Most human groups pretend to practise monogamy, but like swans and gibbons and other creatures that we thought were faithful for life, there are plenty of adulterous relationships and children 'whose legal and biological parentage differ'. In English society, about one child in seven is in that position, and the proportion doesn't differ much between the slums of Liverpool and the stockbroker belt of Maidenhead.45

The most restrained people that we know, in this regard, are the Amish of Eastern Pennsylvania and other parts of the United States, for whom the figure is a mere one in twenty. So, to err on the safe side, let's assume that all of the Mrs Cohens, from the present day back 100 generations to the sons of Aaron, were as well behaved as the Amish. Then the proportion of Cohen males with Aaron's Y-chromo-some should be 0.95100, which is considerably less than one in a hundred. So how can it be as high as one in two?

There is a possible explanation, consonant with what we know about human sexuality, or at least with what John Symons, an expert on human sexual practices, says in his books. According to many surveys of sexual behaviour, going right back to Alfred Kinsey around 1950, women practise adultery with men of both higher and lower status. The two situations frequently occur in different social contexts, with women 'doing favours for' higher status men (think Clinton), but going down-market for 'a bit of rough'. Overwhelmingly often, however, when a baby results the father is of higher status than the woman's husband or regular partner.

This implies that if Mrs Cohen, living in a ghetto or any other predominantly Jewish society, wants to go up-market, her only choices are other Cohens. So the maintenance of the Aaron Y- chromosome may have been assured by sexual snobbery rather than amazing fidelity, and that's a much more likely story.

The breeze shook the willows. And, in the centre of the willows, the lightning-struck tree spoke, in a very faint voice. The Ugs had seen lightning strike the same tree three times. It was the highest point in the area, thanks to the shell mounds.

Even for creatures so preternaturally against thinking any kind of new thoughts, this made an impression. In some way, they'd felt, the tree had importance. It was an important thing. The place of the tree was an important place, where the sky touched the ground.

It wasn't much of an opening, it was a story without a plot and it barely amounted to a belief, but Hex had to make do with what could be found.

Now the wizards were considering the future, or futures.

'Nothing changes?' said the Dean.

'No, sir,' said Ponder, for the fourth time. 'And, yes, this is indeed the same time as the city we were in. But things are different.'

'The city was almost modern!'

'Yes, it had heads on spikes,' said Rincewind.

'It was a bit backward, admittedly,' said Ridcully. 'And the beer was foul. But it had possibilities.'

'But I don't understand! We stopped the elves,' said the Dean.

'And now we've got thousands and thousands of years of this,' said Ponder. 'That's what Hex says. These people won't even have learned how to make fire before the big rock hits. Rincewind is right. They're not exactly stupid, they just don't ... progress. Remember the crab civilisation we found?'

'But they had wars and took prisoners and slaves!' said the Lecturer in Recent Runes.

'Yes. Progress,' said Ponder.

'Heads on spikes,' said Rincewind.

'Do stop going on about that, it was only two heads,' snapped Ponder.

'Perhaps we did something else that changed history,' said the Chair of Indefinite Studies. 'Maybe we trod on the wrong insect or something? Only a thought,' he added, when they glared at him.

'We just saw off the elves, that's all we did,' said Ridcully. 'Elves cause exactly the sort of things we've seen here. Superstition and—'

'The Ugs aren't superstitious,' said Rincewind.

'They didn't like it when I struck that match!'

'They didn't start worshipping you, either. They just don't like things that happen too quickly. But I told you, they don't draw pictures, they don't use body paint, they don't make things ... I asked Ug about the sky and the moon, and as far as I can tell they don't think about them. They're just things in the sky.'

'Oh, come now,' said Ridcully, 'everyone tells stories about the moon.'

'They don't. They don't have any stories at all,' said Rincewind.

There was silence as this sank in.

'Oh dear,' said Ponder.

'No narrativium,' said the Dean. 'Remember? That's what this universe lacks. We never found a trace of it. Nothing knows what it's supposed to be.'

'There must be something like it, surely?' said Ridcully. 'The place looks normal, after all. Seeds grow up into trees and grass, by the look of it. Clouds know they have to stay up in the sky.'

'If you remember, sir,' said Ponder, using the tone that meant I know you've forgotten, sir, 'we found that this universe has things that work instead of narrativium.'

'Then why are these people just sittin' about?'

'Because that's all they have to do!' said Rincewind. There doesn't seem to be much around that can hurt them, there's enough food, the sun is shining ... it's all gravy! They're like ... lions. Lions don't need stories. Eat when you're hungry, sleep when you're tired. That's all they need to know.

What else do they need?'

'But it must get cold in the winter, surely?'

'So? It gets warmer in the spring! It's just like the moon and the stars! Things happen!'

'And they've been like this for hundreds of thousands of years,' said Ponder.

There was some more silence.

'Remember those stupid big lizards?' said the Dean. 'They lasted for more than a hundred million years, I remember. I suppose they were quite successful, in their way.'

'Successful?' said Ridcully.

'I mean they lasted a long time.'

'Really? And did they build a single university?'

'Well, no—'

'Did they draw a single picture? Invent writing? Offer even small classes of elementary tuition?'

'Not that I know—'

'And they all got killed off by a yet another big rock,' said Ridcully. 'They really did not know what hit them. Bein' around for millions of years is not an achievement. Even lumps of stone can manage that.'

The circle of wizards was sunk in gloom.

'And Dee's people were doin' quite well,' muttered Ridcully. 'Terrible beer, of course.'

'I suppose ...' Rincewind began.

'Yes?' said the Archchancellor.

'Well ... how about if we went back and stopped us from stopping the elves? And least we'd be back among people more interesting than cows.'

'Could we do that?' said Ridcully to Ponder.

'I suppose so,' said Ponder. 'Technically, if we stop ourselves, then nothing will change, I assume. All this won't have happened ... I think. That is to say, it will have happened, because we'll remember it, but then it won't have happened.'

'Fair enough,' said Ridcully. Wizards do not have a lot of patience with temporal paradoxes.

'Can we stop ourselves?' said the Dean. 'I mean, how do we do it?'

'We'll just explain the situation to us,' said Ridcully. 'We're reasonable men.'

'Hah!' said Ponder, and then looked up. 'Oh, sorry, Archchancellor. I must have been thinking about something else. Do go on.'

'Ahem. If I was just about to fight elves, and someone who looked very much like me came up and told me not to, I'd assume it was an elvish trick,' said the Lecturer in Recent Runes. 'They can make you think they look like someone else, you know.'

'I'd know me if I saw me!' said the Dean.

'Look, it's easy,' said Rincewind. 'Trust me. Just tell yourself something about yourself that no one else could possibly know.'