selling his own poems and short stories had earned him the grand total of $79.85. He could have bought a pair of shoes. Cheap ones.

Nick had opened the garage door that morning and led the Tortoise out for relief from the heat. Now the family vehicle was waiting in the drive, shaded by nearby trees. Nick had bought it when he was in college and single. It had been young then, with the passenger compartment in the shell just big enough, in a squeeze, for two. And he had squeezed more than one girl in it, he had, until he had found Emily and grown up a bit. As advertised, the Tortoise had grown too, maturing from the sports car stage to coupe. Eventually, timed by gengineers like Emily to match a family's growth, it would gain the capacity of a station wagon.

The Tortoise didn't look like a tortoise. Its chief ancestor had been a lean, low terrapin. The gengineers had given it size and speed, and a cavity beneath the shell. The General Bodies shops had fitted a windshield, side windows, and doors, installed plush seats, added headlights and taillights, and wired the controls into the Tortoise's nervous system. At periodic checkups, they added new fittings and enlarged or refitted the old to keep pace with the creature's growth.

Roachsters, half cockroach and half lobster; Hoppers, derived from grasshoppers; and other Buggies could keep pace with a family's needs just as well. But Nick preferred the more classic lines of the Tortoise. Its shape reminded him of the gas-burners his parents had driven when he had been a child, when the Machine Age had still been vigorous. The oil that had made that Age possible had been on the verge of exhaustion, and most liquid fuels were being produced -- expensively -- from coal. But people had not yet recognized that new forms of technology were essential if civilization were to continue, nor that the replacement technology was already taking shape. The Biological Revolution had by then been fermenting in the world's laboratories for decades, and the gengineers had been on the verge of long-sought success.

As Nick and Andy left the house, the Tortoise's barrellike head turned toward them. The legs on the side facing the house flexed, Nick stepped onto the offered lip of shell, resembling an old-time running board, and opened the door. Andy scooted across the bucket seats to let his father take his position behind the tiller.

Even before the door clicked into its frame, the Tortoise's knees were rising and falling, pistonlike, in Nick's peripheral vision. He steered it onto the greenway that had long since replaced paved streets in his suburb, guided it toward the expressway on-ramp, and accelerated. The Tortoise's knees became a blur, its breathing an audible gale of wind.