[1] Now considered to be thirteen, plus a fourteenth on the Cocos Islands. (Look, people write and complain if we don't point this kind of thing out.)

the development of living creatures, which Paley had denied. The laws of the universe were fixed for all eternity. They had to be, otherwise God's creation was flawed. Paley's analogies were used against him. What kind of artificer made such bad machinery that He had to keep tinkering with it all the time to keep it working?

Science and theology were ripping asunder. The political corruption of the Church was becoming undeniable; now its intellectual claims were also coming under fire. And some radical thinkers, often medics who had studied comparative anatomy and noticed remarkable similarities between the bones of entirely different animals, were engaged in speculation that changed the view of creation itself. According to the Bible, God had created each type of animal as a one-off item - whales and winged fowl on the fifth day, cattle and creeping things and humans on the sixth. But these medical types were starting to think that species could change, `transmute'. Species were not fixed for all time. They realised that there was a rather big gap between, say, a banana and a fish. You couldn't cross that gap in one step. But given enough time, and enough steps ...

Darwin slowly became caught up in the flow. His Red Notebook, where he recorded anything that he saw or that came to mind, began to hint at the `mutability of species'. The hints were incomplete and ill-assorted. Deformed babies resembled new species. The beaks of Galapagos finches were of different shapes and sizes. Rheas were a puzzle, though: two distinct species of the giant birds had overlapping ranges in Patagonia. Why didn't they merge into a single species?

By July, he had secretly started a new notebook, his B Notebook.

It was on the transmutation of species.

By 1839 Darwin was building up a complete picture, and he wrote a 35-page summary of his thinking. A crucial influence was Thomas Malthus, whose 1826 Essay on the Principle of Population pointed out that the unchecked growth of organisms is exponential (or 'geometric', in the old-fashioned phrase of the time), whereas that of resources is linear (`arithmetic'). Exponential growth occurs when each step multiplies the size by some fixed amount, for example 1, 2, 4, 8, 16, 32, where each number is twice the previous one. Linear growth adds some fixed amount at each step, for instance 2, 4, 6, 8, 10, where each number exceeds the previous one by 2. However small the multiplier is in exponential growth, provided it is bigger than 1, and however large the number added in linear growth may be, it turns out that in the long run exponential growth always beats linear. Though it does take some time if the multiplier is close to 1 and the number being added is huge.

Darwin had taken on board Malthus's argument, and he had realised that in practice what keeps populations down is competition for resources, such as food and a place to live. This competition, he wrote, leads to `natural selection', in which those creatures that are victorious in the `war of nature' are the ones that produce the next generation. Individual creatures within a species are not exactly identical; those differences make it possible for the force of natural selection to produce slow, gradual changes. How far might such changes go? In Darwin's view, very far indeed. Far enough to lead to entirely new species, given enough time. And thanks to geology, scientists now knew that the Earth was very, very old.

Darwin, following family tradition, was a Unitarian. This particular branch of Christianity has been aptly described as `people who believe in at most one God'. As a sound Unitarian, he believed that the Deity must work on the grandest of scales. So he finished his summary with a powerful appeal to the Unitarian view of the Deity: It is derogatory that the Creator of countless systems of worlds should have created each of the myriads of creeping parasites and slimy worms which have swarmed each day of life on land and water on this one globe. We cease being astonished, however much we may deplore, that a group of animals should have been directly created to lay their eggs in bowels and flesh of others - that some organisms should delight in cruelty ... From death, famine, rapine, and the concealed war of nature we can see that the highest good, which we can conceive, the creation of the higher animals has directly come.

God surely has better taste than to create nasty parasites directly. They exist only because they are a necessary step along the path that leads to cats, dogs, and us.

Darwin had his hypothesis.

Now he began to agonise about how to bring it to the waiting world.

WIZARDS ON THE WARPATH

IN THE GLOOM OF THE High Energy Magic building, Hex wrote. Every minute another page slid off the writing table.

"`Boat sunk by collision with Spanish fishing vessel",' Ponder Stibbons read out, a tremor in his voice. "`Boat shipwrecked on uncharted reef near Madeira. Boat found drifting minus all crew, with the table laid for a meal. Boat catches fire, all lost. Boat struck by meteorite. Darwin accidentally shot by ship's surgeon and naturalist during a collecting expedition on the island of St Jago. Darwin accidentally shot by ship's captain. Darwin accidentally shot by himself. Darwin loses place on boat. Darwin leaves boat because of seasickness. Darwin loses notebooks. Darwin stung to death by wasps! Darwin bangs head on underside of table and loses memory ..."' He put down the paper. `And these are the more sensible causes.'

`The stone dropping out of the sky was sensible?' said Ridcully.

`Compared to the attack of the giant squid, Archchancellor, I would say so,' said Ponder. `And the enormous waterspout. And the shipwreck off the coast of Norway.'

`Well, ships do get wrecked,' said the Dean.

`Yes, sir. But the country known as Norway is in the wrong direction. The Beagle would only get there by sailing backwards. Hex is right, sir. This is insane. The moment that we decided to change one simple little history, the whole of the universe is trying to stop the voyage happening! And mathematically speaking, this is illegal!'

Ponder thumped the table, his face red. The senior wizards shied. This was as unnerving as hearing a sheep roar.

'My word!' said Ridcully. `Is it?

'Yes! There must be room in phase space for the possibility that Origin gets written! It's not against the physical laws of this universe!'

`That a young inexperienced man takes a voyage around this world and has an insight that changes mankind's view of itself?' said the Dean. `You must admit it looks a bit unlikely - sorry, sorry, sorry!' He backed away as Ponder advanced.

`One of the biggest religions on Roundworld was founded by a carpenter's son!' Ponder snarled. `For years, the most powerful person on the planet was an actor! There's got to be room for Darwin!'

He stamped back to the table and picked up a handful of papers. `Look at this stuff! "Darwin bitten by poisonous spider ... Darwin savaged by kangaroo ... stung by jellyfish ... eaten by shark ... Beagle found floating, table laid for a meal, this time in a different ocean, still no one on board ... Darwin struck by lightning ... killed by volcanic activity ... Beagle sunk by freak wave" ... does anyone expect us to believe this for one minute?'

There was a ringing silence.

`I can see this is worrying you, Mr Stibbons,' said Ridcully.

`Well, yes, I mean, yes, it's so ... wrong! The multiverse is not supposed to change the rules. Anything that's possible to happen has a universe for it to happen in! I mean, here, yes, the rules can be bent in all kinds of ways, but in Roundworld there's no one to bend them!'