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In other words, the choosier the females, the brighter and more elaborate will be male ornaments, which is exactly what you find in nature. Sage grouse are elaborately ornamented and only a few males get chosen; terns are unornamented and most males win mates.

The models also showed that the process could run away from the line of equilibrium with Fisher’s ‘ever-increasing speed’, but only if females vary in their (heritable) preference and if the male’s ornament is not much of an encumbrance to him. These are fairly unlikely conditions, except early in the process, when a new preference and a new trait have just emerged.

But the mathematicians discovered more. It mattered greatly if the process of choosing was costly to females. If, in deciding which male to mate with, a female wastes time that could be more profitably spent incubating eggs, or exposes herself to the risk of being caught by an eagle, then the line no longer stands. For as soon as the species reaches it, and the pros of long tails are balanced by their cons, there is no net advantage left to being choosy, so the costs of choice will drive females into indifference. This looked to be fatal to the whole Fisher idea, and there was brief interest in another version of the idea (one which is known as the sexy-son theory), which suggested that sexy husbands made bad fathers – a clear cost to being a choosy female.27

Luckily, there came another mathematical insight to the rescue. This was that the genes that cause the elaborate ornament or long tail to appear are subject to random mutation. The more elaborate the ornament, the more likely it is that a random mutation will make the ornament less elaborate, not more. Why? A mutation is a spanner thrown into the genetic works. Throwing a spanner into a simple device, like a bucket, may not much alter its function. Throwing a spanner into a more complicated device, like a bicycle, will almost certainly make it a less good bicycle. Thus any change in a gene will tend to make the ornament smaller, less symmetrical or less colourful. This ‘mutational bias’ is sufficient, according to the mathematicians, to make it worth the female’s while to choose an ornamented male, because it means that any defect in the ornament might otherwise be inherited by the sons: by choosing the most elaborate ornament she is choosing the male with fewest mutations. The mutational bias is also, perhaps, sufficient to defeat the central conundrum that we set the theories earlier, the fact that if the best genetic cream of the cream is taken off each generation, there will soon be no separability left in the cream. Mutational bias keeps turning some of the cream back into milk.28

The result, then, of a decade of mathematical games has been to prove that the Fisherians are not wrong. Arbitrary ornaments can grow elaborate for no other reason than that females discriminate between males and end up following arbitrary fashions; and the more they discriminate, the more elaborate the ornaments become. What Fisher said in 1930 was right. But it left a lot of naturalists unconvinced, for two reasons. First, Fisher assumed part of what he set out to prove: that females are already choosy is crucial to the theory. Fisher himself had an answer for this, which was that, initially, females chose long-tailed males for more utilitarian reasons – for example, that it indicated his superior size or vigour. This is not a foolish idea; after all, even the most monogamous species, in which every male wins a female, such as terns, are choosy. But it is an idea borrowed from the enemy camp. And the Good-geners can reply: ‘If you admit that our idea works initially, why rule it out later on?’

The second reason is more mundane. Proving that Fisher’s runaway selection could happen and the ornaments get bigger with ‘ever-increasing speed’ does not prove that it does happen. Computers are not the real world. Nothing but an experiment could satisfy the naturalists: an experiment that demonstrated that the sexiness of sons drove the