“Tell me what that means,” she said. “The alternation.”

“For most of a year the genetic exchanges are relatively local. Summer generations breed in smaller groups as they move north. Some might fly only a few miles from where they are born before mating and dying. But then, in winter, the whole population comes together in one place. The gene pool is thoroughly blended.”

“I get that. Okay. Like mostly swapping your goods at the secondhand store in town, and then once a year doing the international-trade thing at the dollar store.”

Ovid laughed. “You are good. I wish I could put you in front of my students.”

She tried not to smile too hard. Her thermos of coffee was on the table, hidden among the plastic boxes and someone’s raincoat. She shuffled through other junk to find their two stained mugs that stayed on-site as permanent fixtures. She tossed out the grainy dregs of yesterday’s coffee and held the cups outside the shelter to collect a little rain, then wiped them with her shirttail. She unscrewed the thermos and filled both mugs. Housekeeping in the invisible house. She and Ovid liked their coffee black, they had that in common.

He took the mug, nodding his thanks, and sat on an upended section of log they used as furniture. “We don’t know of anything else like it on earth,” he said. “This system of local and universal genetics makes a kind of super-insect. The population can fluctuate fivefold in a year. It’s an insurance policy against environmental surprises.”

Environmental surprise within known limits, he would mean. He grew broody as he drank his coffee, looking out through the rain. He’d left her the lawn chair, but she stayed on her feet. Long clusters of butterflies began to drip. Hangers-on at the bottoms of their strings twisted slowly in an imperceptible wind, like the caricature of a hanged man. A chunk of a cluster near the shelter dropped suddenly onto the ground, severed from the great beast. Grounded butterflies could not hope to lift themselves in a rain like this. She watched this fresh legion of the extinguished, taking their time to die.

“Nobody else came to the site today?” she asked.

He shook his head.

“I’ve left a couple of messages with Vern, but he doesn’t call back. It seems like we’re losing our volunteers. Maybe they’re having exams.”

Ovid said, “Not everyone has the stomach to watch an extinction.”

She noticed the fabric over their heads had begun to droop in spots where the rain pooled. The roof of their invisible house, collapsing. What wouldn’t, under all this? She was slowly submitting to his sense of weather as everything. Not just the moving-picture view out a window. Real, in a way that the window and house were not.

A scattering of butterflies in the fallen mass twitched open and closed, while getting pounded, showing their vivid orange a few last times. Rage, rage against the dying of the light. That was the end of a poem, brought to her by the one bright spot in her education, Mrs. Lake, now dead. Dellarobia suddenly found she could scarcely bear this day at all. She stepped out in the rain to pick up one of the pitiful survivors and bring it under their roof. She held it close to her face. A female. And ladylike, with its slender velvet abdomen, its black eyes huge and dolorous. The proboscis curled and uncurled like a spring. She could feel the hooked tips of the threadlike legs where they gripped her finger. She held it out and the wings opened wide, a small signal.

“So you’re one of the people that can,” she said. “Watch an extinction.”

He did not quite break his communion with the day, his vigil, whatever it was, but asked, “If someone you loved was dying, what would you do?”

She refused that sentence its entrance. Preston and Cordie, no. Not another runaway loss. The Cooks she could think of, barely. Their boy. You do the bone marrow transplants, whatever it takes. She had examined Ovid’s sadness by degrees, but now it hit her fully, the nature of his loss. “You do everything you can,” she said. “And then, I guess, everything you can’t. You keep doing, so your heart won’t stop.”

The butterfly on her hand twitched again, and she held it sideways to the light. On the gloss of the wings she could see every scratch, like the marred lenses of an old pair of glasses. “If they could just breed and lay eggs,” she said. “A few. I’m not saying truck them all to Florida. But just to get them through this one winter?”

He glanced up at her. “It’s not my call, Dellarobia.”

She considered this. To whom did a species belong? She wondered if that kind of law was even on the books. She sat down in the lawn chair, and saw he was getting restless, eyeing a stack of field notes on the table. “I am not a zookeeper,” he said. “I’m not here to save monarchs. I’m trying to read what they are writing on our wall.”

Dellarobia felt stung. “If you’re not, who is?” She could think of some answers: the knitting women, the boys with duct-taped clothes. People Cub and her in-laws thought to be outside the pale of normal adulthood.

“That is a concern of conscience,” he said. “Not of biology. Science doesn’t tell us what we should do. It only tells us what is.”

“That must be why people don’t like it,” she said, surprised at her tartness.

Ovid, too, seemed startled. “They don’t like science?”

“I’m sorry. I’m probably speaking out of turn here. You’ve explained to me how big this is. The climate thing. That it’s taking out stuff we’re counting on. But other people say just forget it. My husband, guys on the radio. They say it’s not proven.”

“What we’re discussing is clear and present, Dellarobia. Scientists agree on that. These men on the radio, I assume, are nonscientists. Why would people buy snake oil when they want medicine?”

“That’s what I’m trying to tell you. You guys aren’t popular. Maybe your medicine’s too bitter. Or you’re not selling to us. Maybe you’re writing us off, thinking we won’t get it. You should start with kindergartners and work your way up.”

“It’s too late for that. Believe me.”

“Don’t say that, ‘too late.’ I hate that. I’ve got my kids to think about.”

Ovid nodded slowly. “We were not always unpopular. Scientists.”

“Herbert Hoover was one! I read that.” Preston’s encyclopedias had already made it to show-and-tell. Flying Ants were making the rounds.