Just one moment, sir, please,' said Ponder. `Did you say another vision?'

Darwin put his head in his hands. `I though it was an epiphany,' he groaned. `I thought that God himself appeared unto me and explained His design. It made so much sense. I had relegated Him to the status of Prime Mover, but now I see that He is immanent in His creation, constantly imparting direction and meaning to it all ... or,' he looked up, blinking, `so I thought ... '

The wizards stood frozen. Then, very carefully, Ridcully said: `Divine visitation, eh? And when was this, exactly?'

`It would have been after breakfast,' moaned Darwin. `It was raining, and then I saw this strange beetle on the window. The room filled up with beetles-'

He stopped, mouth open; a thin blue haze surrounded him.

Ridcully lowered his hand.

`Well, well,' he said. `What about that, Mr Stibbons?'

Ponder was scrabbling desperately at the paper on his clipboard.

`I've no idea!' he said. `Hex hasn't mentioned it!'

The Archchancellor grinned the grim little grin of someone sensing that the game, at last, was afoot.

`Mono Island, remember?' said Ridcully, while Darwin stared blankly at nothing. `A god with a thing about beetles?'

`I'd rather forget,' Ponder shuddered. `But, but ... no, it couldn't be him. How could the God of Evolution get into Roundworld?'

`Same way the Auditors did?' said Ridcully. `All the spacetime continuumuum stuff we're doing, who's to say we aren't leaving a few doors ajar? Well, we can't let the barmy old boy run around there! You and Rincewind, meet me in the Great Hall in one hour!'

Ponder remembered the God of Evolution, who had been so proud of developing a creature even better fitted to survive than mankind. It had been a cockroach.

`We should go right away,' he said, firmly.

`Why? We can move in time!' said Ridcully. `The hour, Mr Stibbons, is for you to come up with some way to kill Auditors!'

`They're indestructible, sir!' `All right - ninety minutes!'

THE SECRETS OF LIFE

THE DISCWORLD VERSION OF DARWIN'S vision may not be quite what Roundworld's historians of science like to tell us, but the two will have been done converged on to the same timeline if the wizards manage to have will defeated the Auditors, so we can concentrate on the after-effects of that convergence. In any case some features are common to both versions of Darwinian history, including apes, beetles, and parasitic wasps. By contemplating these organisms, and many others - especially those con founded barnacles, of course - Darwin was led to his grand synthesis.

Today, no area of biology remains unaffected by the discovery of evolution. The evidence that today's species evolved from different ones, and that this process still continues, is overwhelming. Very little modern biology would make sense without the over-arching framework of evolution. If Darwin were reincarnated today, he would recognise many of his ideas, perhaps slightly reformulated, in the conventional scientific wisdom. The big principle of natural selection would be one of them. But he would also observe debate, perhaps even controversy, about this fundamental pillar of his thinking. Not whether natural selection happens, not whether it drives much of evolution; but whether it is the only driving force.

He would also find many new layers of detail filling some of the gaps in his theories. The most important and far-reaching of these is DNA, the magic molecule that carries genetic `information', the physical form of heredity. Darwin was sure that organisms could pass on their characteristics to their offspring, but he had no idea how this process was implemented, and what physical form it took. Today we are so familiar with the role of genes, and their chemical structure, that any discussion of evolution is likely to focus mainly on DNA chemistry. The role of natural selection, indeed the role of organisms, has been downgraded: the molecule has triumphed.

We want to convince you that it won't stay that way.

Evolution by natural selection, the great advance that Darwin and Wallace brought to public attention, is nowadays considered to be `obvious' by scientists of most persuasions and by most nonspecialists outside the US Bible Belt. This consensus has arisen partly because of a general perception that biology is `easy', it isn't a real, hard-to-understand science like chemistry or physics, and most people think that they know enough about it by a kind of osmosis from the general folk information. This assumption showed up amusingly at the Cheltenham Science Festival in 2001, when the Astronomer Royal Sir Martin Rees and two other eminent astronomers gave talks on `Life Out There'.

The talks were sensible and interesting, but they made no contact with real modem biology. They were based on the kind of biology that is currently taught in schools, most of which is about thirty years out of date. Like almost everything in school science, because it takes at least that long for ideas to `trickle down' from the research frontiers to the classroom. Most `modem mathematics' is at least 150 years old, so thirty-year-old biology is pretty good. But it's not what you should base your thinking on when discussing cutting-edge science.

Jack, in the audience, asked: `What would you think of three biologists discussing the physics of the black hole at the centre of the galaxy?' The audience applauded, seeing the point, but it took a couple of minutes for the scientists on the platform to understand the symmetry. They were then as contrite as they could be without losing their dignity.

This kind of thing happens a lot, because we are all so familiar with evolution that we think we understand it. We devote the rest of this section to a reasonable account of what the average person thinks about evolution. It goes like this.

Once upon a time there was a little warm pond full of chemicals, and they messed about a bit and came up with an amoeba. The amoeba's progeny multiplied (because it was a good amoeba) and some of them had more babies (something funny here ... ) and some had fewer, and some of them invented sex and had a much better time after that. Because biological copying wasn't very good in those days, all of their progeny were different from each other, carrying various copying mistakes called mutations.

Nearly all mutations were bad, on the principle that putting a bullet randomly through a piece of complex machinery is unlikely to improve its performance, but a few were good. Animals with good mutations had many more babies, and those had the good mutation too, so they thrived and bred. Their progeny carried the good mutation into the future. However, many more bad mutations accumulated, so natural selection killed those off. Luckily, another new mutation appeared, which made a new character for a new species (better eyes, or swimming fins, .or scales), which was altogether better and took over.

These later species were fishes, and one of them came out on land, growing legs and lungs to do so. From these first amphibians arose the reptiles, especially the dinosaurs (while the unadventurous fishes were presumably just messing about in the sea for millions of years, waiting to be fish and chips). There were some small, obscure mammals, who survived by coming out at night and eating dinosaur eggs. When the dinosaurs died, the mammals took over the planet, and some evolved into monkeys, then apes, then Stone Age people.

Then evolution stopped, with amoebas in ponds content to remain amoebas and not wanting to be fishes, fishes not wanting to be dinosaurs but just living their little fishy lives, the dinosaurs wiped out by a meteorite. The monkeys and apes, having seen what it was like to be at the peak of evolution, are now just slowly dying out - except in zoos, where they are kept to show us what our progenitors used to be like. Humans now occupy the top branch of the tree of life: since we are perfect, there's nowhere for evolution to go any more, which is why it has stopped.