When I was fifteen, I lived in North Berwick, Scotland for four months. Located on a promontory on the south shore of the Firth of Forth—a wide, tapering bight on Scotland’s east coast that narrows near the capital of Edinburgh, then bends inland—North Berwick was the site of two striking natural features: the Bass Rock, a steep-sided island that dominated the bay in front of the beach; and the North Berwick Law, a mound-shaped hill of volcanic origin, from the top of which Arthur’s Seat, the extinct volcano that looms over Edinburgh, thirty kilometres away, was visible. The writer Robert Louis Stevenson, having spent several summers in North Berwick, devoted a chapter of his novel Catriona—the sequel to Kidnapped—to the Bass Rock. The central character is imprisoned on this island of “crags painted with seabirds’ droppings like a morning frost, the sloping top of it green with grass.” The reaction provoked in me by the North Berwick high school that I attended for almost four months resembled that of Stevenson’s protagonist: I felt imprisoned. I escaped from this dour institution through a regime of solitary walks. The walks that made the deepest impression on me took me along the nearby beaches. During the chilly autumn, North Berwick’s beaches were empty of visitors. Yet they were not vacant: two macabre features drew me back to these otherwise barren stretches of sand. First, they were bolstered with concrete bunkers that had been placed there during the World War II: defences against a German invasion that had failed to appear. Second, they were littered with dead rabbits.

The rabbits had died of a viral disease called myxomatosis. Their corpses lay on their sides, the up-facing eye swollen shut into a black nub that resembled a knot in a sheet of plywood. The cadavers were everywhere. One day I made a two-hour hike west from North Berwick along the beaches to the village of Gullane. My attempt to keep track of the dead rabbits I saw on the way faltered: there were too many to count. The rabbit mortality rate from myxomatosis in Scotland, I later learned, was over ninety-nine per cent. This highly communicable disease caused the rabbits’ faces to swell until they became blind. If they didn’t starve, or weren’t run over by a car, they would die of a secondary infection within two weeks. On asking about the illness, I was told that it had been used to control rabbits in Australia, where the rabbit population had grown into the hundreds of millions. As a result of its success in reducing the impact of rabbits on agriculture, the disease was introduced into Scotland in 1954 at the request of farmers who regarded rabbits as pests. The proliferation of rabbit corpses shocked me, yet the steely Scots were unmoved by the slaughter on their sands. Scottish farmers, whose hilly, rocky land made agriculture precarious at the best of times, had argued forcefully that without a rabbit cull their farms would not survive. In the gentler pastureland of animal-loving England, by contrast, the introduction of myxomatosis became a source of controversy, pitting farmers against animal-rights activists.

The image of beaches strewn with dead rabbits stayed in my mind for decades. Years later, in Guelph, Ontario, I was at a party with a group of veterinary researchers. Their conversation slipped effortlessly from animal viruses to the inevitability of a human virus sweeping the land. “I would expect it to kill three per cent of the human population,” one researcher said in a tombstone voice. The dead rabbits on the beach returned to my mind. They, and the party conversation, returned once more when the COVID-19 pandemic began. By that time I had made a number of return visits to Scotland. During a visit to North Berwick with friends, I was astonished to find that the beaches were pristine. What had happened to myxomatosis? I read that it had grown less virulent and become no more troublesome to the rabbits than a slight fever was to humans. I hoped that the coronavirus, too, would evolve toward milder, non-lethal strains.

Alexander Van Tulleken wrote recently in the Times Literary Supplement, “The standard story is that myxomatosis became milder, allowing the rabbits to once again flourish.” Yet this, it turns out, is not what happened. Recent DNA research, based on samples taken from rabbits over the last one hundred fifty years, has refuted the theory that myxomatosis grew less fatal. The research demonstrates that in response to myxomatosis European rabbits began an accelerated process of natural selection. Those few rabbits who had genetic mutations that made them immune to the disease became the breeding stock for future rabbits. The same genetic evolution in rabbits’ immune systems occurred simultaneously in Australia, Great Britain and France. The disease was defeated by evolution—until it came back. In Australia, in 217, myxomatosis returned in a more deadly form, killing off even genetically fortified rabbits. In both Australia and Europe, rabbit populations are once again in decline.

I returned to Scotland in October 219, my last trip prior to being grounded by the pandemic. In Edinburgh, I climbed to the top of Arthur’s Seat. From the summit I looked far down the coast until I found North Berwick. I thought about the time I had spent there; foolishly, I didn’t think about the dead rabbits. Life’s continuities felt as immutable then as the stones of the Bass Rock, the North Berwick Law, or Arthur’s Seat. By early 22 we were reminded that, contrary to what these indomitable geographical formations might lead us to believe, nature is not fixed and eternal; it is restless and ever-changing. Like the rabbits, we inhabit a society that is founded on an assumption of a healthy immune system. Unlike that of the rabbits, our social organization enables us to take measures to mitigate the effects of a virus: medicines, social distancing, mask-wearing. Our biology, though, is not superior to that of other mammals. COVID-19 may not be as devastating to human demographics as myxomatosis was to that of rabbits, yet, like them, we are caught in the eternal, see-sawing struggle between the evolution of our genetic defences and the equally relentless evolution of disease.