“I’m assuming that you’re an exception,” Miller said, perhaps intending to pay Lisa a compliment. “I suppose that as a policeman, you’ll at least be uncommonly dutiful, if not overly willing to challenge authority.”

That seemed to Lisa to be marginally more offensive than the remark for which he’d issued his offhand non-apology. “I’ll come along to your lectures,” she assured him. As a postgraduate, she was obliged to attend a quota of second- and third-year courses in order to make up ground that had fallen outside her own undergraduate specialties. Those that her supervisor taught had to be on her list, if only for diplomatic reasons.

“If you cared to set an example and ask some searching questions in the seminars, I’d be grateful,” he said. “It might save me from having to go quite so far over the top in the hope of eliciting a response. Feel free to be as aggressive as you like. It’s a postgrad’s responsibility to play the Judas goat, after all.”

It wasn’t, but Lisa didn’t know whether he was joking or being provocative, so she didn’t laugh and didn’t rise to the bait. “Twenty-eight years is a long time to run an experiment,” she said instead. “And the running costs can’t be trivial. Even if the food’s cheap, equipment maintenance and waste disposal must consume quite a budget.”

“Animal population dynamics is a difficult field in which to do experiments,” Miller agreed, seeming to lose half an inch of height as he bowed to the force of her fascination with the four cities and slumped into patient resignation. “Even organisms that can get through a generation in thirty days or so have to be observed for years if you’re to get any worthwhile data about the way their populations respond to changes in circumstance. Anything with a yearly life cycle is out of the question for lab work, although there are teams all over the world that send people out every spring to collect data on wild populations of all kinds of species, and have been doing so for twenty years and more. Most of what we know about mammalian population dynamics in nature is based on the records kept by hunters and fur trappers, and the data is prejudiced by the fact that the killing of their members by humans is by far the most important variable impacting on the populations. Lab-based observations are virtually restricted to rats, rabbits, and mice—and if you think the running costs of thissetup are an unacceptable burden, imagine what it would cost to keep a similar number of rats or rabbits.”

“So why keep them in such large numbers?” Lisa asked.

“Because you can’t do experiments on the effects of overpopulation with small numbers,” Miller observed, without loading the comment with more scorn that was actually necessary.

“I see,” Lisa said, wishing that she’d seen it a little earlier.

“The American experiments set up in advance of this one were all terminated after a couple of years,” Miller told her, perhaps by way of repentance. “Even when they began to produce interesting results, the practical and political difficulties of keeping them going were insuperable. The whole point of this one was to build something sustainable over the long term, in the hope that it would clarify some of the puzzles Calhoun and McKendrick had to leave unsolved.”

“And has it?” Lisa asked, determined not to be forced into a humiliating confession that she had no idea of who Calhoun and McKendrick were. Fortunately, Miller knew perfectly well that she was a biochemical geneticist whose background in population biology was likely to be exceedingly sketchy, and he didn’t try to make her look foolish.

“Calhoun was one of the first people to investigate what would happen to a population limited only by space,” he said. “His experiments gained a certain anecdotal notoriety in the sixties, when even I was but a child, but that overestimated both their scope and their importance. To simplify brutally, he put a few rats into a fairly spacious but limited complex, gave them as much food and water as they needed, and did what he could to keep pollution within reasonable limits. The population did pretty much what he expected it to do: rose exponentially to a peak, then collapsed again. When the crowding became unbearable, the rats’ social system—such as it was—completely disintegrated. They fought continually and destructively, began to eat their own young, and showed every known symptom of environmental stress: ulceration, heart disease, hair loss … you name it, the observers saw it. It was never really intended as an experiment in the scientific sense, of course. If I remember correctly, Calhoun was working for the National Institutes of Health. It was a demonstration—a parable to supplement the natural parables of the lemming and the snowshoe hare.”

“I read about the snowshoe hare,” Lisa put in helpfully. “They’re responsible for the lynx cycle in Canada—and the lemmings are famous. There used to be a cinema ad that showed them pouring over a cliff, but I can’t remember what it was for.”

“It was an antismoking ad,” Miller reminded her. “People misunderstood the lemmings for a hundred years, just as they misunderstood the lynx cycle. The myth was that the lemmings were committing suicide, just like smokers who wouldn’t stop. There were all kinds of crackpot theories. One suggested that some atavistic instinct was forcing them to follow an ancient migration route to land that had been inundated by the sea. In much the same spirit, people tried to correlate the lynx cycle with the sunspot cycle, as if that would somehow provide an explanation. Even within the scientific community, there was a well-established myth of predator-prey cycles suggesting that the number of lynx pelts recovered by the Hudson Bay Company’s trappers varied cyclically because of the feedback effects of the trappers’ own activity, or because every time the lynx numbers increased, they sent the populations of their prey into steep decline. All nonsense, of course. The lynx population and the snowshoe hare population went up and down together—the population crashes that caused the hares to decline were entirely independent of the intensity of predation, but every time the hare population crashed, the lynx population crashed too.”

“But they can’t have been in the same situation as the experimental rats,” Lisa pointed out, glad for an opportunity to show that she was on the ball. “They had unlimited space.”

“That’s the curious thing,” Miller agreed. “You’d think so, wouldn’t you? The snowshoe hares had all of Canada, the lemmings all of Siberia and Scandinavia. You’d think that the limiting factor controlling their population size would be the availability of food—but it wasn’t. When the cases were actually investigated, it immediately became obvious that the peak populations could endure the winters, despite the scarcity of food. The populations didn’t collapse until the spring, when food was becoming much more abundant.”

He paused, inviting Lisa to catch on. She had to hesitate for six or seven seconds, but then she figured it out. “The mating season,” she said.

“Exactly,” Miller conceded, favoring her with a smile of pure but not particularly abundant generosity. “They could tolerate the density of population when their attention was fixed exclusively on the business of survival, but when the breeding season came around, the males became fiercely territorial. It wasn’t the absolute limitation of space that was important, but the perceived limitation. The competition for territory became so intense so suddenly that the animals couldn’t handle the consequent physiological stress. Their systems became permanently adremdinized. Snowshoe hares are relatively meek, so they just drop dead in droves, mostly from heart attacks. Lemmings aren’t—when they get into fighting mode, they simply can’t stop. The lemmings that died in the last couple of so-called lemming years were mostly killed on the roads, and human activity has had such a profound effect on their numbers that there’ll probably never be another, but the lemmings that had attracted the most attention back in the famous lemming years were the ones that carried their territorial squabbles to the limits of the available territory. They fought on cliff tops for every last meter, sometimes to the death. Suicide wasn’t a factor, although sheer frustration was.”