Why the butterflies, why now. Why here?

Ovid had three theories. Not at first. In the beginning he resisted, wielding nonanswers with the best of them: untestable hypotheses, too many variables. Herbicides, for example. Their sole larval food is milkweed, a plant whose last name is “weed.” Pesticides too, spraying on the increase, as warming temperatures bring in the West Nile mosquito. New weather patterns affect everything in the migratory pathways. Both the fire and the flood. But at length he consented to certainty about these few things: It has become much too warm at the Mexican roost sites. With climate change the whole forest moves up those mountain slopes, a slow-motion slipping uphill, a thing she could imagine. The trees have their requirements. With arboreal stoicism they edge toward the peaks, and from there they cannot levitate.

But that explains why not there. That is not why here.

His second line of thinking was the OE parasites he’d shown her under the microscope. They stunt wingspan and lifespan. Monarchs highly infected with this parasite cannot fly very far. The annual trip to Mexico seems to weed out the most burdened, keeping the population healthy. But west of the Rockies is a different group, an outsider’s club of monarchs that are very infected, and do not fly to Mexico but seek their winter shelter in scattered groves of trees along the California coast. Maybe they portend what is coming. Warmer temperatures correlate with rising infection rates. If the parasite reached a critical level in eastern populations, natural selection might favor short migrations and dispersed winter roosting everywhere, not just in California. The hypothesis is immense, with its multiple bonds of cause and effect, some of them testable. To this end she cut small squares of cellophane tape, pressed them against the abdomens of one hundred live monarchs, and under the scope, counted the dark parasite spores nestled among the ridged, translucent scales. It took hours of acute ocular focus, a headache beyond all known proportions, and an appointment with the eye doctor for new glasses (overdue). Counting the microscopic dots on every centimeter-square of tape was not unlike counting butterflies on squares of forest floor, except that the numbers kept rising. Measuring and counting are the tasks of science. Not guessing, and not wishing. The potential answers are infinite, and no preference among them is allowed: there will be no just-because, or unjust-because.

She understood. But still, it’s why not there. Not why here.

His third theory concerned devastation in the “spring range,” which is what he called a funnel-shaped area on the map, fundamentally Texas. Monarchs that eke out winter in the Mexican Neovolcanics awaken from their torpor to an unruly sexual madness. Males are hormonally driven to assail anything—a quaking leaf, other males—eventually enclosing within their embrace the host of congregated females, and afterward they are spent, fulfilled. Their mates flee with gorged ovaries toward a nonnegotiable deadline, the deposit of perfectly timed egg on the first unfurling leaf of a Texan milkweed, moving inside the consecrated clock of a ticking earth. This, he said, tapping the map on the glass screen of his computer, is all our eggs in one basket. The spring range. Steady through the ages, now its rhythms abruptly faltered, ransacked by drought and unquenchable fires. By fire ants marching north, consuming 100 percent of the monarch caterpillars they chance to meet. Suppose a genetic mishap sent a handful of fall migrants just to the northern edge of this realm of fire ant and firestorm. This far south in the autumn, and no farther, he said, drawing one long finger from the Texas panhandle to the Carolinas, a scattering of migrants overwintering here, where they would not be forced to come back across that desert. A Bible Belt latitude, favorable for its mildness, but a mountainous place high enough to cool an insect pulse to dormancy for the winter wait. Suppose there is only one such place. And that they had been coming here for years, in small numbers, cloaked by this forest, mostly unsurviving. Until precipitous natural selection against the Mexican migrants destroyed most of the population, shedding favor on these pioneers. Their gene, suddenly, the inheritance of a species.

The explanation was far from complete. A population was only as valid as its habitat. Winter nectar sources remained problematic, when repeated warm spells broke their dormancy, and so did the spring milkweed emergence. There are always more questions. Science as a process is never complete. It is not a foot race, with a finish line. He warned her about this, as a standard point of contention. People will always be waiting at a particular finish line: journalists with their cameras, impatient crowds eager to call the race, astounded to see the scientists approach, pass the mark, and keep running. It’s a common misunderstanding, he said. They conclude there was no race. As long as we won’t commit to knowing everything, the presumption is we know nothing.

And even while he warned her of these caveats, Dellarobia felt a settling down of her lifelong plague of impatience. He did not claim that God moves in mysterious ways. Instead he seemed to believe, as she did, though they never could have discussed it, that everything else is in motion while God does not move at all. God sits still, perfectly at rest, the silver dollar at the bottom of the well, the question.

On the way to the study site a pine cone war broke out among the kindergartners. The boys took it more to heart, predictably, although the instigator was a big, rough girl in a decrepit parka whose fake fur hood was matted like old shag carpet. She shimmied up a pine trunk and fired away, ignoring Miss Rose’s escalating threats about getting sent right straight home with what she called a pink note. Dellarobia had a whole new impression of Miss Rose and what she was up against, in general. This girl, Comorah, exemplified a category of children whose parents, if applicable, would not be impressed by a pink note. She came down when she was good and ready, with gummy black stains all over her clothes and hands that Dellarobia knew would not give in to soap and water. She’d had her own tangles with pine sap up here. Preston seemed both thunderstruck and distressed by Comorah, needing to tell her the munitions were cones, not “pine combs.” Undaunted by her indifference, he sidled up to her with this information again and again, just the way Roy would carry around his old tooth-punctured Frisbee to drop at your feet while you did yard work, all afternoon if need be.

Dellarobia held herself a little apart from her son, curious to observe this ecosystem he regularly navigated without her. She saw that he was reserved but not shy, that other kids ran to him with their special finds such as beetles, and that he stuck close to the willowy, confident Josefina. She was his partner or protector—Dellarobia couldn’t quite read it. For all she knew, they might be the only two free-lunch kids, though she doubted it. Some of these youngsters appeared to be well-heeled—she’d actually spotted a cell phone—and others, like Comorah, were turned out in gear that had seen many generations of hand-me-down. But Josefina and Preston seemed to represent some subtle divide of maturity, like the automatic segregation of seniors from sophomores at a dance. Dellarobia recalled their spontaneous hug, that first day Josefina’s family showed up on their porch. In retrospect she saw in it some element of rescue.