The Science of Discworld Revised Edition

witches can be quite vindictive.’
    ‘The Omnians believe something like this, you know,’ said the Senior Wrangler. ‘Om started off making simple things like snakes, they say, and worked his way up to Man.’
    ‘As if life was like modelling clay?’ said Ridcully, who was not a patient man with religion. ‘You start out with simple things and then progress to elephants and birds which don’t stand up properly when you put them down? We’ve
met
the God of Evolution, gentlemen … remember? Natural evolution merely improves a species. It can’t
change
anything.’
    His finger stabbed at the next page in the brightly coloured book.
    ‘Gentlemen, this is merely some sort of book of magic, possibly about the Morphic Bounce Hypothesis. 2 Look at this.’ The picture showed a very large lizard followed by a big red arrow, followed by a bird. ‘Lizards don’t turn into birds. If they did, why have we still got lizards? Things can’t decide for
themselves
what shape they’re going to be. Ain’t that so, Bursar?’
    The Bursar nodded happily. He was halfway through H EX ’s write-out of the theoretical physics of the project universe and, so far, had understood every word. He was particular happy with the limitations of light speed. It made absolute sense.
    He took a crayon and wrote in the margin: ‘Assuming the universe to be a negatively curved non-Paramidean manifold – which is more or less obvious – you could deduce its topology by observing the same galaxies in several different directions.’ He thought for a moment, and added: ‘Some travel will be involved.’
    Of course, he was a natural mathematician, and one thing a natural mathematician wants to do is get away from actual damn sums as quickly as possible and slide into those bright sunny uplands where everything is explained by letters in a foreign alphabet, and no one shouts very much. This was even better than that. The hard-to-digest idea that there were dozens of dimensions rolled up where you couldn’t see them was sheer jelly and ice cream to a man who saw
lots
of things no one else saw.
    1 ‘Reddish-brown’.
    2 … which had engrossed wizards for many years. The debate ran like this: it was quite easy to turn someone into a frog, and fairly easy to turn them into, say, a white mouse. Strangely, considering the basic similarity of size and shape, turning someone into an orangutan took a vast amount of power and it was only an explosion in the intense thaumic confines of the Library which had managed the trick. Turning someone into a tree was much, much harder even than that, although turning a pumpkin into a coach was so easy that even a crazy old woman with a wand could do it. Was there some kind of framework into which all this fitted?
    The current hypothesis was that most Change spells unravelled the victim’s morphic field down to some very basic level and then ‘bounced’ them back. A frog was quite simple, so they wouldn’t have to bounce far. An ape, being quite human-like in many respects, would mean a very long return journey indeed. You couldn’t turn someone into a tree because there was no way to get there from here, but a pumpkin could be turned into a wooden coach because it was quite close to it in vegetable space.
    The wizards agreed that this all seemed to fit nicely, and was therefore true.
    If William of Occam had been a wizard at Unseen University, he would have grown a beard.

TWENTY-SIX
THE DESCENT OF DARWIN

    THE WIZARDS MET the God of Evolution in
The Last Continent
. He made things the way a god ought to:
    ‘“Amazin’ piece of work,” said Ridcully, emerging from the elephant. “Very good wheels. You paint these bits before assembly, do you?”’
    The God of Evolution builds creatures piece by piece, like a butcher in reverse. He likes worms and snakes because they’re very easy – you can roll them out like a child with modelling clay. But once the God of Evolution has
made
a species, can it change? It does on Discworld, because the God runs around making hurried adjustments … but how does it work without such divine intervention?
    All societies that have domestic animals, be they hunting dogs or edible pigs, know that living creatures can undergo gradual changes in form from one generation to the next. Human intervention, in the form of ‘unnatural selection’, can
breed
long thin dogs to go down holes and big fat pigs that provide more bacon per trotter. 1 The wizards know this, and so