`No, we sorted it all out,' said Ridcully. `Remember? All the people did get away before the planet froze.'

`Yes, Archchancellor,' said Stibbons. `And, then again, no.'

`If I ask you to explain that, would you tell me in words I can understand?' said Ridcully.

Ponder stared at the wall for a moment. His lips moved as he tried out sentences. `Yes,' he said at last. `We changed the history of the world, sending it towards a future where the people could escape before it froze. It appears that something has happened to change it back since then.'

`Again? Elves did it last time!' [2]

'I doubt if they've tried again, sir.'

`But we know the people left before the ice,' said the Lecturer in

[1] See The Science of Discworld (Ebury Press, 1999, revd 2000).

[2] See The Science of Discworld II (Ebury Press, Recent Runes. He looked from face to face, and added uncertainly, `Don't we?'

`We thought we knew before,' said the Dean, gloomily.

`In a way, sir,' said Ponder. `But the Roundworld universe is somewhat ... soft and mutable. Even though we can see a future happen, the past can change so that from the point of view of Roundworlders it doesn't. It's like ... taking out the last page of a book and putting a new one in. You can still read the old page, but from the point of view of the characters, the ending has changed, or ... possibly not.'

Ridcully slapped him on the back. `Well done, Mr Stibbons! You didn't mention quantum even once!' he said.

`Nevertheless, I suspect it may be involved,' sighed Ponder.

PALEY'S WATCH

THE SCENE: A RADIO CHAT-SHOW in the Bible Belt of the United States, a few years ago. The host is running a phone-in about evolution, a concept that is anathema to every God-fearing southern fundamentalist. The conversation runs something like this: HOST: So, Jerry, what do you think about evolution? Should we take any notice of Darwin's theories?