+++ The Chair of Indefinite Studies is correct +++ The wizards clustered round.
`Right about what?' said Ponder.
+++ Charles Darwin of Theology of Species was for much of his life a Rector in the Church of England, a sub-set of the British nation +++ the computer scrawled. +++ The chief function of the priests of that religion at the time was to further the arts of archaeology, local history, lepidoptery, botany, palaeontology, geology and the making of fireworks +++
`Priests did that?' said the Dean. `What about the praying and so on?'
+++ Some of them did that too, yes, although it was considered to be showing off. The God of the English did not require much in the way of sacrifice, only that people acted decently and kept the noise down. Being a priest in that church was a natural job for a young man of good breeding and education but no very specific talent. In the rural areas they had much free time. My calculations suggest that Theology of Species was the book that he was destined to write. In all the histories of third-level phase space, there is only one in which he writes The Origin of Species +++
`Why is that?' said Ponder.
+++ The explanation is complex +++
`Well, out with it,' said Ridcully. `We're all sensible men here.'
Another piece of paper slid off Hex's tray. It read: +++ Yes. That is the problem. You understand that every possibility of choice gives birth to a new universe in which that choice is manifest? +++
`This is the Trousers of Time again, isn't it?' said Ridcully.
+++ Yes. Except that every leg of the Trousers of Time branches into many other legs, and so do those legs and every following leg, until everywhere is full of legs, which often pass through one another or join up again +++
`I think I'm losin' track,' said Ridcully.
+++ Yes. Language is not good at this. Even mathematics gets lost.
But a little story might work. I will tell you the story. It will be not completely inaccurate +++
`Go ahead,' said Ridcully.
+++ Imagine an unimaginably large number +++
`Right. No problem there,' said Ridcully, after the wizards had consulted among themselves.
+++ Very well +++ Hex wrote. +++ From the moment that the Roundworld universe was made, it began to split into almost identical copies of itself, billions of times a second. That unimaginably large number represents all possible Roundworld universes that there are +++
`Do all these universes really exist?' said the Dean.
+++ Impossible to prove. Assume that they do. In all those universes there are hardly any in which a man called Charles Darwin exists, takes a momentous ocean voyage, and writes a hugely influential book about the evolution of life on the planet. Nevertheless, that number is still unimaginably large +++
`But imagined by a smaller imagination?' said Ridcully. `I mean, is it half as many as the other unimaginable number?'
+++ No. It is unimaginably large. But compared to the first number, it is unimaginably small +++
The wizards debated this in whispers.
`Very well,' said Ridcully, at last. `Keep goin' and we'll kind of join in when we can.'
+++ Even so, it is not so unimaginable as the number of universes in which the book was The Origin of Species. That number is quite strange and can only be imagined at all in very unusual circumstances +++
`It's unimaginably larger?' said Ridcully.
+++ just unimaginably unique. The number one. Gentlemen. All by itself. One is one and all alone. One. Yes. In third-level phase space there is only one history where he gets on the boat, completes the voyage, considers the findings and writes that book. All the other alternative Darwins either did not exist, did not stay on the boat, did not survive the journey, did not write any book at all or wrote, in a large number of cases, Theology of Species and entered the Church +++
`Boat?' said Ponder. `What boat? What've boats got to do with it?'
+++ I explained, in the successful timeline which led to humanity leaving the planet, Mr Darwin makes a significant voyage. It is one of nineteen pivotal events in the history of the species. It is almost as important as Joshua Goddelson leaving his house by the back door in 1734 +++
`Who was he?' said Ponder. `I don't recall the name.'
+++ A shoemaker living in Hamburg, Germany +++ wrote Hex.
+++ Had he left his house by the front door that day, commercial nuclear fusion would not have been perfected 283 years later +++ `That was important, was it?' said Ridcully.
+++ Vastly. Major technomancy +++
`Did it need much in the way of shoes, then?' said Ridcully, mystified.
+++ No. But the chain of causality, though complex, is clear +++ `How hard is it to get on this boat?' said the Dean. +++ In the case of Charles Darwin, very hard +++ `Where did it go?'
+++ It sailed from England to England. But there were crucial stops along the way. Even in those histories where he did embark on the boat, he did not complete the voyage and complete The Origin of Species in every case but one +++
`Just one version of history, you say,' said Ponder Stibbons. `Do you know why?'
+++ Yes. It is the one where you intervene +++ `But we haven't intervened,' said Ridcully.
+++ In a primitive subjective sense this is the case. However, you are going to will have already soon +++ Hex wrote.
`What? And I am not a primitive subject, Mr Hex!'
+++ I am sorry. It is hard to convey five-dimensional ideas in a language evolved to scream defiance at the monkeys in the next tree +++
The wizards looked at one another.
`Getting a man on a ship can't be hard, surely?' said the Dean.
`Is it dangerous in Darwin's time?' said Rincewind.
+++ Inevitably. The centre of the Globe is an inferno, humanity is protected from being fried alive by nothing more than a skin of air and magnetic forces, and the chance of an asteroid strike is ever present +++
`I think Rincewind was referring to more immediate concerns,' said Ridcully.
+++ Understood. The major city you must visit has many squalid areas and open sewers. The river bisecting it is noxious. Your destination could be considered a high-crime drainage ditch in a dangerous and dirty world +++
`Pretty much like here, you mean?'
+++ The similarity is noticeable, yes +++
The writing arms stopped moving. Bits of Hex rattled and shook. The ants ceased their purposeful scurrying and began to mill about aimlessly in their glass tubing. Hex appeared to have something on his mind.
Then one writing arm dipped its pen into the ink and wrote, slowly:
+++ There is an additional problem. It is not clear to me why Darwin did not write Origin somewhere in the multiple universes without your forthcoming assistance +++
`We haven't decided that we will-' Ridcully began.
+++ But you are going to have done +++
`Well, probably-'
+++ Across the entire phase space of this world Charles Darwin did many things. He became an expert watchmaker. He ran a pottery factory. In many worlds he was a country priest. In others, he was a geologist. In yet others, he did make the important voyage and, as a result wrote Theology of Species. In some he began to write The Origin of Species only to give up. Only in one timeline was Origin published. This should not be possible. I detect ... +++
+++ I detect ... +++
The wizards waited politely.
`Yes?' said Ponder.
The single pen moved across the paper.
+++ MALIGNITY +++
BORROWED TIME
THE EVER-BRANCHING LEGS OF the Trousers of Time are a metaphor (unless you are a quantum physicist, in which case they represent a certain mathematical view of reality) for the many paths that history might have taken if events had been slightly different. Later, we'll think about all those legs, but for now, we restrict attention to one trouser. One timeline. What exactly is time?
We know what it is on Discworld. `Time', states The New Discworld Companion, `is one of the Discworld's most secretive anthropomorphic personifications. It is hazarded that time is female (she waits for no man) but she has never been seen in the mundane worlds, having always gone somewhere else just a moment before. In her chronophonic castle, made up of endless glass rooms, she does at, er, times, materialise into a tall woman with dark hair, wearing a long red-and-black dress.'