Science of Discworld III

voyage, considers the findings and writes that book. All the otheralternative 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 alanguage 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 voyageand, 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 +++
    1 Phase space, in a given context, is the space of everything that might have happened, not just what did. See The Science of Discworld .

SIX
BORROWED TIME
    T HE 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.