'That only works if you have narrativium,' said Ponder 'We've got no evidence, sir, that anything on the planet has any concept of causality. Things just live and die.'

And then, on Thursday, the Senior Wrangler spotted a fish. A real, swimming fish.

'There you are,' he said triumphantly. 'The seas are the natural home of life. Look at the land. It's just rubbish, quite frankly.'

'But the sea's not getting anywhere,' said Ridcully. 'Look at those tentacled shellfish you were trying to educate yesterday. Even if you so much as made a sudden movement they just squirted ink at you and swam away.'

'No, no, they were trying to communicate,' the Senior Wrangler insisted. 'Ink is a natural medium, after all. Don't you get the impression that everything is striving? Look at them. You can see them thinking, can't you?'

There were a couple of the things in a tank behind him, peering out of their big spiral shells. The Senior Wrangler had the idea that they could be taught simple tasks, which they would then pass on to the other ammonites. They were turning out to be rather a dis­appointment. They might be good at thinking, ran the general view, but they were pants at actually doing anything about it.

'That's because here's no point in being able to think if you haven't got much to think about,' said the Dean. 'Damn all to think about in the sea. Tide comes in, tide goes out, everything's damp, end of philosophical discourse.'

'Now these are the chaps,' he went on, strolling along to another tank. The Luggage had been quite good as a collector, provided the specimens didn't appear to be threatening Rincewind.

'Hmph,' sniffed the Senior Wrangler. 'Underwater woodlice.'

'But there's a lot of them,' said the Dean. 'And they have legs. I've seen them on the seashore.'

'By accident. And they haven't got anything to use as hands.'

'Ah, well, I'm glad you've pointed that out ...' said the Dean, walking along to the next aquarium.

It contained crabs.

The Senior Wrangler had to admit that crabs looked a good con­tender for Highest Lifeform status. HEX had located some on the other side of the world that were moving along very well indeed, with small underwater cities guarded by carefully transplanted sea-anemones and what appeared to be shellfish farms. They had even invented a primitive form of warfare and had built statues, of sand and spit, apparently to famous crabs who had fallen in the struggle.

The wizards went and had another look fifty thousand years later, after coffee. To the Dean's glee, population pressure had forced the crabs on to the land as well. The architecture hadn't improved, but there were now seaweed farms in the lagoons, and some apparently more stupid crabs had been enslaved for transport purposes and use in inter-clan campaigns. Several large rafts with crudely woven sails were moored in one lagoon, and swarming with crabs. It seemed that crabkind was planning a Great Leap Sideways..

'Not quite there yet,' said Ridcully. 'But definitely very promis­ing, Dean.'

'You see, water's too easy,'said the Dean. 'Your food floats by, there's not much in the way of weather, there's nothing to kick against... mark my words, the land is the place for building a bit of backbone ...'

There was a clatter from HEX, and the field of vision of the omniscope was pulled back rapidly until the world was just a mar­ble floating in space.

'Oh dear,' said the Archchancellor, pointing to a trail of gas, 'Incoming.'

The wizards watched gloomily as a large part of one hemisphere became a cauldron of steam and fire.

'Is this going to happen every time?' said the Dean, as the smoke died away and spread out across the seas.

'I blame the over-large sun and all those planets,' said Ridcully.

'And you fellows should have cleared out the snowballs. Sooner or later, they fall in.'

'It'd just be nice for a species to make a go of things for five min­utes without being frozen solid or broiled,' said the Senior Wrangler.

'That's life,' said Ridcully.

'But not for long,' said the Senior Wrangler.

There was a whimper from behind them.

Rincewind hung in the air, the outline of the virtually-there suit shimmering around him.

'What's up with him?' said Ridcully.

'Er ... I asked him to investigate the crab civilization, sir.'

"The one the comet just landed on?'

'Yes, sir. A billion tons of rock have just evaporated around him, sir.'

'It couldn't have hurt him, though, could it?' 'Probably made him jump, sir.'

UNIVERSALSAND PAROCHIALS

CHANCE MAY HAVE PLAYED A GREATER ROLE than we imagine in ensuring our presence on the Earth. Not only aren't we the pinnacle of evolution: it's conceivable that we very nearly didn't appear at all. On the other hand, if life had wandered off the particular evolutionary track that led to us, it might well have blun­dered into something similar instead. Intelligent crabs, for example. Or very brainy net-weaving jellyfish.

We have no idea how many promising species got wiped out by a sudden drought, a collapse of some vital resource, a meteorite strike, or a collision with a comet. All we have is a record of those species that happen to have left fossils. When we look at the fossil record, we start to see a vague pattern, a tendency towards increas­ing complexity. And many of the most important evolutionary innovations seem to have been associated with major catastrophes…

When we look at today's organisms, some of them seem very sim­ple while others seem more complex. A cockroach looks a lot simpler than an elephant. So we are liable to think of a cockroach as being 'primitive' and an elephant as 'advanced', or we may talk of 'lower' and 'higher' organisms. We also remember that life has evolved, and that today's complex organisms must have had simpler ancestors, and unless we are very careful we think of today's 'prim­itive' organisms as being typical of the ancestors of today's complex organisms. We are told that humans evolved from something that looked more like an ape, and we conclude that chimpanzees are more primitive, in an evolutionary sense, than we are.

When we do this, we confuse two different things. One is a kind of catalogue-by-complexity of today's organisms. The other is a catalogue-by-time of today's organisms, yesterday's ancestors, the day before's ancestors-of-ancestors, and so on. Although today's cock­roach may be primitive in the sense that it is simpler than an elephant, it is not primitive in the sense of being an ancient ances­tral organism. It can't be: it's today's cockroach, a dynamic go-ahead cockroach that is ready to face the challenges of the new millennium.

Although ancient fossil cockroaches have the same appearance as modern ones, they operated against a different backgrounds. What you needed to be a viable cockroach in the Cretaceous was probably rather different from what you need to be a viable cock­roach today. In particular, the DNA of a Cretaceous cockroach was probably significantly different from the DNA of a modern cock­roach. Your genes have to run very fast in order for your body to stand still.

The general picture of evolution that theorists have homed in on resembles a branching tree, with time rising like the sap from the trunk at the bottom, four billion years in the past, to the tips of the topmost twigs, the present. Each bough, branch, or twig represents a species, and all branches point upwards. This 'Tree of Life' pic­ture is faithful to one key feature of evolution, once a branch has split, it doesn't join up again. Species diverge, but they can't merge.