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TO THE WIZARDS OF UNSEEN UNIVERSITY, the heavens include two obviously different types of body: stars, which are tiny pinpricks of light, and the sun, which is a hot ball, not too far away, and passes over the Disc during the day and under it at night. It's taken humanity a while to realize that in our universe it's not like that. Our Sun is a star, and like all stars it's huge, so those tiny pinpricks must be a very long way off. Moreover, some of the pinpricks that seem to be stars aren't: they betray themselves by moving differently from the rest. These are the planets, which are a lot closer and a lot smaller, and together with the Earth, Moon, and Sun they form the solar system. Our solar system may look like a lot of balls whizzing around in some kind of cosmic game of pool, but that doesn't mean that it started out as balls or rock and ice. It is the outcome of a physical process, and the ingredients that went into that process are not obliged to resemble the result that comes out. The more we learn about the solar system, the more difficult it is to give a plausible answer to the question: how did it start? It is not the 'answer' part that gets harder, it's the plausibility. As we learn more and more about the solar system, the reality-check that our theories have to pass becomes more and more stringent. This is one reason why scientists have a habit of opening up old questions that everybody assumed were settled long ago, and deciding that they weren't. It doesn't mean that scientists are incompetent: it demon¬strates their willingness to contemplate new evidence and re-examine old conclusions in its light. Science certainly does not claim to get things right, but it has a good record of ruling out ways to get things wrong.
What must a theory of the formation of the solar system explain? Principally, of course, the planets, nine of them, dotted rather randomly in space; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. It must explain their differences in size. Mercury is a mere 3,032 miles (4,878 km) in diameter, whereas Jupiter is 88,750 miles (142,800 km) in diameter, 29 times as big, 24,000 times the volume, an enormous discrepancy. It must explain their differences in chemical composition: Mercury is made of iron, nickel, and silicate rock; Jupiter is made from hydrogen and helium. It must explain why the planets near the Sun are generally smaller than those further out, with the exception of tiny Pluto, out in the cold and the dark. We don't know a great deal about Pluto, but most of what we do know is strange. For instance, all the other planets lie pretty close to a single plane through the centre of the Sun, but Pluto's orbit is inclined at a noticeable angle. All the other planets have orbits that are pretty close to circles, but Pluto's orbit is much more elongated, to the extent that some of the time it is closer to the Sun than Neptune is.
But that's not all that a theory of the origin of the solar system has to get right. Most planets have smaller bodies in orbit around them, our own familiar Moon; Phobos and Deimos, the diminutive twin satellites of Mars; Jupiter's 16 satellites; Saturn's 17 ... Even Pluto has a satellite, called Charon, and that's weird too. Saturn goes one better and also has entire rings of smaller bodies surrounding it, a broad, thin band of encircling rocks that breaks up into a myriad distinct ringlets, with satellites mixed up among them as well as more conventional satellites elsewhere. Then there are the asteroids, thousands of small bodies, some spherical like planets, others irreg¬ular lumps of rock, most of which orbit between Mars and Jupiter, except for quite a few that don't. There are comets, which fall in towards the Sun from the huge 'Oort cloud' way out beyond the orbit of Pluto, a cloud that contains trillions of comets. There is the Kuiper belt, a bit like the asteroid belt but outside Pluto's orbit: we know over 30 bodies out there now, but we suspect there are hun¬dreds of thousands. There are meteorites, lumps of rock of various sizes that wander erratically through the whole thing ...
Each of these celestial objects, moreover, is a one-off. Mercury is a blisteringly hot lump of cratered rock. Venus has a sulphuric acid atmosphere, rotates the wrong way compared to nearly every¬thing else in the solar system, and is believed to resurface itself every hundred million years or so in a vast, planetwide surge of volcanic activity. Earth has oceans and supports life; since we live on it we find it the most congenial of the planets, but many aliens would probably be aghast at its deadly, poisonous, corrosive oxygen atmos¬phere. Mars has rock-strewn deserts and dry ice at its poles. Jupiter is a gas giant, with a core of hydrogen compressed so much that it has become metallic, and maybe a small rocky core inside that, 'small' compared to Jupiter, but about three times the diameter of the Earth. Saturn has its rings, but so do Jupiter, Uranus, and Neptune, though these are nowhere near as extensive or spectacu¬lar. Uranus has an icy mantle of methane and ammonia, and its axis of rotation is tilted so far that it is slightly upside down. Neptune is similar to Uranus but without that ridiculous axial tilt. Pluto, as we've said, is just crazy. We don't even know accurately how big it is or how massive it is, but it's a Lilliputian in the country of the Gas Giants.
Right... all that is what a theory of the origins of the solar sys¬tem has to explain. It was all a lot easier when we thought there were six planets, plus the Sun and the Moon, and that was it. As for the solar system being an act of special creation by a supernatural being, why would any self-respecting supernatural being make the thing so complicated?

Because it makes itself complicated, that's why. We now think that the solar system was formed as a complete package, starting from quite complicated ingredients. But it us took a while to realize this.
The first theory of planetary formation that makes any kind of sense by modern standards was thought up by the great German philosopher Immanuel Kant about 250 years ago. Kant envisaged it all starting as a vast cloud of matter, big lumps, small lumps, dust, gas, which attracted each other gravitationally and clumped together.
About 40 years later the French mathematician Pierre-Simon de Laplace came up with an alternative theory of enormous intrinsic beauty, whose sole flaw is that it doesn't actually work. Laplace thought that the Sun formed before the planets did, perhaps by some cosmic aggregation process like Kant's. However, that ancient Sun was much bigger than today's, because it hadn't fully collected together, and the outer fringes of its atmosphere extended well beyond what is now the orbit of Pluto. Like the wizards of Unseen University, Lapkce thought of the Sun as a gigantic fire whose fuel must be slowly burning away. As the Sun aged, it would cool down. Cool gas contracts, so the Sun would shrink.
Now comes a neat peculiarity of moving bodies, a consequence of another of Newton's laws, the Law(s) of Motion. Associated with any spinning body is a quantity called 'angular momentum', a combination of how much mass it contains, how fast it is spinning, and how far out from the centre the spinning takes place. According to Newton, angular momentum is conserved, it can be redistrib¬uted, but it neither goes away nor appears of its own accord. If a spinning body contracts, but the rate of spin doesn't change, angu¬lar momentum will be lost: therefore the rate of spin must increase to compensate. This is how ice skaters do rapid spins: they start with a slow spin, arms extended, and then bring their arms in close to their body. Moreover, spinning matter experiences a force, cen¬trifugal force, which seems to pull it outwards, away from its centre.
Laplace wondered whether centrifugal force acting on a spin¬ning gascloud might throw off a belt of gas round the equator. He calculated that this ought to happen whenever the gravitational force attracting that belt towards the centre was equal to the cen¬trifugal force trying to fling it away. This process would happen not once, but several times, as the gas continued to contract, so the shrinking Sun would surround itself with a series of rings of mate¬rial, all lying in the same plane as the Sun's equator. Now suppose that each belt coalesced into a single body ... Planets!
What Laplace's theory got right, but Kant's did not, was that the planets lie roughly in a plane and they all rotate round the Sun in the same direction that the Sun spins. As a bonus, something rather similar might have occurred while those belts were coalescing into planets, in which case the motion of satellites is explained as well.
It's not hard to combine the best features of Kant's and Laplace's theories, and this combination satisfied scientists for about a cen¬tury. However, it slowly became clear that our solar system is far more unruly than either Kant or Laplace had recognized. Asteroids have wild orbits, and some satellites revolve the wrong way. The Sun contains 99% of the solar system's mass, but the planets pos¬sess 99% of its angular momentum: either the Sun is rotating too slowly or the planets are revolving too quickly.
As the twentieth century opened, these deficiencies of the Laplacian theory became too great for astronomers to bear, and sev¬eral people independently came up with the idea that a star developed a solar system when it made a close encounter with another star. As the two stars whizzed past each other, the gravita¬tional attraction from one of them was supposed to draw out a long cigar-shaped blob of matter from the other, which then condensed into planets. The advantage of the cigar shape was that it was thin at the ends and thick at the middle, just as the planets are small close to the Sun or out by Pluto, but big in the middle where Jupiter and Saturn live. Mind you, it was never entirely clear why the blob had to be cigar-shaped ...
One important feature of this theory was the implication that solar systems are rather uncommon, because stars are quite thinly scattered and seldom get close enough together to share a mutual cigar. If you were the sort of person who'd be comforted by the idea that human beings are unique in the universe, then this was a rather appealing suggestion: if planets were rare, then inhabited planets would be rarer still If you were the sort of person who preferred to think that the Earth isn't especially unusual, and neither are its life-forms, then the cigar theory definitely put a crimp on the imagination.

By the middle of the twentieth century, the shared-cigar theory had turned out to be even less likely than the Kant-Laplace theory. If you rip a lot of hot gas from the atmosphere of a star, it doesn't con¬dense into planets, it disperses into the unfathomable depths of interstellar space like a drop of ink in a raging ocean. But by then, astronomers were getting a much clearer idea of how stars origi¬nated, and it was becoming clear that planets must be created by the same processes that produce the stars, A solar system is not a Sun that later acquires some tiny companions: it all comes as one pack¬age, right from the start. That package is a disc, the nearest thing in our universe (so far as we know) to Discworld. But the disc begins as a cloud and eventually turns into a lot of balls (Stibbons's Third Rule).
Before the disc formed, the solar system and the Sun started out as a random portion of a cloud of interstellar gas and dust. Random jigglings triggered a collapse of the dustcloud, with everything heading for roughly, but not exactly, the same central point. All it takes to start such a collapse is a concentration of matter some¬where, whose gravity then pulls more matter towards it: random jigglings will produce such a concentration if you wait long enough. Once the process has started, it is surprisingly rapid, taking about ten million years from start to finish. At first the collapsing cloud is roughly spherical. However, it is being carried along by the rotation of the entire galaxy, so its outer edge (relative to the centre of the galaxy) moves more slowly than its inner edge. Conservation of angular momentum tells us that as the cloud collapses it must start spinning, and the more it collapses, the faster it spins. As its rate of spin increases, the cloud flattens out into a rough disc.
More careful calculations show that near the middle this disc thickens out into a dense blob, and most of the matter ends up in the blob. The blob condenses further, its gravitational energy gets traded for heat energy, and its temperature goes up fast. When the temperature rises enough, nuclear reactions are ignited: the blob has become a star. While this is happening, the material in the disk undergoes random collisions, just as Kant imagined, and coalesces in a not terribly ordered way. Some clumps get shoved into wildly eccentric orbits, or swung out of the plane of the disc; most clumps, however, are better behaved and turn into decent, sensible planets. A miniature version of the self-same processes can equip most of those planets with satellites.
The chemistry fits, too. Near the Sun, those incipient planets get very hot, too hot for solid water to form. Further out, around the orbit of Jupiter for a dustcloud suitable for making our Sun and solar system, water can freeze into solid ice. This distinction is important for the chemical composition of the planets, and we can see the main outlines if we focus on just three elements: hydrogen, oxygen, and silicon. Hydrogen and oxygen happen to be the two most abundant elements in the universe, apart from helium which doesn't undergo chemical reactions. Silicon is less abundant but still common. When silicon and oxygen combine together, you get silicates, rocks. But even if the oxygen can mop up all the available silicon, some 96% of the oxygen is still unattached, and it combines with hydrogen to make water. There is so much hydrogen, a thou¬sand times as much as oxygen, that virtually all of the oxygen that doesn't go into rocks gets locked away in water. So by far the most common compound in the condensing disc is water.
Close to the star, that water is liquid, even vapour, but out at Jovian distances, it's solid ice. You can pick up a lot of solid mass if you're condensing in a region where ice can form. So the planets there are bigger, and (at least to begin with) they are icy Nearer the star, the planets are smaller, and rocky. But now the big guys can parky their initial weight advantage into an even bigger one. Anything that is ten times the mass of the Earth, or greater, can attract and retain the two most abundant elements of the disc, hydrogen and helium. So the big balls soak up large amount of extra mass in the form of these two gases. They can also retain com¬pounds like methane and ammonia, which are volatile gases closer to the star.
This theory explains rather a lot. It gets all the main features of the solar system pretty much right. It allows for the odd exceptional motion, but not too many. It agrees with observations of condens¬ing gas clouds in distant regions of space. It may not be perfect, and some special pleading might be necessary to explain odd things like Pluto, but most of the important features click neatly into place.

The future of the solar system is at least as interesting as its past. The picture of the solar system that emerged from the ideas of Newton and his contemporaries was very much that of a clockwork universe, a celestial machine that, once set ticking, would continue to follow some simple mathematical rules and continue ticking mer¬rily away forever. They even built celestial machines, called orreries, with lots and lots of cogwheels, in which little brass planets with ivory moons went round and round when you turned a handle.
We now know that the cosmic clockwork can go haywire. It won't happen quickly, but there may be some big changes to the solar system on the way. The underlying reason is chaos, chaos in the sense of 'chaos theory', with all those fancy multicoloured 'frac¬tal' things, a rapidly expanding area of mathematics which is invading all of the other sciences. Chaos teaches us that simple rules need not lead to simple behaviour, something that Ponder Stibbons and the other wizards are in the process of discovering. In fact, simple rules can lead to behaviour that in certain respects has distinct elements of randomness. Chaotic systems start out behav¬ing predictably, but after you cross some 'prediction horizon' all predictions fail. Weather is chaotic, with a prediction horizon of about four days. The solar system, we now know, is chaotic, with a prediction horizon of tens of millions of years. For example, we can't be sure which side of the Sun Pluto will be in a hundred mil¬lion years' time. It will be in the same orbit, but its position in that orbit is completely uncertain.
We know this because of some mathematical work that was done, in part, with an orrery, but this was a 'digital orrery', a custom-built computer that could do celestial mechanics very fast. The digital orrery was developed by Jack Wisdom's research group, which, in competition with its rival headed by Jaques Laskar, has been extending our knowledge of the solar system's future. Even though a chaotic system is unpredictable in the long run, you can make a whole series of independent attempts at predicting it and then see what they agree about. According to the mathematics, you can be pretty sure those things are right.
One of the most striking results is that the solar system is due to lose a planet. About a billion years from now, Mercury will move outwards from the Sun until it crosses the orbit of Venus. At that point, a close encounter between Venus and Mercury will fling one or the other, possibly both, out of the solar system altogether -unless they hit something on the way, which is highly unlikely, but possible. It might even be the Earth, or the passing Venus might join with us in a cosmic dance whose end result is the Earth being flung out of the solar system. The details are unpredictable, but the gen¬eral scenario is very likely.
This means that we've got the wrong picture of the solar system. On a human timescale it's a very simple place, in which nothing much changes. On its own timescale, hundreds of millions of years, it's full of drama and excitement, with planets roaring all over the place, whirling around each other, and dragging each other out of orbit in a mad gravitational dance.
This is vaguely reminiscent of Worlds in Collision, a book pub¬lished in 1950 by Immanuel Velikovsky, who believed that a giant comet was once spat out by Jupiter, passed close to the Earth twice, had a love affair with Mars (giving rise to a brood of baby comets), and finally retired to live in peace as Venus. Along the way it gave rise to many strange effects that became stories in the Bible. Velikovsky was right about one thing: the orbits of the planets are not fixed forever. He wasn't right about much else.

Do other solar systems encircle distant stars, or are we unique? Until a few years ago there was a lot of argument about this ques¬tion, but no hard evidence. Most scientists, if they had to bet, would have backed the existence of other solar systems, because the collapsing dustcloud mechanism could easily get going almost any¬where there's cosmic dust, and there are a hundred billion stars in our own galaxy, let alone the billions upon billions of others in the universe, all of which once were cosmic dust. But that's only indi¬rect evidence. Now the position is much clearer. Characteristically, however, the story involves at least one false start, and a critical re-examination of evidence that at first looked rather convincing.
In 1967 Jocelyn Bell, a graduate student at the University of Cambridge, was working for a doctorate under the direction of Anthony Hewish. Their field was radio astronomy Like light, radio is an electromagnetic wave, and like light, radio waves can be emit¬ted by stars. Those radio waves can be detected using parabolic dish receivers, today's satellite TV dishes are a close relative, rather misleadingly called 'radio telescopes', even though they work on very different principles from normal optical telescopes. If we look at the sky in the radio part of the electromagnetic spectrum, we can often 'see' things that are not apparent using ordinary visible light. This should be no surprise: for example military snipers can 'see in the dark' using infra-red waves, detecting things by the heat they emit. The technology in those days wasn't terribly slick, and the radio signals were recorded on long rolls of paper using automatic pens that drew wiggly curves in good old-fashioned ink. Bell was given the task of looking for interesting things on the paper charts, carefully scanning about 400 feet of chart per week. What she found was very strange, a signal that pulsated about thirty times per second. Hewish was sceptical, suspecting that the signal was somehow generated by their measuring instruments, but Bell was convinced it was genuine. She searched through three miles of pre¬vious charts and found several earlier instances of the same signal, which proved she was right. Something out there was emitting the radio equivalent of a reverberating whistle. The object responsible was named a 'pulsar', a pulsating starlike object.
What could these strange things be? Some people suggested they were radio signals from an alien civilization, but all attempts to extract the alien equivalent of The Jerry Springer Show failed (which was possibly just as well). There seemed to be no structured messages hidden in the signals. In fact, what they are now believed to be is even stranger than an alien TV programme. Pulsars are thought to be neutron stars, stars composed of highly degenerate matter containing only neutrons, usually a mere 12 miles (20 km) in diameter. Recall that neutron stars are incredibly dense, formed when a larger star undergoes gravitational collapse. That initial star, as we have seen, will be spinning, and because of conservation of angular momentum, the resulting neutron star has to spin a lot faster In fact, it typically spins through about thirty complete revolutions every second. For a star, that's pretty speedy. Only a tiny star like a neutron star can do it: if an ordinary star were to revolve that fast, its surface would have to be travelling faster than light, which wouldn't greatly please Einstein. (More realistically, a normal star would be torn apart at much lower speeds.) But a neutron star is small, and its angular momentum is comparatively large, and pirou¬etting thirty times a second is no problem at all.
For a helpful analogy, contemplate our own Earth. Like a pulsar, it spins on an axis. Like a pulsar, it has a magnetic field. The mag¬netic field has an axis too, but it's different from the axis of rotation, that's why magnetic north is not the same as true north. There's no good reason for magnetic north to be the same as true north on a pulsar, either. And if it isn't, that magnetic axis whips round thirty times every second. A rapidly spinning magnetic field emits radia¬tion, known as synchrotron radiation, and it emits it in two narrow beams which point along the magnetic axis. In short, a neutron star projects twin radio beams like the spinning gadgetry on top of a ter¬restrial lighthouse. So if you look at a neutron star in radio light, you see a bright flash as the beam points towards you, and then vir¬tually nothing until the beam comes round again. Every second, you see thirty flashes. That's what Bell had noticed.
If you're a living creature of remotely orthodox construction, you definitely do not want your star to be a pulsar. Synchrotron radiation is spread over a wide range of wavelengths, from visible light to x-rays, and x-rays can seriously damage the health of any creature of remotely orthodox construction. But no astronomer ever seriously suspected that pulsars might have planets, anyway. If a big star collapses down to an incredibly dense neutron star, surely it will gobble up all the odd bits of matter hanging around nearby. Won't it?
Perhaps not. In 1991 Matthew Bailes announced that he had detected a planet circling the pulsar PSR 1829-10, with the same mass as Uranus, and lying at a distance similar to that of Venus from the Sun. The known pulsars are much too far away for us to see planets directly, indeed all stars, even the nearest ones, are too far away for us to see planets directly. However, you can spot a star that has planets by watching it wiggle as it walks. Stars don't sit motion¬less in space, they generally seem to be heading somewhere, presumably as the result of the gravitational attraction of the rest of the universe, which is lumpy enough to pull different stars in dif¬ferent directions. Most stars move, near enough, in straight lines. A star with planets, though, is like someone with a dancing partner. As the planets whirl round the star, the star wobbles from side to side. That makes its path across the sky slightly wiggly. Now, if a big fat dancer whirls a tiny feather of a partner around, the fat one hardly moves at all, but if the two partners have equal weight, they both revolve round a common centre. By observing the shape of the wig¬gles, you can estimate how massive any encircling planets are, and how close to the star their orbits are.
This technique first earned its keep with the discovery of dou¬ble stars, where the dancing partner is a second star, and the wobbles are fairly pronounced because stars are far more massive than planets. As instrumentation has become more accurate, ever tinier wobbles can be detected, hence ever tinier dancing partners. Bailes announced that pulsar PSR 1829-10 had a dancing partner whose mass was that of a planet. He couldn't observe the wiggles directly, but he could observe the slight changes they produced in the timing of the pulses in the signal. The only puzzling feature was the rotational period of the planet: exactly six Earth months. Bit of a coincidence. It quickly turned out that the supposed wiggles were not caused by a planet going round the pulsar, but by a planet much closer to home, Earth. The instruments were doing the wiggling at this end, not the pulsar at the far end.
Scarcely had this startling claim of a pulsar planet been with¬drawn, however, when Aleksander Wolszczan and Dale Frail announced the discovery of two more planets, both circling pulsar PSR 1257+12. A pulsar solar system with at least two worlds! The way you wiggle when you have two dancing partners is more com¬plex than the way you do it with one, and it's difficult to mistake such a signal for something generated at the receiving end by the motion of the Earth. So this second discovery seems to be fairly solid, unless there is a way for pulsars to vary their output signals in just such a complex manner without having planets, maybe the radio beam could be a bit wobbly? We can't go there to find out, so we have to do the best we can from here; and from here it looks good.
So there do exist planets outside our solar system. But it's the possibility of life that really makes distant planets interesting, and a pulsar planet with all those x-rays is definitely not a place for any¬thing that wants to be alive for very long. But now conventional stars are turning out to have planets, too. In October 1995 Michel Mayor and Didier Queloz found wobbles in the motion of the star 51 Pegasi that were consistent with a planet of about half Jupiter's mass. Their observations were confirmed by Geoffrey Marcy and Paul Butler, who found evidence for two more planets, one seven times the mass of Jupiter orbiting 70 Virginis, and one two or three times Jupiter's mass orbiting 47 Ursae Majoris. By 1996 seven such planets had been found; right now there are about ten. The exact number fluctuates because every so often astronomers discover problems with previous measurements that cast doubt on some¬body else's favourite new planet, but the general trend is up. And our nearest sunlike neighbour, epsilon Eridani, is now known to possess an encircling dustcloud, perhaps like our Sun's Oort cloud, thanks to observations made in 1998 by James Greaves and col¬leagues. We can't see any wobbles, though, so if it has planets, their mass must be less than three times that of Jupiter. A year earlier, David Trilling and Robert Brown used observations of a similar dustcloud round 55 Cancri, which does wobble, to show that it has a planet whose mass is at most 1.9 Jupiters. This definitely rules out alternative explanations of the unseen companion, for example that it might be a 'brown dwarf', a failed star.

Although today's telescopes cannot detect an alien planet directly, future telescopes might. Conventional astronomical telescopes use a big, slightly dish-shaped mirror to focus incoming light, plus lenses and prisms to pick up the image and send it to what used to be an eypiece for an astronomer to look down, but then became a photographic plate, and is now likely to be a 'charge-coupled device', a sensitive electronic light-detector, hooked up to a com¬puter. A single telescope of conventional design would need a very big mirror indeed to spot a planet round another star, a mirror some 100 yards (100 m) across. The biggest mirror in existence today is one-tenth that size, and to see any detail on the alien world you'd need an even bigger mirror, so none of this is really practica¬ble.
But you don't have to use just one telescope.
A technique known as 'interferometry' makes it possible, in principle, to replace a single mirror 100 yards wide by two much smaller mirrors 100 yards apart. Both produce images of the same star or planet, and the incoming light waves that form those images are aligned very accurately and combined. The two-mirror system gathers less light than a complete 100-yard mirror would, but it can resolve the same amount of tiny detail. And with modern electron¬ics, very small quantities of incoming light can be amplified. In any case, what you actually do is use dozens of smaller mirrors, together with a lot of clever trickery that keeps them aligned with each other and combines the images that they receive in an effective manner.
Radio astronomers use this technique all the time. The biggest technical problem is keeping the length of the path from the star to its image the same for all of the smaller telescopes, to within an accuracy of one wavelength. The technique is relatively new in optical astronomy, because the wavelength of visible light is far shorter than that of radio waves, but for visible light the real killer is that it's not worth bothering if your telescopes are on the ground. The Earth's atmosphere is in continual turbulent motion, bending incoming light in unpredictable ways. Even a very powerful ground-based telescope will produce a fuzzy image, which is why the Hubble Space Telescope is in orbit round the Earth. Its planned successor, the Next Generation Space Telescope, will be a million miles away, orbiting the Sun, delicately poised at a place called Lagrange point L2. This is a point on the line from the Sun to the Earth, but further out, where the Sun's gravity, the Earth's gravity, and the centrifugal force acting on the orbiting telescope all cancel out. Hubble's structure includes a heavy tube which keeps out unwanted light, especially light reflected from our own planet. It's a lot darker out near L2, and that cumbersome tube can be dis¬pensed with, saving launch fuel. In addition, L2 is a lot colder than low Earth orbit, and that makes infra-red telescopy much more effective.
Interferometry uses a widely separated array of small telescopes instead of one big one, and for optical astronomy the array has to be set up in space. This produces an added advantage, because space is big, or, in more Discworldly terms, a place to be big in. The biggest distance between telescopes in the array is called the baseline. Out in space you can create interferometers with gigantic baselines, radio astronomers have already made one that is bigger than the Earth by using one ground-based telescope antenna and one in orbit. Both NASA and the European Space Agency ESA have mis¬sions on the drawing-board for putting prototype optical interferometer arrays, 'flocks' is a more evocative term, into space. Some time around 2002 NASA will launch Deep Space 3, involving two spacecraft flying 1 kilometre apart and maintaining station relative to each other to a precision of less than half an inch (1 cm). Another NASA venture, the Space Interferometry Mission, will employ seven or eight optical telescopes bolted to a rigid arm 10-15 yards (10-15 m) long. In 2009 ESA hopes to launch its Infrared Space Interferometer, not to image distant planets but to find out what their atmospheres are made of by looking for telltale absorption lines in their spectra.
The biggest dream of all, though, is NASA's Planet Imager, pen¬cilled in for 2020. A squadron of spacecraft, each equipped with four optical telescopes, will deploy itself into an interferometer with a baseline of several thousand miles, and start mapping alien plan¬ets. The nearest star is just over four light years away; computer simulations show that 50 telescopes with a baseline of just 95 miles (150 km) can produce images of a planet 10 light years away that are good enough to spot continents and even moons the size of ours. With 150 telescopes and the same baseline, you could look at the Earth from 10 light years away and see hurricanes in its atmosphere. Think what could be done with a thousand-mile baseline.
Planets outside our solar system do exist, then, and they probably exist in abundance. That's good news if you're hoping that some¬where out there are alien lifeforms. The evidence for those, though, is controversial.
Mars, of course, is the traditional place where we expect to find life in the solar system, partly because of myths about Martian 'canals' which astronomers thought they'd seen in their telescopes but which turned out to be illusions when we sent spacecraft out there to take a close look, partly because conditions on Mars are in some ways similar to those on Earth, though generally nastier, and partly because dozens of science-fiction books have subliminally prepared us for the existence of Martians. Life does show up in nasty places here, finding a foothold in volcanic vents, in deserts, and deep in the Earth's rocks. Nevertheless, we've found no signs of life on Mars.
For a while, some scientists thought we had. In 1996 NASA announced signs of life on Mars. A meteorite dug up in the Antarctic with the code number ALH84001 had been knocked off Mars 15 million years ago by a collision with an asteroid, and plunged to Earth 13,000 years ago. When it was sliced open and the interior examined at high magnification we found three possible signs of life. These were markings like tiny fossil bacteria, crystals containing iron like those made by certain bacteria, and organic molecules resembling some found in fossil bacteria on Earth. It all pointed to: Martian bacteria! Not surprisingly, this claim led to a big argument, and the upshot is that all three discoveries are almost certainly not evidence for life at all. The fossil 'bacteria' are much too small and most of them are steps on crystal surfaces that have caused funny shapes to form in the metal coatings used in electron microscopy; the iron-bearing crystals can be explained without invoking bacteria at all; and the organic molecules could have got there without the aid of Martian life.
However, in 1998 the Mars Global Surveyor did find signs of an ancient ocean on Mars. At some point in the planet's history, huge amounts of water gushed out of the highlands and flowed into the northern lowlands. It was thought that this water just seeped away or evaporated, but it now turns out that the edges of the northern lowlands are ail at much the same height, like shorelines eroded by an ocean. The ocean, if it existed, covered a quarter of Mars's sur¬face. If it contained life, there ought to be Martian fossils for us to find, dating from that period.
The current favourite for life in the solar system is a surprise, at least to people who don't read science fiction: Jupiter's satellite Europa. It's a surprise because Europa is exceedingly cold, and cov¬ered in thick layers of ice. However, that's not where the life is suspected to live. Europa is held in Jupiter's massive gravitational grasp, and tidal forces warm its interior. This could mean that the deeper layers of the ice have melted to form a vast underground ocean. Until recently this was pure conjecture, but the evidence for liquid water beneath Europa's surface has now become very strong indeed. It includes the surface geology, gravitational measurements, and the discovery that Europa's interior conducts electricity. This finding, made in 1998 by K.K.Khurana and others, came from observations of the worldlet's magnetic field made by the space probe Galileo, The shape of the magnetic field is unusual, and the only reasonable explanation so far is the existence of an under¬ground ocean whose dissolved salts make it a weak conductor of electricity. Callisto, another of Jupiter's moons, has a similar mag¬netic field, and is now also thought to have an underground ocean. In the same year, T.B.McCord and others observed huge patches of hydrated salts (salts whose molecules contain water) on Europa's surface. This might perhaps be a salty crust deposited by upwelling water from a salty ocean.
There are tentative plans to send out a probe to Europa, land it, and drill down to see what's there. The technical problems are for¬midable, the ice layer is at least ten miles (16 km) thick, and the operation would have to be carried out very carefully so as not to disturb or destroy the very thing we're hoping to find: Europan organisms. Less invasively, it would be possible to look for tell-tale molecules of life in Europa's thin atmosphere, and plans are afoot to do this too. Nobody expects to find Europan antelopes, or even fishes, but it would be surprising if Europa's water-based chemistry, apparently an ocean a hundred miles (160 km) deep, has not pro¬duced life. Almost certainly there are sub-oceanic 'volcanoes' where very hot sulphurous water is vented through the ocean floor. These provide a marvellous opportunity for complicated chemistry, much like the chemistry that started life on Earth.
The least controversial possibility would be an array of simple bacteria-like chemical systems forming towers around the hot vents, much as Earthly bacteria do in the Baltic sea. More complicated creatures like amoebas and parameciums would be a pleasant sur¬prise; anything beyond that, such as multicellular organisms, would be a bonus. Don't expect plants, there's not enough light that far from the sun, even if it could filter down through the layers of ice. Europan life would have to be powered by chemical energy, as it is around Earth's underwater volcanic vents. Don't expect Europan lifeforms to look like the ones round our vents, though: they will have evolved in a different chemical environment.



PONDER OPENED HIS EYES and looked up into a face out of time. A mug of tea was thrust towards him. It had a banana stuck in it. 'Ah ... Librarian,' said Ponder weakly, taking the cup. He drank, stabbing himself harmlessly in the left eye. The Librarian thought that practically everything could be improved by the addition of soft fruit, but apart from that he was a kindly soul, always ready with a helping hand and a banana.
The wizards had put Ponder to sleep on a bench in the store¬room. Dusty items of magical gear were stacked from floor to ceiling. Most of it was broken, and all of it was covered in dust.
Ponder sat up and yawned.
'What time is it?'
'Gosh, that late?'
As the warm clouds of sleep ebbed, it dawned on Ponder that he had left the Project entirely in the hands of the senior faculty. The Librarian was impressed at how long the door kept swinging.
Most of the main laboratory was empty, except for the pool of light around the Project.
The Dean's voice said, 'Mappin Winterley ... that's a nice name?'
'Owen Houseworthy?'
'Shut up, Dean. That's not funny. It never was funny.' This was the voice of the Archchancellor.
'Just as you say, Gertrude.'
Ponder advanced towards the glowing Project.
'Ah, Ponder,' said the Senior Wrangler, stepping in front of it hurriedly. 'Good to see you looking so…'
'You've been ... doing things, haven't you,' said Ponder, trying to see around him.
'I'msure everything can be mended,' said the Lecturer in Recent Runes.
'And it's still nearly circular,' said the Dean, 'Just ask Charlie Grinder here. His name's definitely not Mustrum Ridcully, I know that.'
'I'm warning you, Dean...’
'What have you done?'
Ponder looked at his globe. It was certainly warmer now, and also rather less globular. There were livid red wounds across one side, and the other hemisphere was mainly one big fiery crater. It was spinning gently, wobbling as it did so.
'We've saved most of the bits,' said the Senior Wrangler, watch¬ing him hopefully
'What did you do?'
'We were only trying to be helpful,' said the Dean. 'Gertrude here suggested we make a sun, and...’
'Dean?' said Ridcully
'Yes, Archchancellor?'
'I would just like to point out, Dean, that it was not a very funny joke to begin with. It was a pathetic attempt, Dean, at dragging a sad laugh out of a simple figure of speech. Only four-year-olds and people with a serious humour deficiency keep on and on about it. I just wanted to bring this out into the open, Dean, calmly and in a spirit of reconciliation, for your own good, in the hope that you may be made well. We are all here for you, although I can't imagine what you are here for.' Ridcully turned to the horrified Ponder. 'We made a sun...’
‘...some suns...’ muttered the Dean.
‘...some suns, yes, but ... well, this "falling in circles" business is very difficult, isn't it? Very hard to get the hang of.'
'You crashed a sun into my world?' said Ponder.
'Some suns,' said Ridcully.
'Mine bounced off,' said the Dean.
'And created this rather embarrassingly large hole here,' said the Archchancellor. 'And incidentally knocked a huge lump out of the place.'
'But at least bits of my sun burned for a long time,' said the Dean.
'Yes, but inside the world. That doesn't count.' Ridcully sighed. 'Yet your machine, Mister Stibbons, says a sun sixty miles across won't work. And that's ridiculous.'
Ponder stared hollow-eyed at his world, wobbling around like a crippled duck.
'There's no narrativium,' he said dully. 'It doesn't know what size a sun should be.'
'Ook,' said the Librarian.
'Oh dear,' said Ridcully. 'Who let him in here?'
The Librarian was informally banned from the High Energy Magic building, owing to his inherent tendency to check on what things were by tasting them. This worked very well in the Library, where taste had become a precision reference system, but was less useful in a room occasionally containing bus bars throbbing with several thousand thaums. The ban was informal, of course, because anyone capable of pulling the dooknob right through an oak door can obviously go where he likes.
The orangutan knuckled over to the dome and tasted it. The wizards tensed as delicate black fingers twiddled the knobs of the omniscope, bringing into focus the furnace that had exploded yes¬terday. It was a tiny point of light now, surrounded by coruscating streamers of glowing gas.
The focus moved in to the glowing ember.
'Still too big,' said Ridcully. 'Nice try, old chap.'
The Librarian turned towards him, the light of the explosion moving across his face, and Ponder held his breath.
It came out in a rush. 'Someone give me a light!'
The globes on his desk rolled off and bounced on the floor as he tried to grab one. He held it as the Senior Wrangler obligingly lit a match, and waggled it this way and that. 'It'll work!' 'Jolly good!' said Ridcully. 'What will?'
'Days and nights!' said Ponder. 'Seasons, too, if we do it right! Well done, sir! I'm not sure about the wobble, but you might have got it just right!'
'That's the kind of thing we do,' said Ridcully, beaming. 'We're the chaps for getting things right, sure enough. What things did we get right this time?'
'The spin!'
'That was my sun that did that,' the Dean pointed out, smugly.
Ponder was almost dancing. And then, suddenly, he looked grave.
'But it all depends on fooling people down there,' he said. 'And there isn't anyone down there ,.. HEX?'
There was a mechanical rattle as HEX paid attention.
+++Yes? +++
'Is there any way we can get onto the world?'
+++ Nothing Physical May Enter The Project +++
'I want someone down there to observe things from the surface.'
+++ That Is Possible. Virtually Possible +++
+++ But You Will Need A Volunteer. Someone To Fool +++
'This is Unseen University,' said the Archchancellor 'That should present no problem.'

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WE DON'T KNOW IF THE EARTH IS A TYPICAL PLANET. We don't know how common 'aqueous' planets with oceans and continents and atmospheres are. In our solar system, Earth is the only one. And we'd better be careful about phrases like 'earthlike planet', because for about half of Earth's history it has not been the familiar blue-green planet that we see in satellite photos, with its oxygen atmosphere, white clouds, and everything else that we are used to. In order to get an earthlike planet, in today's sense, you have to start with an unearthlike planet and wait a few billion years. And what you get is quite different from what, only a few decades ago, we thought the Earth was like.
We thought it was a very stable place, that if you could go back to the time when the oceans and continents first separated out, they'd have been in the same places they are now. And we thought that the interior of the Earth was pretty simple. We were wrong.
We know a lot about the surface of the Earth, but we still know much less about what's inside it. We can study the surface by going there, which is usually fairly easy, unless we want to look at the top of Everest. We can also penetrate the ocean depths using vehicles that can protect frail humans against the huge pressures of the deep seas, and we can dig holes down into the ground and send people down those too. We can get further information about the top few miles of the Earth's crust by drilling, but that's just a thin skin, comparatively speaking. We have to infer what it's like deeper down from indirect observations, of which the most important are shock-waves emitted by earthquakes, laboratory experiments, and theory. The surface of our planet generally seems fairly placid, apart from weather and the sometimes severe effects of the seasons, but there are plenty of volcanoes and earthquakes to remind us that not so far below our feet it's a lot less hospitable. Volcanoes form where the molten rocks inside the Earth well up to the surface, often accompanied by massive clouds of gas or ash, all of it emerging under high pressure. In 1980 Mount St Helens in Washington State, USA blew up like a pressure-cooker whose lid had been tied down, and about half of a large mountain simply disappeared. Earthquakes happen when the Earth's crustal rocks slide past each other along deep cracks. Later we'll see what drives these two things, but they need to be put into perspective: despite occasional disasters, the surface of the Earth has been sufficiently hospitable for life to have evolved and survived for several billion years.
The Earth is nearly spherical, having a diameter of 7,928 miles (12,756 km) at the equator but only 7,902 miles (12,714 km) from pole to pole. The slight broadening at the equator is the result of centrifugal forces from the Earth's spin, and originally set in when the planet was molten. The Earth is the densest planet in the solar system, with an average density 5.5 times that of water. When the Earth condensed from the primal dustcloud the chemical elements and compounds that formed it separated into layers: the denser materials sank to the centre of the Earth and the lighter ones floated to the top, much as a layer of light oil floats on denser water.
In 1952 the American geophysicist Francis Birch set out a description of the general structure of our planet which has been modified in only minor ways since. The inside of the Earth is hot, but the pressure there is also very high: the most extreme condi¬tions occur at the centre where the temperature is about 6,000°C and the pressure is 3 million times atmospheric pressure. Heat tends to make rocks and metals melt, but pressure tends to solidify them, so it is the combination of these two conflicting factors that determines whether the materials are liquid or solid. The centre of the Earth is a rather lumpy spherical core, mainly made of iron, with a radius of roughly 2,220 miles (3,500 km). The innermost regions of the core, out to a radius of 600 miles (1000 km), are solid, but a thick outer layer is molten. The very top layers of the Earth form a thin skin, the crust, which is only a few miles thick. Between crust and core lies the mantle, which is solid, formed from a variety of silicate rocks. The mantle also divides into an inner layer and an outer layer, with the division occurring at a radius of about 3,600 miles (5,800 km). Above this 'transition zone' the main rocks are olivine, pyroxine, and garnet; below it their crystal structures become more tightly packed, forming such minerals as perovskite. The outermost parts of the mantle, and the deeper parts of the crust where the two join, are again molten.
The crust is between 3 and 12 miles (5 and 20 km) thick, and there's a lot going on there. Those parts of the crust that form the continental land masses are mainly made of granite. Beneath the oceans, the crustal layer is predominately basalt, and this basalt layer continues underneath the continental granite. So the conti¬nents are broad, thin sheets of granite stuck on top of a basalt skin. From the Earth's surface the most evident features of the granite layers are mountains. The highest ones look big to us, but they rise no more than 5 miles (9 km) above sea level, a mere seventh of a per cent of the Earth's radius. The deepest part of the ocean, the Mariana Trench in the northwest Pacific, plunges 7 miles (11 km) beneath the waves. The overall deviation from an ideal sphere (strictly, spheroid, because of the flattening of the poles) is about one-third of a per cent, about as irregular as the shallow indenta¬tions you find on a basketball, which add to its grip. Our home planet, give or take a bit of squashing, is remarkably round and sur¬prisingly smooth. Gravity made it that way, and it keeps it that way, except that some small but interesting movements in the mantle and the crust add a few wrinkles.
How do we know all this? Mainly because of earthquakes. When an earthquake hits, the whole Earth rings like a bell hit by a ham¬mer. Shockwaves, vibrations emitted by the earthquake, travel through the Earth. They are deflected by transition zones between different kinds of material, such as that between core and mantle, or lower and upper mantle. They bounce off the Earth's crust and head back down again. There are several kinds of wave, and they travel with different speeds. So the short sharp shock of an earth¬quake gives rise to a very complex pattern of waves. When the waves hit the surface they can be detected and recorded, and recordings made in different places can be compared. Working backwards from these recorded signals, it is possible to deduce a certain amount about the underground geography of our planet.

* * *

One consequence of the Earth's internal structure is a magnetic field. A compass needle points roughly north. The standard 'lie-to-children' is that the Earth is a giant magnet. Let's unpack the next layer of explanation.
The Earth's magnetic field has long been something of a puzzle since magnets are seldom made out of rock, but once you realize that the Earth has a whopping great lump of iron inside it, every¬thing makes much more sense. The iron doesn't form a 'permanent' magnet, like the ones you inexplicably buy to stick plastic pigs and teddy bears on the fridge door; it's more like a dynamo. In fact it's called the geomagnetic dynamo. The iron in the core is, as we've said, mostly molten, except for a slightly lumpy solid bit in the mid¬dle. The liquid part is still heating up, the old explanation of this was that radioactive elements are denser than most of the rest of the Earth, and therefore sank to the middle where they became trapped, and their radioactive energy is showing up as heat. The current theory is quite different: the molten part of the core is heat¬ing up because the solid part is cooling down. The liquid iron that is in contact with the solid core is itself slowly solidifying, and when it does so it loses heat. That heat has to go somewhere, and it can't just waft away unnoticed as warm air because everything is thou¬sands of miles underground. So it goes into the molten part of the core and heats it up.
You're probably wondering how the part that is in contact with the solid core can simultaneously be getting cooler, so that it solid¬ifies too, and be getting hotter as a result of that solidification, but what happens is that the hot iron moves away as soon as it's been warmed up. For an analogy, think about a hot air balloon. When you heat air, it rises: the reason is that air expands when it gets hot, so becomes less dense, and less dense things float on top of denser things. A balloon traps the hot air in a huge cloth bag, usually brightly coloured and emblazoned with adverts for banks and estate agents, and floats up along with the air. Now hot iron rises, just as hot air does, and that takes the newly heated iron away from the solid core. It heads upwards, cooling slowly as it does so, and when it gets to the top it cools down, comparatively speaking, and starts to sink again. The result is that the Earth's core circulates up and down, being heated at the bottom and cooling at the top. It can't all go up at the same time, so in some regions it's heading up, and in others it's heading back down again. This kind of heat-driven cir¬culation is called convection.
According to physicists, a moving fluid can develop a magnetic field provided three conditions hold. First, the fluid must be able to conduct electricity, which iron can do fine. Secondly, there has to be at least a tiny magnetic field present to begin with, and there are good reasons to suppose that the Earth had a bit of personal mag¬netism, even early on. Thirdly, something has to twist the fluid, distorting that initial magnetic field, and for the Earth this twist¬ing happens by way of Coriolis forces, which are like centrifugal forces but a bit more subtle, caused by the Earth's rotation on its axis. Roughly speaking, the twisting tangles the original, weak mag¬netic field like spaghetti being twirled on to a fork; then the magnetism bubbles upwards, trapped in the rising parts of the iron core. As a result of these motions, the magnetic field becomes a lot stronger.
So, yes, the Earth does behave a bit as though it had a huge bar magnet buried inside it, but there's rather more going on than that. Just to paint the picture in a little more detail, there are at least seven other factors that contribute to the Earth's magnetic field. Some of the materials of the Earth's crust can form permanent magnets. Like a compass needle pointing north, these materials align themselves with the stronger field from the geomagnetic dynamo and reinforce it. In the upper regions of the atmosphere is a layer of ionized gas, gas bearing an electrical charge. Until satel¬lites were invented, this 'ionosphere' was crucial for radio communications, because radio waves bounced back down off the charged gas instead of beaming off into space. The ionosphere is moving, and moving electricity creates a magnetic field. About 15,000 miles (24,000 km) out lies the ring current, a low-density region of ionized particles forming a huge torus. This slightly reduces the strength of the magnetic field. The next two factors, the magnetopause and the magnetotail, are created by the interaction of the Earth's magnetic field with the solar wind, a continual stream of particles outward bound from our hyperactive sun. The magne¬topause is the 'bow wave' of the Earth's magnetic field as it heads into the solar wind; the magnetotail is the 'wake' on the far side of the Earth, where the Earth's own field streams outwards getting ever more broken up by the solar wind. The solar wind also causes drag along the direction of the Earth's orbit, creating a further kind of motion of magnetic field lines known as field-aligned currents. Finally, there are the convective electrojets. The 'northern lights', or aurora borealis, are dramatic, eerie sheets of pale light that waft and shimmer in the northern polar skies: there is a similar display, the aurora australis, near the south pole. The auroras are generated by two sheets of electrical current that flow from magnetopause to magnetotail; these in turn create magnetic fields, the westward and eastward electrojets.
Yes, like a bar magnet, in the sense that an ocean is like a bowl of-water.

Magnetic materials found in ancient rocks show that every so often, about once every half a million years, but with no sign of regular¬ity, the Earth's magnetic field flips polarity, reversing magnetic north and south. We're not sure exactly why, but mathematical models suggest that the magnetic field can exist in these two orien¬tations, with neither of them being totally stable. So whichever one it's in, it eventually loses stability and flips to the other one. The flips are rapid, taking about 5,000 years; the periods between flips are about a hundred times as long.
Most of the other planets have magnetic fields, and these can be even more complicated and difficult to explain than that of the Earth. We've still got a lot to learn about planetary magnetism.
One of the most dramatic features of our planet was discovered in 1912 but wasn't accepted by science until the 1960s, and some of the most compelling evidence was left by those flips in the Earth's magnetism. This is the notion that the continents are not fixed in place, but wander slowly over the surface of the planet. According to Alfred Wegener, the German who first publicized the idea, all of today's separate continents were originally part of a single super-continent, which he named Pangea ('All-Earth'). Pangea existed about 300 million years ago.
Wegener surely wasn't the first person to speculate along such lines, because he got the idea, in part, at least, from the curious similarity between the shapes of the coasts of Africa and South America. On a map the resemblance is striking. That wasn't Wegener's only source of inspiration, however. He wasn't a geolo¬gist; he was a meteorologist, specializing in ancient climates. Why, he wondered, do we nowadays find rocks in regions with cold cli¬mates that were clearly laid down in regions with warm climates? And why, for that matter, do we nowadays find rocks in regions with warm climates that were clearly laid down in regions with cold cli¬mates? For example, remains of ancient glaciers 420 million years old can still be seen in the Sahara Desert, and fossil ferns are found in Antarctica. Pretty much everyone else thought that the climate must have changed: Wegener became convinced that the climate had stayed much the same, give or take the odd ice age, and the con¬tinents had shifted. Perhaps they'd been driven apart by convection in the mantle, he wasn't sure.
This was considered a crazy idea: it wasn't suggested by a geol¬ogist, and it ignored all sorts of inconvenient evidence, and the alleged fit between South American and Africa wasn't all that good anyway, and-to top it all, there was no conceivable mechanism for carting continents around. Certainly not convection, which was too weak. Great A'Tuin may lug a planet around on its back, but that's fantasy: in the real world, there seemed to be no conceivable way for it to happen.
We use the word 'conceivable' because a number of very bright and very reputable scientists were busily making one of the subject's worst, and commonest, errors. They were confusing 'I can't see a way for this to happen' with There is no way for this to hap¬pen.' One of them, it pains one of us to admit, was a mathematician, and a brilliant one, but when his calculations told him that the Earth's mantle couldn't support forces strong enough to move con¬tinents, it didn't occur to him that the theories on which those calculations were based might be wrong. His name, was Sir Harold Jeffreys, and he really should have been more imaginative, because it wasn't just the shapes of the land on either side of the Atlantic that fitted. The geology fitted too, and so did the fossil record. There is, for example, a fossil beast called Mesosaurus. It lived 270 million years ago, and is found only in South America and Africa. It could¬n't have swum the Atlantic, but it could have evolved on Pangea and spread to both continents before they drifted apart.
In the 1960s, however, Wegener's ideas became orthodox and the theory of 'continental drift' became established, though the ancient supercontinent was renamed Gondwanaland because it dif¬fered in some ways from Wegener's conception of Pangea. At a meeting of leading geologists, a Ponder Stibbons-like young man named Edward Bullard and two colleagues enlisted the aid of a new piece of kit called a computer They instructed the machine to find the best fit between Africa and South America, and North America, and Europe, allowing for a bit of breakage but not too much. Instead of using today's coastline, which was never a very sensible idea but made it possible to claim that the fit wasn't actually that good, they used the contour corresponding to a depth of 3200 feet (1000 m) underwater, whose shape is less likely to have been changed by ero¬sion. The fit was good, and the geology across the join matched amazingly well. And even though the people at the conference came out just as divided in their opinions as they'd been when they went in, somehow continental drift had become the consensus.
Today we have much more evidence, and a fair idea of the mech¬anism. Down the middle of the Atlantic Ocean, and elsewhere in other oceans, there runs a ridge, roughly north-south and about midway between South America and Africa. Volcanic material is welling up along that ridge, and spreading sideways. It's been spreading for 200 million years, and it's still doing it today: we can even send deep-sea submarines down there to watch. It's not spreading at speeds humans can see, America moves about three-quarters of an inch (2 cm) further away from Africa every year, about the same rate that your fingernails grow, but today's instru¬ments can easily measure such a change.
The most striking evidence for continental drift is magnetic: the rocks on either side bear a curious pattern of magnetic stripes, reversing polarity from north to south and back again, and that pat¬tern is symmetric on either side of the ridge, making it clear that the stripes were frozen in place as the rocks cooled in the Earth's magnetic field. Whenever the Earth's dynamo flipped polarity, as it does from time to time, the rock immediately adjacent to the ridge-line, on either side, got the same new polarity. As the rocks then spread apart, they took the same patterns of stripes with them.
The surface of the Earth is not a solid sphere. Instead, the con¬tinents and the ocean-beds float on top of large, essentially solid plates, and those plates can be driven apart by upwelling magma. (Oh, but mostly by convection in the mantle. Jeffreys didn't know what we now know about how the mantle moves.) There are about a dozen plates, ranging from 600 miles (1000 km) across to 6000 miles (10,000 km), and they twist and turn. Where plate boundaries rub against each other, sticking and slipping and sticking and slip¬ping, you get a lot of earthquakes and volcanoes. Especially along the 'Pacific rim', the edge of the Pacific Ocean up along the west coast of Chile, central America, the USA, along down past Japan, and back round New Zealand, which is all the edge of a single gigantic plate. Where plate boundaries collide you get mountain ranges: one plate burrows under the other, lifting it up and crush¬ing and folding its edges. India was once not part of the main Asian continent at all, but came crashing into it, creating the world's high¬est mountain range, the Himalayas. India hasn't fully stopped even now, and the Himalayas are still being pushed up by the force of the impact.



A FIGURE WAS FROGMARCHED through the early-morning corridors, surrounded by the senior wizards. It wore a long white nightshirt, and a nightcap with the word 'Wizzard' embroidered, inexpertly, on it. It was Unseen University's least qualified but most well-travelled member, usually away from some¬thing. And it was in trouble.
'This won't hurt a bit,' said the Senior Wrangler.
'It's right up your street,' said the Lecturer in Recent Runes.
'It's on a log and in your face,' explained the Dean.
'That isn't what HEX said, is it?' said the Senior Wrangler, as the sleepy figure was hustled around a corner.
'Very similar, but what HEX said made less sense,' said the Dean.
They hurried across the lawn and barged through the doors in the High Energy Magic Building.
Mustrum Ridcully finished filling his pipe, and struck a match on the dome of the Project. Then he turned, and smiled.
'Ah, Rincewind,' he said. 'Good of you to come.'
'I was dragged, sir.'
'Well done. And I have good news. I intend to appoint you Egregious Professor of Cruel and Unusual Geography. The post is vacant.'
Rincewind looked past him. On the far side of the room some of the junior wizards were working in a haze of magic that made it hard to see exactly what it was they were working on, but it looked almost like ... some sort of skeleton.
'Oh,' he said. 'Er ... but I'm very happy as assistant librarian. I'm getting really good at peeling the bananas.'
'But the new post offers you room, board and all your laundry done,' said the Archchancellor.
'But I get that already, sir.'
Ridcully drew leisurely on his pipe and blew out a cloud of blue smoke.
'Up until now,' he said.
'Oh. I see. And you're about to send me somewhere really dan¬gerous, yes?'
Ridcully beamed. 'How did you guess?'
'It wasn't a guess.'
Fortunately the Dean had been forewarned and had grabbed the back of Rincewind's nightshirt, and so he was ready. The wizard's bedroom slippers skidded uselessly on the tiles as he tried to make for the door.
'It's best to let him run for a little while,', said the Senior Wrangler. 'It's a nervous reaction.'
'And the best thing is,' said Ridcully, to Rincewind's back, 'that although we are sending you to a place of immense danger where no living thing could possibly survive, you will not, in so many words, actually be there. Won't that be nice?'
Rincewind hesitated.
'How many words?'
'It'll be like being in a ... story,' said the Archchancellor. 'Or ... or a dream, as far as I can understand it. Mister Stibbons! Come and explain!'
'Oh, hello, Rincewind,' said Ponder, stepping out of the mist and wiping his hands on a rag. 'Twelve spells HEX has amalgamated for this! It's an amazing piece of thaumaturgical engineering! Do come and see!'
There are creatures which have evolved to live in coral reefs and simply could not survive in the rough, tooth-filled wastes of the open sea. They continue to exist by lurking among the dangerous tentacles of the sea anemone or around the lips of the giant clam and other perilous crevices shunned by all sensible fish.
A university is very much like a coral reef. It provides calm waters and food particles for delicate yet marvellously constructed organisms that could not possibly survive in the pounding surf of reality, where people ask questions like 'Is what you do of any use?' and other nonsense.
In fact Rincewind in his association with UU had survived dan¬gers that would have stripped a hero to the bone, but he nevertheless believed, despite all the evidence to the contrary, that he was safe in the university. He would do anything to stay on the roll.
At the moment this involved looking at some sort of skeletal armour made out of smoke while Ponder Stibbons gabbled incom¬prehensible words in his ear. As far as he could understand it, the thing put all your senses somewhere else when you stayed here. So far, that sounded quite acceptable, since it had always seemed to Rincewind that if you had to go a long way away it'd be nice to stay at home while you did it, but people seemed a little unclear about where pain fitted in.
'We'll send you, that is, your senses, somewhere,' said Ridcully.
'Where?' said Rincewind.
'Somewhere amazing,' said Ponder. 'We just want you to tell us what you see. And then we'll bring you back.'
'At what point will things go wrong?' said Rincewind.
'Nothing can possibly go wrong.'
'Oh,' Rincewind sighed. There was no point in arguing with a statement like that. 'Could I have some breakfast first?'
'Of course, dear fellow,' said Ridcully, patting him on the back. 'Have a hearty meal!'
'Yes, I thought that'd probably be the case,' said Rincewind gloomily.
When he'd been taken away, under escort by the Dean and a cou¬ple of college porters, the wizards clustered around the project.
'We've found a suitably large "sun", sir,' said Ponder, taking care to annunciate the inverted commas. 'We're moving the world now,'
'A very suspicious idea, this,' said the Archchancellor. 'Suns go around. We see it happen every day. It's not some kind of optical illusion. This is a bit of a house of cards we're building here.'
'It's the only one available, sir.'
'I mean, things fall down because they're heavy, you see? The thing that causes them to fall down because they're heavy is, in fact, the fact that they're heavy. 'Heavy' means inclined to fall down. And, while you can call me Mr Silly...’
'Oh, I wouldn't do that, sir,' Ponder said, glad that Ridcully couldn't see his face.
‘...I somehow feel that a crust of rock floating around on a ball of red-hot iron should not be thought of as "solid ground".'
'I think, sir, that this universe has a whole parcel of rules that take the place of narrativium,' said Ponder. 'It's ... sort of ... copy¬ing us, as you so perspicaciously pointed out the other day. It's making the only kind of suns that can work in it, and the only worlds that can exist if you don't have chelonium.'
'Even so ... going around a sun ,.. that's the sort of thing the Omnian priests used to teach, you know. Mankind is so insignificant that we just float around on some speck, and all that superstitious stuff. You know they used to persecute people for saying the turtle existed? And any fool can see it exists.'
'Yes, sir It certainly does.'

There were problems, of course
'Are you sure it's the right sort of sun?' said Ridcully.
'You told HEX to find one that was "nice and yellow, nice and dull, and not likely to go off bang", sir,' said Ponder. 'It seems to be a pretty average one for this universe.'
'Even so ... tens of millions of miles ... that's a long way away for our world.'
'Yes, sir. But we tried some experimental worlds close to and they fell in, and we tried one a bit further out and that's baked like a biscuit, and there's one ... well, it's a bit of an armpit, really. The students have got quite good at making different sorts. Er ... we're calling them planets.'
'A planet, Stibbons, is a lump of rock a mere few hundred yards across which gives the night sky a little, oh, I don't know, what's the word, a little je ne sais quoi’
'These will work, sir, and we've such a lot of them. As I said, sir, I've come to agree with your theory that, within the Project, matter is trying to do all by itself what in the real world is done by purpose, probably conveyed via narrativium.'
'Was that my theory?' said Ridcully.
'Oh, yes, sir,' said Ponder, who was learning the particular sur¬vival skills of the academic reef.
'It sounds rather a parody to me, but I dare say we will under¬stand the joke in time. Ah, here comes our explorer. 'Morning, Professor,' said Ridcully. 'Are you ready?'
'No,' said Rincewind.
'It's very simple,' said Ponder, leading the reluctant traveller across the floor. 'You can think of this assemblage of spells as a suit of very, very good armour. Things will flicker , and then you'll be ... somewhere else. Except you'll really be here, you see? But every¬thing you see will be somewhere else. Absolutely nothing will hurt you because HEX will buffer all extreme sensations and you'll sim¬ply received a gentle analogue of them. If it's freezing you'll feel rather chilly, if it's boiling you'll feel a little hot. If a mountain falls on you it'll be a bit of a knock. Time where you're going is moving very fast but HEX can slow it down while you are there, HEX says that he can probably exert small amounts of force within the Project, so you will be able to lift and push things, although it will feel as though you're wearing very large gloves. But this should not be required because all we want you to do to start with ... Professor ... is tell us what you see.'
Rincewind looked at the suit. It was, being largely make up of spells under HEX's control, shimmery and insubstantial. Light reflected off it in odd ways. The helmet was far too large and com¬pletely covered the face.
'I have three ... no, four ... no, five questions,' he said.
'Can I resign?'
'Do I have to understand anything you just told me?'
'Are there any monsters where I'm being sent?'
'Are you sure?' 'Yes.'
'Are you totally positive about that?'
'I've just thought of another question,' said Rincewind.
'Fire away.'
'Are you really sure?'
'Yes!' snapped Ponder 'And even if there were any monsters, it wouldn't matter.'
'It'd matter to me.'
'No it wouldn't! I have explained! If some huge toothed beast came galloping towards you, it'd have no effect on you at all.'
'Another question?'
'Is there a toilet in this suit?'
'Because there will be if a huge toothed beast comes galloping towards me.'
'In that case, you just say the word and you can come back and use the privy down the hall,' said Ponder. 'Now, stop worrying, please. These gentlemen will help you, er, insert yourself into the thing, and we'll begin ...'
The Archchancellor wandered up as the reluctant professor was enveloped in the glittering, not-quite-there stuff.
'A thought occurs, Ponder,' he said.
'Yes, sir?'
'I suppose there's no chance that there is life anywhere in the Project?'
Ponder looked at him in frank astonishment.
'Absolutely not, sir! It can't happen. Simple matter is obeying a few rather odd rules. That's probably enough to get things ... spin¬ning and exploding and so on, but there's no possibility that they could cause anything so complex as...’
'The Bursar, for example?'
'Not even the Bursar, sir.'
'He's not very complicated, though. If only we could find a parrot that was good at sums, we could pension the old chap off.'
'No, sir. There's nothing like the Bursar. Not even an ant or a blade of grass. You might as well try to tune a piano by throwing rocks at it. Life does not turn up out of nowhere, sir. Life is a lot more than just rocks moving in circles. The one thing we're not going to run into is monsters.'

Two minutes later Rincewind blinked and found, when he opened his eyes, that they were somewhere else. There was a rather grainy redness in front of them, and he felt rather warm.
'I don't think it's working,' he said.
'You should be seeing a landscape,' said Ponder, in his ear.
'It's all just red.'
There was the sound of distant whispering. Then the voice said, 'Sorry. The aim wasn't very good. Wait a moment and we'll soon have you out of that volcanic vent.'
In the HEM Ponder took the ear trumpet away from his ear. The other wizards heard it sizzling, as if a very angry insect was trapped therein.
'Curious language,' he said, in mild surprise, 'well, let's raise him somewhat and let time move on a little ...'
He put the trumpet to his ear and listened.
'He says it's pissing down,' he announced.

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IT'S CERTAINLY A SURPRISE that the rigid rules of physics permit anything as flexible as life, and the wizards can hardly be blamed for not anticipating the possibility that living creatures might come into being on the barren rocks of Roundworld. But Down Here is not as different from Up There as it seems. Before we can talk about life, though, we need to deal with a few more features of our home planet: atmosphere and oceans. Without them, life as we know it could not have arisen; without life as we know it, our oceans and atmosphere would be distinctly different. The story of the Earth's atmosphere is inextricably intertwined with that of its oceans. Indeed, the oceans can reasonably be viewed as just a rather damp, dense layer of the atmosphere. The oceans and the atmosphere evolved together, exerting strong influences on each other, and even today such an 'obviously' atmospheric phe¬nomenon as weather turns out to be closely related to what happens in the oceans. One of the main recent breakthroughs in weather prediction has been to incorporate the oceans' ability to absorb, transport, and give off heat and moisture. To some extent, the same point can be made about the solid regions of the Earth, which also co-evolved with the air and the seas, and also interact with them. But the link between oceans and atmosphere is stronger.
The Earth and its atmosphere condensed together out of the primal gascloud that gave rise to the Sun and to the solar system. As a rough rule of thumb, the denser materials sank to the bottom of the condensing clump of matter that we now inhabit, and the lighter ones floated to the top. Of course there was, and still is, a lot more going on than that, so the Earth is not just a series of concen¬tric shells of lighter and lighter matter, but the general distribution of solids, liquids, and gases makes sense if you think about it that way. And so, as the molten rocks of Earth began to cool and solid¬ify, the nascent planet found itself already enveloped in a primordial atmosphere.
It was almost certainly very different from the atmosphere today, which is a mixture of gases, the main ones being the elements nitro¬gen, oxygen and the inert gas argon, and the compounds carbon dioxide and water (in the form of vapour). The primordial atmos¬phere also differed considerably from the gas cloud out of which it condensed, it wasn't just a representative sample of what was around. There are several reasons for this. One is that a solid planet and a gas cloud retain different gases. Another is that a solid planet can generate gases, by chemical or even nuclear reactions, or by other physical processes, which can escape from its interior into its atmosphere.
The early cloud was rich in hydrogen and helium, the lightest of elements. The speed with which a molecule moves becomes slower as the molecule gets heavier, a molecule with one hundred times the mass moves at about one-tenth the speed. Anything that moves faster than the Earth's escape velocity, about 7 miles per second (11 km/sec), can overcome the planet's gravity and disappear into space. Molecules in the atmosphere whose molecular weight, what you get by adding up the atomic weights of the component atoms -is less than about 10 should therefore disappear into the void. Hydrogen has molecular weight 2, helium 4, so neither of these otherwise abundant gases should be expected to hang around. The most abundant molecules in the primal gas cloud, with molecular weight greater than 10, are methane, ammonia, water, and neon. This is similar to what we find today on the gas giants Jupiter, Saturn, Uranus, and Neptune, except that they are more massive, so have a greater escape velocity, and can retain lighter gases such as hydrogen and helium as well. We can't be certain that the Earth of 4 billion years ago possessed a methane-ammonia atmosphere, because we don't know exactly how the primal gas cloud condensed, but it is clear that if the ancient Earth ever possessed such an atmos¬phere, it lost nearly all of it. Today there is little methane or ammonia, and what there is has a biological origin.
Shortly after the Earth was formed, the atmosphere contained very little oxygen. Around 2 billion years ago, the proportion of oxy¬gen in the atmosphere increased to about 5%. The most likely cause of this change, though perhaps not the only one, was the evolu¬tion of photosynthesis. At some stage, probably around 2 billion years ago, bacteria in the oceans evolved the trick of using the energy of sunlight to turn water and carbon dioxide into sugar and oxygen. Plants use the same trick today, and they use the same molecules as one of the early bacteria did: chlorophyll. Animals proceed in pretty much the opposite direction: they power themselves by using oxy¬gen to burn food, producing carbon dioxide instead of using it up. Those early photosynthesizing bacteria used the sugar for energy, and multiplied rapidly, but to them the oxygen was just a form of toxic waste, which bubbled up into the atmosphere. The oxygen level then stayed roughly constant until about 600 million years ago, when it underwent a rapid increase to the current level of 21%.
The amount of oxygen in today's atmosphere is far greater than could ever be sustained without the influence of living creatures, which not only produce oxygen in huge quantities but use it up again, in particular locking it up in carbon dioxide. It is startling how far 'out of balance' the atmosphere is, compared to what would happen if life were suddenly removed and only inorganic chemical processes could act. The amount of oxygen in the atmosphere is dynamic, it can change on a timescale that by geological standards is extremely rapid, a matter of centuries rather than millions of years. For example, if some disaster occurred which killed off all the plants but left all the animals, then the proportion of oxygen would halve in about 500 years, to the level on mountain peaks in the Andes today. The same goes for the scenario of 'nuclear winter' introduced by Carl Sagan, in which clouds of dust thrown into the atmosphere by a nuclear war stop most of the sunlight from reach¬ing the ground. In this case, plants may still eke out some kind of existence, but they don't photosynthesize: they do use oxygen, though, and so do the microorganisms that break down dead plants.
The same screening effect could also occur if there were unusual numbers of active volcanoes, or if a big meteorite or comet hit the Earth. When comet Shoemaker-Levy 9 hit Jupiter in 1994, the impact was equivalent to half a million hydrogen bombs.
The 'budget' of income and expenditure for oxygen, and the associated but distinct budget for carbon, is still not understood. This is an enormously important question because it is vital back¬ground to the debate about global warming. Human activities, such as electrical power plants, industry, use of cars, or simply going about one's usual business and breathing while one does so, gener¬ate carbon dioxide. Carbon dioxide is a 'greenhouse gas' which traps incoming sunlight like the glass of a greenhouse. So if we pro¬duce too much carbon dioxide, the planet should warm up. This would have undesirable consequences, ranging from floods in low-lying regions such as Bangladesh to big changes in the geographical ranges of insects, which could inflict serious damage on crops. The question is: do these human activities actually increase the Earth's carbon dioxide, or does the planet compensate in some way? The answer makes the difference between imposing major restrictions on how people in developed (and developing) countries live their lives, and letting them continue along their current paths. The cur¬rent consensus is that there are clear, though subtle, signs that human activities do increase the carbon dioxide levels, which is why major international treaties have been signed to reduce carbon diox¬ide output. (Actually taking that action, rather than just promising to do so, may prove to be a different matter altogether.)
The difficulties involved in being sure are many. We don't have good records of past levels of carbon dioxide, so we lack a suitable 'benchmark' against which to assess today's levels, although we're beginning to get a clearer picture thanks to ice cores drilled up from the Arctic and Antarctic, which contain trapped samples of ancient atmospheres. If 'global warming' is under way, it need not show up as an increase in temperature anyway (so the name is a bit silly). What it shows up as is climatic disturbance. So even though the six warmest summers in Britain this century have all occurred in the nineties, we can't simply conclude that 'it's getting warmer', and hence that global warming is a fact. The global climate varies wildly anyway, what would it be doing if we weren't here?
A project known as Biosphere II attempted to sort out the basic science of oxygen/carbon transactions in the global ecosystem by setting up a 'closed' ecology, a system with no inputs, beyond sun¬light, and no outputs whatsoever. In form it was like a gigantic futuristic garden centre, with plants, insects, birds, mammals, and people living inside it. The idea was to keep the ecology working by choosing a design in which everything was recycled.
The project quickly ran into trouble: in order to keep it running, it was necessary to keep adding oxygen. The investigators therefore assumed that somehow oxygen was being lost. This turned out to be true, in a way, but for nowhere near as literal a reason. Even though the whole idea was to monitor chemical and other changes in a closed system, the investigators hadn't weighed how much carbon they'd introduced at the start. There were good reasons for the omission, mostly, it's extremely difficult, since you have to esti¬mate carbon content from the wet weight of live plants. Not knowing how much carbon was really there to begin with, they couldn't keep track of what was happening to carbon monoxide and carbon dioxide. However, 'missing' oxygen ought to show up as increased carbon dioxide, and they could monitor the carbon diox¬ide level and see that it wasn't going up.
Eventually it turned out that the 'missing' oxygen wasn't escap¬ing from the building: it was being turned into carbon dioxide. So why didn't they see increased carbon dioxide levels? Because, unknown to anybody, carbon dioxide was being absorbed by the building's concrete as it 'cured'. Every architect knows that this process goes on for ten years or so after concrete has set, but this knowledge is irrelevant to architecture. The experimental ecologists knew nothing about it at all, because esoteric properties of poured concrete don't normally feature in ecology courses, but to them the knowledge was vital.
Behind the unwarranted assumptions that were made about Biosphere II was a plausible but irrational belief that because car¬bon dioxide uses up oxygen when it is formed, then carbon dioxide is opposite to oxygen. That is, oxygen counts as a credit in the oxy¬gen budget, but carbon dioxide counts as a debit. So when carbon dioxide disappears from the books, it is interpreted as a debt can¬celled, that is, a credit. Actually, however, carbon dioxide contains a positive quantity of oxygen, so when you lose carbon dioxide you lose oxygen too. But since what you're looking for is an increase in carbon dioxide, you won't notice if some of it is being lost.
The fallacy of this kind of reasoning has far wider importance than the fate of Biosphere II. An important example within the gen¬eral frame of the carbon/oxygen budget is the role of rainforests. In Brazil, the rainforests of the Amazon are being destroyed at an alarming rate by bulldozing and burning. There are many excellent reasons to prevent this continuing, loss of habitat for organisms, production of carbon dioxide from burning trees, destruction of the culture of native Indian tribes, and so on. What is not a good rea¬son, though, is the phrase that is almost inevitably trotted out, to the effect that the rainforests are the 'lungs of the planet'. The image here is that the 'civilized' regions, that is, the industrialized ones, are net producers of carbon dioxide. The pristine rainforest, in contrast, produces a gentle but enormous oxygen breeze, while absorbing the excess carbon dioxide produced by all those nasty people with cars. It must do, surely? A forest is full of plants, and plants produce oxygen.
No, they don't. The net oxygen production of a rainforest is, on average, zero. Trees produce carbon dioxide at night, when they are not photosynthesizing. They lock up oxygen and carbon into sug¬ars, yes, but when they die, they rot, and release carbon dioxide. Forests can indirectly remove carbon dioxide by removing carbon and locking it up as coal or peat, and by releasing oxygen into the atmosphere. Ironically, that's where a lot of the human production of carbon dioxide comes from, we dig it up and burn it again, using up the same amount of oxygen.
If the theory that oil is the remains of plants from the carbonif¬erous period is true, then our cars are burning up carbon that was once laid down by plants. Even if an alternative theory, growing in popularity, is true, and oil was produced by bacteria, then the prob¬lem remains the same. Either way, if you burn a rainforest you add a one-off surplus of carbon dioxide to the atmosphere, but you do not also reduce the Earth's capacity to generate new oxygen. If you want to reduce atmospheric carbon dioxide permanently, and not just cut short-term emissions, the best bet is to build up a big library at home, locking carbon into paper, or put plenty of asphalt on roads. These don't sound like 'green' activities, but they are. You can cycle on the roads if it makes you feel better.
Another important atmospheric component is nitrogen. It is a lot easier to keep track of the nitrogen budget. Organisms, plants especially, as every gardener knows, need nitrogen for growth, but they can't just absorb it from the air. It has to be 'fixed', that is, combined into compounds that organisms can use. Some of the fixed nitrogen is produced as nitric acid, which rains down after thunderstorms, but most nitrogen fixation is biological. Many sim¬ple lifeforms 'fix' nitrogen, using it as a component of their own amino-acids. These amino-acids can then be used in everybody else's proteins.

The Earth's oceans contain a huge quantity of water, about a third of a billion cubic miles (1.3 billion cubic km). How much water there was in the earliest stages of the Earth's evolution, and how it was distributed over the surface of the globe, we have little idea, but the existence of fossils from about 3.3 billion years ago shows that there must have been water around at that time, probably quite a lot. As we've already explained, the Earth, along with the rest of the solar system, Sun included, condensed from a vast cloud of gas and dust, whose main constituent was hydrogen. Hydrogen com¬bines readily with oxygen to form water, but it also combines with carbon to form methane and with nitrogen to form ammonia.
The primitive Earth's atmosphere contained a lot of hydrogen and a fair quantity of water vapour, but initially the planet was too hot for liquid water to exist. As the planet slowly cooled, its surface passed a critical temperature, the boiling point of water. That tem¬perature was probably not exactly the same as the one at which water boils now; in fact even today it's not one inflexible tempera¬ture, because the boiling point of water depends on pressure and other circumstances. Nor was it just a simple matter of the atmosphere's getting colder: its composition also changed because the Earth was spouting out gases from its interior through volcanic activity.
A crucial factor was the influence of sunlight, which split some of the atmospheric water vapour into oxygen and hydrogen. The hydrogen escaped from the Earth's relatively weak gravitational field, so the proportion of oxygen got bigger while that of water vapour got smaller. The effect of this was to increase the tempera¬ture at which the water vapour could condense. So as the temperature of the atmosphere slowly fell, the temperature at which water vapour would condense rose to meet it. Eventually the atmosphere going down passed the boiling point of water going up, and water vapour began to condense into liquid water ... and to fall as rain.
It must have absolutely bucketed down.
When the rain hit the hot rocks beneath, it promptly evaporated back into vapour, but as it did so it cooled the rocks. Heat and tem¬perature are not the same. Heat is equivalent to energy: when you heat something, you input extra energy. Temperature is one of the ways in which that energy can be expressed: it is the vibration of molecules. The faster those vibrations are, the higher the tempera¬ture. Ordinarily, the temperature of a substance goes Up if you heat it: all the extra heat is expressed as more vibration of the molecules. However, at transitions from solid to liquid, or liquid to vapour or gas, the extra heat goes into changing the state of the substance, not into making its temperature higher. So you can throw in a lot of heat and instead of the stuff getting hotter, it changes state, a so-called phase transition. Conversely, when a substance cools through a phase transition, it gives off a lot of heat. So the cooling water vapour put more heat back into the upper atmosphere, from which it could be radiated away into space and lost. When the hot rocks turned the water back into vapour, the rocks got a lot cooler very suddenly. In a geologically short space of time, the rocks had cooled below the boiling point of water, and now the falling rain no longer got turned back into vapour, at least, not much of it did.
It may well have rained for a million years. So it's not surprising that Rincewind noticed that it was a bit wet.
Thanks to gravity, water goes downhill, so all that rain accumu¬lated in the lowest depressions in the Earth's irregular surface. Because the atmosphere had a lot of carbon dioxide in it, those early oceans contained a lot of dissolved carbon dioxide, making the water slightly acidic. There may have been hydrochloric and sul¬phuric acids too. The acid ate away at the surface rocks, causing minerals to dissolve in the oceans; the sea began to get salty.
At first the amount of oxygen in the atmosphere increased slowly, because the effect of incoming sunlight isn't particularly dramatic. But now life got in on the act, bubbling off oxygen as a by¬product of photosynthesis. The oxygen combined with any remaining hydrogen in the atmosphere, whether on its own or com¬bined inside methane, to produce more water. This also fell as rain, and increased the amount of ocean, leading to more bacteria, more oxygen, and so it continued until the available hydrogen pretty much ran out.
Originally it used to be thought that the oceans just kept dis¬solving the rocks of the continents, accumulating more and more minerals, getting saltier and saltier until the amount of salt reached its current value of about 3.5%. The evidence for this is the per¬centage of salt in the blood of fishes and mammals, which is about 1%. In effect, it was believed that fish and mammal blood were 'fos¬silized' ocean. Today we are still often told that we have ancient seas in our blood. This is probably wrong, but the argument is far from settled. It is true that our blood is salty, and so is the sea, but there are plenty of ways for biology to adjust salt content. That 1% may just be whatever level of salt makes best sense for the creature whose blood it is. Salt, more properly, the ions of sodium and chlorine into which it decomposes, have many biological uses: our nervous systems, for instance, wouldn't work without them. So while it is entirely believable that evolution took advantage of the existence of salt in the sea, it need not be stuck with the same proportion. On the other hand, there is good reason to think that cells first evolved as tiny free-floating organisms in the oceans, and those early cells weren't sophisticated enough to fight against a difference in salt concentration between their insides and their outsides, so they may well have settled on the same concentration because that was all they could initially manage, and having done so, they were rather stuck with it.
Can we decide by taking a more careful look at the oceans? Oceans have ways to lose salt as well as gaining it. Seas can dry out; the Dead Sea in Israel is a famous example. There are salt mines all over the place, relics of ancient dried-up seas. And just as living creatures, bacteria, took out carbon dioxide, turning it into oxy¬gen and sugar, so they can take out other dissolved minerals too. Calcium, carbon and oxygen go into shells, for instance, which fall to the ocean floor when their owner dies. The clincher is ... time. The oceans are thought to have reached their current composition, and in particular their current degree of saltiness, about 2 to 1.5 bil¬lion years ago. The evidence is the chemical composition of sedimentary rocks, rocks formed from deposits of shells and other hard parts of organisms, which seems not to have changed much in the interim. (Though in 1998 Paul Knauth presented evidence that the early ocean may have been more salty than it is now, with some¬where between 1.5 to 2 times as much salt. His calculations indicate that salt could not have been deposited on the continents until about 2.5 billion years ago.) Simple calculations based on how much mate¬rial dissolves in rivers and how fast rivers flow show that the entire salt content of the oceans can be supplied from dissolved continen¬tal rocks in twelve million years, the twinkling of a geological eye. If salt had just built up steadily, the oceans would now be far more salt than water So the oceans are not simply sinks for dissolved min¬erals, one-way streets into which minerals flow and get trapped. They are mineral-processing machines. The geological evidence of the similarity of ancient and modern sedimentary rocks suggests that the inflow and the outflow pretty much balance each other.
So do we have ancient seas in our blood? In a way. The propor¬tions of magnesium, calcium, potassium, and sodium are exactly the same as they were in the ancient seas from which our blood may have evolved, but cells seem to prefer a salt concentration of 1%, not 3%.



'HE'S RIGHT ABOUT THE RAIN,' said the Senior Wrangler, who was at the omniscope. 'You've got clouds again. And there's lots of volcanoes.'
'I'mmoving him on further ... Oh. Now he says it's dark and cold and he's got a headache ...' 'Not very graphic, is it?' said the Dean. 'He says it's a splitting headache.' HEX wrote something.
'Oh,' said Ponder 'He's under water. I'm sorry about that, I'm afraid he's a little hard to position accurately. We're still not sure what size he should be. How's this?'
The trumpet rattled, 'He's still under water, but he says he can see the surface. I think that's as good as we're going to get. Just walk forward.'
As one wizard, they turned to watch the suit. It hung in the air, a few inches above the floor. As they watched, the figure inside made hesitant walking motions.

It was not a nice day.
It was still raining, although it had slackened off recently, with sporadic outbreaks during the early part of the millennium and scattered showers during the last couple of decades. Now ten thou¬sand rivers were finding their way to the sea. The light was grey and gave the beach a flat, monochrome, and certainly very damp look.
Whole religions have been inspired by the sight of a figure emerging, miraculously, from the sea. It would be hard to guess at what strange cult might be inspired by the thing now trudging out of the waves, although avoidance of strong drink and certainly of seafood would probably be high on its list of 'don'ts'.
Rincewind looked around.
There was no sand underfoot. The water sucked at an expanse of rough lava. There was no seaweed, no seabirds, no little crabs -nothing potentially dangerous at all.
'There's not a lot going on,' he said. 'It's all rather dull.'
'It'll be dawn in a moment,' said Bonder's voice in his ear. 'We'll be interested to see what you think of it.'
Strange way of putting it, Rincewind thought, as he watched the sun come up. It was hidden behind the clouds, but a greyish-yellow light picked its way across the landscape.
'It's all right,' he said. 'The sky's a dirty colour. Where is this? Llamedos? Hergen? Where aren't there any seashells? Is this high tide?'

All the wizards were trying to speak at once.
'I can't think of everything, sir!'
'But everyone knows about tides!'
'Perhaps some mechanism for raising and lowering the sea bed would be acceptable?'
'If it comes to that, what causes tides here?'
'Can we all please stop shouting?'
The babble died down.
'Good,' said Ridcully. 'Over to you, Mister Stibbons.'
Stibbons stared at the notes in front of him.
'I'm ... there's ... it's a puzzler, sin On a round world the sea just sits there. There's no edge for it to pour off.'
'It's always been believed that the sea is in some way attracted to the moon,' the Senior Wrangler mused. 'You know ... the attraction of serene beauty and so on.'
Dead silence fell.
Finally, Ponder managed: 'No one said anything to me about a moon.'
'You've got to have a moon,' said Ridcully.
'It should be easy, shouldn't it?' said the Dean. 'Our moon goes around the Disc.'
'But where can we put it?' said Ponder. 'It's got to be light and dark, we've got to move it for phases, and it's got to be almost as big as the sun and we know that if you try to make things sun-sized here they, well, become suns.'
'Our moon is closer than the sun,' said the Dean. 'That's why we get eclipses.'
'Only about ninety miles,' said Ponder That's why it's burned black on one side.'
'Dear me, Mister Stibbons, I'm surprised at you,' said Ridcully. 'The damn great sun looks pretty big even though it's a long way away. Put the moon nearer.'
'We've still got the big lump that the Dean knocked out of the planet,' said the Senior Wrangler. 'I made the students park it around the Target.'
'Target?' said Ponder.
'It's the big fat planet with the coloured lines on,' said the Senior Wrangler 'I made them bring the whole lot out to the new, er, sun because frankly they were a nuisance where they were. At least when they're spinning round you know where they're coming from.'
'Are the students still sneaking in here at night to play games?' said Ridcully.
'I've put a stop to that,' said the Dean. 'There's too many rocks and snowballs around this sun in any case. Masses of the things. Such a waste.'
'Well, can we get the lost lump here soon?'
'HEX can manipulate time from Rincewind's point of view,' said Ponder. 'For us, Project time is very fast ... we should get it here before the coffee arrives.'

'Can you hear me, Rincewind?'
'Yes. Any chance of some lunch?'
'We're getting you some sandwiches. Now, can you see the sun properly?'
'It's all very hazy, but yes.'
'Can you tell me what happens if I do ... this?'
Rincewind squinted into the grey sky. Shadows were racing across the landscape.
'You're not going to tell me you've just caused an eclipse of the sun, are you?'
Rincewind could hear faint cheering in the background.
'And you're quite certain it's an eclipse?' said Ponder.
'What else is it? A black disc is covering the sun and there's no birdsong.'
'Is it about the right size?'
'What kind of question is that?'
'All right, all right. Ah, here are your san, what? How? Excuse me ... now what? ...'
The senior wizards were puzzled again, and demonstrated this by prodding Ponder while he was trying to talk. The wizards were great ones for the prod as a means of getting attention.
'You can see there's only one moon,' said the Senior Wrangler, for the third time.
'All right ... how about this?' said Ponder. 'Let us suppose that in some way this world has got both water that likes moons and water that can't stand moons at any price. If it's got about the same amount of both, then that at least explains why there seem to be high tides on both sides at once. I think we can dispose of the Invisible Moon theory, interesting though it was, Dean.'
'I like that explanation,' said Ridcully. 'It is elegant, Mister Stibbons.'
'It's only a guess, sir.'
'Good enough for physics,' said Ridcully.
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HUMANITY HAS ALWAYS KNOWN the Moon is impor¬tant. It often comes out at night, which is useful; it changes, in a sky where change is rare; some of us believe our ancestors live there. That last one might not be capable of experimental verification, but nevertheless humanity in general got it right. The Moon reaches out ghostly tentacles, gravity and light; it may even be our protector.
The wizards are right to worry that they've forgotten to give Roundworld a Moon, though as usual they're worried for the wrong reasons.
The Moon is a satellite of the Earth: we go round the Sun, but the Moon goes round us. It's been up there for a long time, and in its quiet way it's been exceedingly busy. The Moon affects people as well as baby turtles. The main way it affects us is by causing tides. It may affect us in other, less obvious ways, although many common beliefs about the moon are, to say the least, scientifically controver¬sial. The female menstrual cycle repeats roughly every four weeks, much the same time that it takes the moon to go round the Earth -one month, in fact, a word that comes from 'moon'. In popular belief this numerical similarity is no coincidence, as for example in 'the wrong time of the month'. On the other hand, the Moon is the epitome of regularity, as predictable as the date of Christmas day, which cannot be said of the menstrual cycle. Lovers, of course, swoon and spoon beneath the Moon in June ... It is also widely held that people go mad when there is a full Moon, or, a more extreme type of madness, those who are suitably afflicted turn into wolves for a night.
The werewolf legend plays a central role in Men at Arms. Most of the time lance-constable Angua of the Ankh-Morpork city watch is a well-built ash-blonde, but when the Moon is full she turns into a wolf who can smell colours and rip out people's jugular veins. But it does play havoc with her private life. 'It was always a problem, growing fangs and hair every full moon. Just when she thought she'd been lucky before, she'd found that few men are happy in a relationship where their partner grows hair and howls.' Fortunately Corporal Carrot is unperturbed by these occasional changes. He likes a girlfriend who enjoys long walks.
The Moon is unusual, and it is quite likely that without it, none of us would be here at all. Not because of the alleged effect on lovers, who find a way Moon or no, but because the Moon protects the Earth from some nasty influences that might have made it dif¬ficult for life to have arisen, or at least to have got beyond the most rudimentary forms. What makes the Moon unusual is not that it is a companion to a planet: all of the planets except Mercury and Venus have moons. It is remarkable because it is so big in compari¬son to its parent planet. Only Pluto has a satellite, Charon, discovered in 1978 by Jim Christy, that is comparable in relative size to our Moon. It's not stretching things much to say that we live on one half of a double planet.
We know the Moon is very different from the Earth in all sorts of ways. Its gravity is weaker, so it wouldn't be able to keep an atmosphere for very long, even if it had one, which it doesn't by any sensible use of the term. The Moon's surface is rock and rock dust, with no seas anywhere (water easily escapes too), although in 1997 NASA probes discovered substantial quantities of water ice at the Moon's poles, hidden from the warmth of the Sun by the perma¬nent shadows of crater walls. That's good news for future lunar colonies, which could act as bases for the exploration of the solar system. The Moon is a good place to start from, because your spaceship doesn't need much fuel to escape the Moon's pull; the Earth is of course a bad place to start from, because down here gravity is so much stronger. How typical of humans to have evolved in the wrong place ...
How was the Moon formed? Did it condense out of the primal dustclouds along with the Earth? Did it form separately and get captured later? Are the craters extinct volcanoes, or are they marks made by lumps of rock smashing into the Moon? We know rather more about the Moon than we do about most other bodies in the solar system, because we've been there. In April 1969, Neil Armstrong stepped down on to the surface of the Moon, fluffed his lines, and made history. Between 1968 and 1972 the United States sent ten Apollo missions to the Moon and back. Of these, Apollos 8,9, and 10 were never intended to land; Apollo-11 was that historic first landing; and Apollo-13 never made it down to the surface, suf¬fering a disastrous explosion early in its flight and turning into an excellent movie.
The rest of Apollos 11-17 landed, and between them they brought back 800 lb (400 kg) of moon rock. Most of it is still stored in the Lunar Curatorial Facility in NASA's Johnson Space Center at Clear Lake, Houston; a lot of it has never been seriously looked at at all, but what has been analysed has taught us a lot about the ori¬gins and nature of the Moon.
The Moon is about a quarter of a million miles (400,000 km) from the Earth. It is less dense than the Earth, on average, but the Moon's density is very similar to that of the Earth's mantle, a curi¬ous fact that may not be coincidence. The same side of the Moon always faces the Earth, though it wobbles a bit. The dark markings on it are called maria, Latin for 'seas', but they're not. They're flat-tish plains of rock which at one time was molten and flowed across the lunar surface like lava from a volcano. Nearly all of the craters are impact craters, where meteorites have smashed into the Moon. There are lots of them because there's a lot of rocks floating about in space, the Moon has no atmosphere to shield it by burning up the rocks through frictional heating, and the Moon has no weather to grind them back down again until they disappear. The Earth's atmosphere is a pretty good shield, but once geologists started looking they found remains of 160 impact craters down here, which is interesting given that a lot of them will have eroded away in the wind and the rain. But more of that when we get to dinosaurs.
Today the Moon always turns the same face to the Earth, which means that it rotates once round its axis every month, the same time that it takes to revolve around the Earth. (If it didn't rotate at all, it would always be pointing in the same direction, not the same direction relative to the Earth, but the same direction period. Imagine someone walking round you in a circle but always facing north, say. Then they don't always face you. In fact, you see all sides of them.) It wasn't always like this. Over hundreds of millions of years, the effect of tides has been to slow down the rotation rates of both Earth and Moon. Once the moon's rotation became synchro¬nized with its revolutions round the Earth, the system stabilized. The moon also used to be quite a bit closer to the Earth, but over long periods of time it has moved further and further out.

Between 1600 and 1900 three theories of the formation of the Moon came into vogue and out again. One was that the Moon had formed at the same time as the Earth when the dustcloud con¬densed to form the solar system, Sun, planets, satellites, the whole ball of wax ... or rock, anyway. This theory, like early theories of the solar system's formation, falls foul of angular momentum. The Earth is spinning too fast, and the moon is revolving too fast, to be consistent with the Moon condensing from a dustcloud. (We mis¬led you earlier when we said that the dustcloud theory explained the satellites too. Mostly it does, but not our enigmatic Moon. Lies-to-children, you see, now you're ready for the next layer of complication.)
Theory two was that the Moon is a piece of the Earth that broke away, maybe when the Earth was still completely molten and spin¬ning rather fast. That theory bounced into the bin because nobody could find a pkusible way for a spinning molten Earth to eject any¬thing that would remotely resemble the Moon, even if you waited a bit for things to cool down.
According to theory three, the Moon formed elsewhere in the solar system, and was wandering along when it happened to come within the Earth's gravitational clutches and couldn't get out again. This theory was very popular, even though gravitational capture is distinctly tricky to arrange. It's a bit like trying to throw a golfball into the hole so that it goes round and round just inside the rim. What usually happens is that it falls to the bottom (collides with the Earth) or does what every golfer has experienced to their utter hor¬ror, and goes in for a split second before climbing back out again (escapes without being captured).
The rock samples from Apollo missions added to the mystery of the Moon's origins. In some respects, Moon rock is astonishingly similar to Earth rock. If they were similar in most respects, this would be evidence for a common origin, and we'd have to take another look at the theory that they both condensed from the same dustcloud. But Moon rock doesn't resemble all Earth rock, only the mantle. The current theory, which dates from the early 1980s, is that the Moon was once part of the Earth's mantle. It wasn't ejected as a result of the Earth's spin: it was knocked into space about four billion years ago when a giant body, about the size of Mars, struck the early Earth a glancing blow. Computer calculations show that such an impact can, if conditions are right, strip a large chunk of mantle from the Earth, and sort of smear it out into space. This takes about 13 minutes (aren't computers good?). Then the ejected mantle, which is molten, begins to condense into a ring of rocks of various sizes. Some of it forms a big lump, the proto-Moon, and this quickly sweeps up most of the rest. What's left doesn't go away so easily, however, but over 100 million years nearly all of it crashes into either the Moon or the Earth, because of gravity.
Because Earth has weather, especially back then, oh boy, did it have weather then, the resulting impact craters all got eroded away; but because the Moon has no weather, the lunar impact craters did¬n't get eroded away, and a lot of them are still there now. The great charm of this theory is that it explains many different features of the Moon in one go, its similarity to the Earth's mantle, the fact that its surface seems to have undergone a sudden and extreme amount of heating about 4 billion years ago, its craters, its size, its spin, even those sea-like maria, released as the proto-Moon slowly cooled. The early solar system was a violent place.
In fact, the Dean's mis-designed sun might have done us some good after all ...

The Moon affects life on Earth in at least two or three ways that we know of, probably dozens more that we haven't yet appreciated.
The most obvious effect of the Moon on the Earth is the tides -a fact that the wizards are stumbling towards. Like most of science, the story of the tides is not entirely straightforward, and only loosely connected to what common sense, left to its own devices, would lead us to expect. The common sense bit is that the Moon's gravity pulls at the Earth, and it pulls more strongly on the bit that is closest to the Moon. When that bit is land, nothing much hap¬pens, but when it's water, and more than half our planet's surface is ocean, it can pile up. This explanation is a lie-to-children, and it doesn't agree with what actually happens. It leads us to expect that at any given place on Earth, high tide occurs when the Moon is overhead, or at least at its highest point in the sky. That would lead to one high tide every day, or, allowing for a little complexity in the Earth-Moon system, one high tide every 24 hours 50 minutes.
Actually, high tides occur twice a day, 12 hours and 25 minutes apart. Exactly half the figure.
Not only that: the pull of the Moon's gravity at the surface of the Earth is only one ten millionth of the Earth's surface gravity; the pull of the Sun is about half that. Even when combined together, these two forces are not strong enough to lift masses of water through heights of up to 70 feet (21m)- the biggest tidal move¬ment on Earth, occurring in the Bay of Fundy between Nova Scotia and New Brunswick.
An acceptable explanation of the tides eluded humanity until Isaac Newton worked out the law of gravity and did the necessary calculations. His ideas have since been refined and improved, but he had the basics.
For simplicity, ignore everything except the Earth and the Moon, and assume that the Earth is completely made of water. The watery Earth spins on its axis, so it is subjected to centrifugal force and bulges slightly at the equator. Two other forces act on it: the Earth's gravity and the Moon's. The shape that the water takes up in response to these forces depends on the fact that water is a fluid. In normal circumstances, the surface of a standing body of water is horizontal, because if it wasn't, then the fluid on the higher bits would slosh sideways into the lower bits. The same kind of thing happens when there are extra forces acting: the surface of the water settles at right angles to the net direction of the combined forces.
When you work out the details for the three forces we've just mentioned, you find that the water forms an ellipsoid, a shape that is close to a sphere but very slightly elongated. The direction of elongation points towards the Moon. However, the centre of the ellipsoid coincides with the centre of the Earth, so the water 'piles up' on the side furthest from the Moon as well as on the side near¬est it. This change of shape is only partly caused by the Moon's gravity 'lifting' the water closest to it. Most of the motion, in fact, is sideways rather than upwards. The sideways forces push more water into some regions of the oceans, and take it away from others. The total effect is tiny, the surface of the sea rises and falls through a distance of 18 inches (half a metre).
The coast, where land meets sea, is what creates the big tidal movements. Most of the water is moving sideways (not up) and its motion is affected by the shape of the coastline. In some places the water flows into a narrowing funnel, and then it piles up much more than it does elsewhere. This is what happens in the Bay of Fundy. This effect is made even bigger because coastal waters are shallow, so the energy of the moving water gets concentrated into a thinner layer, creating bigger and faster movements.
Finally, let's put the sun back. This has the same kind of effect as the Moon, but smaller. When Sun and Moon are aligned, either both on the same side of the Earth, in which case we see a new moon, or both on opposite sides (full moon), their gravitational pulls reinforce each other, leading to so-called 'spring tides' in which high tide is higher than normal and low tide is lower. These have nothing to do with the season Spring. When the Sun and Moon are at right angles as seen from Earth, at half moon, the Sun's pull cancels out part of the Moon's, leading to 'neap tides' with less movement than normal (these presumably have nothing to do with the season Neap ...).
By putting all these effects together, and keeping good records of past tides, it is possible to predict the times of high and low tide, and the amount of vertical movement, anywhere on Earth.
There are similar tidal effects (large) on the Earth's atmosphere, and (small) on the planet's land masses. Tidal effects occur on other bodies in the solar system, and beyond. It is thought that Jupiter's moon lo, whose surface is mostly sulphur and which has numerous active volcanoes, is heated by being 'squeezed' repeatedly by tidal effects from Jupiter.

Another effect of the Moon on the Earth, discovered in the mid-'90s by Jaques Laskar, is to stabilize the Earth's axis. The Earth spins like a top, and at any given moment there is a line running through the centre of the Earth around which everything else rotates. This is its axis. The Earth's axis is tilted relative to the plane in which the Earth orbits the Sun, and this tilt is what causes the seasons. Sometimes the north pole is closer to the sun than the south pole is, and six months later it's the other way round. When the northern end of the axis is tilted towards the Sun, more sunlight falls on the northern half of the planet than on the southern half, so the north gets summer and the south gets winter. Six months later, when the axis points the other way relative to the sun, the reverse applies.
Over longer periods of time, the axis changes direction. Just as a top wobbles when it spins, so does the Earth, and over 26,000 years its axis completes one full circle of wobble. At every stage, however, the axis is tilted at the same angle (23°) away from the perpendicu¬lar to the orbital plane. This motion is called precession, and it has a small effect on the timing of the seasons, they slowly shift by a total of one year in 26,000. Harmless, basically. However, the axes of most other planets do something far more drastic: they change their angle to the orbital plane. Mars, for example, probably changes this angle by 90° over a period of 10-20 million years. This has a dramatic effect on climate.
Suppose that a planet's axis is at right angles to the orbital plane. Then there are no seasonal variations at all, but everywhere except the poles there is a day/night cycle, with equal amounts of day and night. Now tilt the axis a little: seasonal variations appear, and the days are longer in summer and shorter in winter. Suppose that the axis tilts 90°, so that at some instant the north pole, say, points directly at the sun. Half a year later, the south pole points at the Sun. At either pole, there is a 'day' of half a year followed by a 'night' of half a year. The seasons coincide with the day/night cycle. Regions of the planet bake in high heat for half a year, then freeze for the other half. Although life can survive in such circum¬stances, it may be harder for it to get going in the first place, and it may be more vulnerable to extremes of climate, vulcanism, or meterorite impacts.
The Earth's axis can change its angle of tilt over very long peri¬ods of time, much longer than the 26,000 year cycle of precession, but even over hundreds of millions of years the angle doesn't change much. Why? Because, as Laskar discovered when he did the calculations, the Moon helps keep the Earth's axis steady. So it is at least conceivable that life on Earth owes quite a lot to the calming influence of its sister world, however much it may madden us indi¬vidually.
A third influence of the Moon was discovered in 1998: a clear association between tides and the rate of growth of trees. Ernst Ziircher and Maria-Giulia Cantiani measured the diameters of young spruce trees grown in containers in the dark. Over periods of several days the diameters changed in step with the tides. The sci¬entists interpret this as an effect of the Moon's gravity on the transport of water within the tree. It can't be variations in moon¬light, which would perhaps affect photosynthesis, because the trees were grown in darkness. But the effect may be similar to one that occurs with creatures that live on the seashore. Because they evolved to live there, they have to respond to the tides, and evolu¬tion sometimes achieves this by creating an internal dynamic that runs in step with the tides. If you remove the creatures to the labo¬ratory, this internal dynamic makes them continue to 'follow' the tides.
The Moon has been important in another way. The Babylonians and Greeks knew that the Moon is a sphere; the phases are obvious, and there is also a slight wobble which means that, over time, humans see rather more than one half of the Moon's surface. There it was, hanging in the sky, a big ball, not a disc like the sun, and a hint that perhaps 'big balls in space' is a much better way of think¬ing about the Earth and its neighbours than 'lights in the sky'.
All this is a long way from lance-constable Angua, even a long way from the female menstrual cycle. But it shows how much we are creatures of the universe. Things Up There really do affect us Down Here, every day of our lives.



THERE WAS NO DARK. This came as such a shock to Ponder Stibbons that he made HEX look again. There had to be Dark, surely? Otherwise, what was there for the light to show up against?
Eventually, he reported this lack to the other wizards.
'There should be lots of Dark and there isn't,' he said flatly. There's just Light and ... no light. And it's a pretty strange light, too.'
'In what way?' said the Archchancellor.
'Well, sir, as you know, there's ordinary light, which travels at about the same speed as sound . . . '
'That's right. You've only got to watch shadows across a land¬scape to realize that.'
'Quite, sir ... and then there's meta-light, which doesn't really travel at all because it is already everywhere.'
'Otherwise we wouldn't even be able to see darkness,' said the Senior Wrangler.
'Exactly. But the Project universe has just got the one sort of light. HEX thinks it moves at hundreds of thousands of miles a sec¬ond.'
'What use is that?'
'Er . . . in this universe, that's as fast as you can go.'
'That's nonsense, because...’ Ridcully began, but Ponder held up a hand. He had not been looking forward to this one.
'Please, Archchancellor. It's doing the best it can. Just trust me on this one. Please? Yes, I can see all the reasons why it's impossi¬ble. But, in there, it seems to work. HEX has written pages of stuff about it, if anyone's interested. Just don't ask me about any of it. Please, gentlemen? It's all supposed to be logical but you'll find your brain squeaking around until the ends point out of your ears.'
He placed his hands together and tried to look wise.
'It really is almost as if the Project is aping the real universe...’
'I beg your pardon,' said Ponder. 'A figure of speech.'
The Librarian nodded at him and knuckled his way across the floor. The wizards watched him carefully.
'You really believe that that thing,' said the Dean, pointing, 'with its moon-hating water and worlds that go around suns...’
'As far as I can see from this,' interrupted the Senior Wrangler, who'd been reading HEX's write-out on the more complex physics of the Project, 'if you were travelling in a cart at the speed of light, and threw a ball ahead of you ...' he turned over the page, read on silently for a moment, creased his brows, turned the page over to see if any enlightenment was to be found on the other side, and went on '... your twin brother would ... be fifty years older than you when you got home ... I think.'
'Twins are the same age,' said the Dean, coldly. 'That's why they are twins.'
'Look at the world we're working on, sir,' said Ponder. 'It could be thought of as two turtle shells tied together. It's got no top and bottom but if you think of it as two worlds, bent around, with one sun and moon doing the work of two ... it's similar.'
He fried in their gaze.
'In a way, anyway,' he said.
Unnoticed by the others, the Bursar picked up the write-out on the physics of the Roundworld universe. After making himself a paper hat out of the title page, he began to read ...

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Zastava Srbija


THAT BLUE IN THE ROUNDWORLD SEA isn't a chemical, well, not in the usual Simple chemical' sense of the word. It's a mass of bacteria, called cyanobac-teria. Another name for them is 'blue-green algae', which is wonderfully confusing. Modern so-called blue-green algae are usually red or brown, but the ancient ones probably were blue-green. And blue-green algae are really bac¬teria, whereas most other algae have cells with a nucleus and so are not bacteria. The blue-green colour comes from chlorophyll, but of a different kind from that in plants, together with yellow-orange chemicals called carotenoids.
Bacteria appeared on Earth at least 3.5 billion years ago, only a few hundred million years after the Earth cooled to the point at which living creatures could survive on it. We know this because of strange layered structures found in sedimentary rocks. The layers can be flat and bumpy, they can form huge branched pillars, or they can be highly convoluted like the leaves in a cabbage. Some deposits are half a mile thick and spread for hundreds of miles. Most date from 2 billion years ago, but those from Warrawoona in Australia are 3.5 billion years old.
To begin with, nobody knew what these deposits were, In the 1950s and 1960s they were revealed as traces of communities of bacteria, especially cyanobacteria.
Cyanobacteria collect together in shallow water to form huge, floating mats, like felt. They secrete a sticky gel as protection against ultraviolet light, and this causes sediment to stick to the mats. When the layer of sediment gets so thick that it blocks out the light, the bacteria form a new layer, and so on. When the layers fossilize they turn into stromatolites, which look rather like big cushions. The wizards haven't been expecting life. Roundworld runs on rules, but life doesn't, or so they think. The wizards see a sharp discontinuity between life and non-life. This is the problem of expecting becomings to have boundaries, of imagining that it ought to be easy to class all objects into either the category 'alive' or the category 'dead'. But that's not possible, even ignoring the flow of time, in which 'alive' can become 'dead', and vice versa. A 'dead' leaf is no longer part of a living tree, but it may well have a few revivable cells.
Mitochondria, now the part of a cell that generates its chemical energy, once used to be independent organisms. Is a virus alive? Without a bacterial host it can't reproduce, but neither can DNA copy itself without a cell's chemical machinery.
We used to build 'simple' chemical models of living processes, in the hope that a sufficiently complex network of chemistry could 'take off', become self-referential, self-copying, by itself There was the concept of the 'primal soup', lots of simple chemicals dis¬solved in the oceans, bumping into each other at random, and just occasionally forming something more complicated. It turns out that this isn't quite the way to do it. You don't have to work hard to make real-world chemistry complex: that's the default. It's easy to make complicated chemicals. The world is full of them. The problem is to keep that complexity organized.
What counts as life? Every biologist used to have to learn a list of properties: ability to reproduce, sensitivity to its environment, utilization of energy, and the like. We have moved on. 'Autopoeisis', the ability to make chemicals and structures related to one's own reproduction, is not a bad definition, except that modern life has evolved away from those early necessities. Today's biologists prefer to sidestep the issue and define life as a property of the DNA mol¬ecule, but this begs the deeper question of life as a general type of process. It may be that we're now defining life in the same way that 'science fiction' is defined, it's what we're pointing at when we use the term.
The idea that life could somehow be self-starting is still contro¬versial to many people. Nevertheless, it turns out that finding plausible routes to life is easy. There must be at least thirty of them.
It's hard to decide which, if any, was the actual route taken, because later lifeforms have destroyed nearly all the evidence. This may not matter much: if life hadn't taken the route that it did, it could eas¬ily have taken one of the others, or one of the hundred we haven't thought of yet.
One possible route from the inorganic world to life, suggested by Graham Cairns-Smith, is clay. Clay can form complicated micro¬scopic structures, and it often 'copies' an existing structure by adding an extra layer to it, which then falls off and becomes the starting point of a new structure. Carbon compounds can stick on to clay surfaces, where they can act as catalysts for the formation of complex molecules of the kind we see in living creatures, proteins, even DNA itself. So today's organisms may have hitched an evolu¬tionary ride on clay.
An alternative is Gunther Wachterhauser's suggestion that pyrite, a compound of iron and sulphur, could have provided an energy source suitable for bacteria. Even today we find bacteria miles underground, and near volcanic vents at the bottom of the oceans, which power themselves by iron/sulphur reactions. These are the source of the 'upflow of poisonous minerals' noticed by Rincewind. It's entirely conceivable that life started in similar envi¬ronments.
A potential problem with volcanic vents, though, is that every so often they get blocked, and another one breaks out somewhere else. How could the organisms get themselves safely across the interven¬ing cold water? In 1988 Kevin Speer realized that the Earth's rotation causes the rising plumes of hot water from vents to spin, forming a kind of underwater hot tornado that moves through the deep ocean. Organisms could hitch a ride on these. Some might make it to another vent. Many would not, but that doesn't matter -all that would be required would be enough survivors.
It is interesting to note that back in the Cretaceous, when the seas were a lot warmer than now, these hot plumes could even have risen to the ocean's surface, where they may have caused 'hyper-canes', like hurricanes but with a windspeed close to that of sound. These would have caused major climatic upheavals on a planet which, as we shall see, it not the moderately peaceful place we tend to believe it is.

Bacteria belong to the grade of organisms known as prokaryotes. They are often said to be 'single-celled', but many single-celled creatures are far more complex and very different from bacteria. Bacteria are not true cells, but something simpler; they have no cell wall and no nucleus. True cells, and creatures both single-celled and many-celled, came later, and are called eukaryotes. They probably arose when several different prokaryotes joined forces to their mutual benefit, a trick known as symbiosis. The first fossil eukary¬otes are singe-celled, like amoebas, and appear about 2 billion years ago. The first fossils of many-celled creatures are algae from 1 bil¬lion years ago ... maybe even as old as 1.8 billion years.
This was the story as scientists understood it up until 1998: ani¬mals like arthropods and other complex beasts came into being a mere 600 million years ago, and that until about 540 million years ago the only creatures were very strange indeed, quite unlike most of what's around today.
These creatures are known as Ediacarans, after a place in Australia where the first fossils were found. They could grow to half a metre or more, but as far as can be told from the fossil record, seem not to have had any internal organs or external orifices like a mouth or an anus (they may have survived by digesting symbiotic bacteria in their selves, or by some other process we can only guess at). Some were flattened, and clustered together in quilts. We have no idea whether the Ediacarans were our distant ancestors, or whether they were a dead end, a lifestyle doomed to failure. No matter: they were around then, and as far as anyone knew, not much else was. There are hints of fossil wormcasts, though, and some very recent fossils look like ... but we're getting ahead of the story. The point is that nearly all Ediacaran life was apparently unrelated to what came later.
About 540 million years ago the Pre-Cambrian Ediacarans were succeeded by the creatures of the Cambrian era. For the first ten million years, these beasties were also pretty weird, leaving behind fragments of spines and spikes which presumably are the remains of prototype skeletons that hadn't yet joined up. At that point, nature suddenly learned how to do joined-up skeletons, and much else: this was the time known as the Cambrian Explosion. Twenty mil¬lion years later virtually every body-plan found in modern animals was already in existence: everything afterwards was mere tinkering. The real innovation of the Cambrian Explosion, though, was less obvious than joined-up skeletons or tusks or shells or limbs. It was a new kind of body plan. Diploblasts were overtaken by triploblasts ...
Sorry, Archchancellor. We mean that creatures began to put another layer between themselves and the universe. Ediacarans and modern jellyfish are diploblasts, two-layered creatures. They have an inside and an outside, like a thick paper bag. Three-layered crea¬tures like us and practically everything else around are called triploblasts. We have an inner, an outer, and a within.
The within was the big leap forward, or at least the big slither. Within you can put the things you need to protect, like internal organs. In one sense, you are not part of the environment any more, there is a you as well. And, like someone who now has a piece of property of their very own, you can begin to make improvements. This is a lie-to-children, but as lies go it is a good one. Triploblasts played a crucial role in evolution, precisely because they did have internal organs, and in particular they could ingest food and excrete it. Their excreta became a major resource for other creatures; to get an interestingly complicated world, it is vitally important that shit happens.
But where did all those triploblasts come from? Were they an offshoot of the Ediacarans? Or did they come from something else that didn't leave fossils?
It's hard to see how they could have come from Ediacarans. Yes, an extra layer of tissue might have appeared, but as well as that extra layer you need a lot of organization to exploit it. That organization has to come from somewhere. Moreover, there were these occa¬sional tantalizing traces of what might have been pre-Cambrian triploblasts, fossils not of worms, which would have clinched it, but of things that might have been trails made by worms in wet mud.
And then again, might not.
In February 1998, we found out.
The discovery depended upon where, and in this case how -you look for fossils. One way for fossils to form is by petrification. There is a poorly known type of petrification that can happen very fast, within a few days. The soft parts of a dead organism are replaced by calcium phosphate. Unfortunately for palaeontologists, this process works only for organisms that are about a tenth of an inch (2 mm) long. Still, some interesting things are that tiny. From about 1975 onwards scientists found wonderfully preserved speci¬mens of tiny ancient arthropods, creatures like centipedes with many segments. In 1994 they found fossilized balls of cells from embryos, early stages in the development of an organism, and it is thought that these come from embryonic triploblasts. However, all of these creatures must have come after the Ediacarans. But in 1998 Shuhai Xiao, Yun Zhang, and Andrew Knoll discovered fos¬silized embryos in Chinese rock that is 570 million years old -smack in the middle of the Ediacaran era. And those embryos were triploblasts.
Forty million years before the Cambrian explosion, there were triploblasts on Earth, living right alongside those enigmatic Ediacarans.
We are triploblasts. Somewhere in the pre-Cambrian, sur¬rounded by mouthless, organless Ediacarans, we came into our inheritance.

It used to be thought that life was a delicate, highly unusual phe¬nomenon: difficult to create, easy to destroy. But everywhere we look on Earth we find living creatures, often in environments that we would have expected to be impossibly hostile. It's beginning to look as if life is an extremely robust phenomenon, liable to turn up almost anywhere that's remotely suitable. What is it about life that makes it so persistent?
Earlier we talked about two ways to get off the Earth, a rocket and a space elevator. A rocket is a thing that gets used up, but a space elevator is a process that continues. A space elevator requires a huge initial investment, but once you've got it, going up and down is essentially free. A functioning space elevator seems to contradict all the usual rules of economics, which look at individual transac¬tions and try to set a rational price, instead of asking whether the concept of a price might be eliminated altogether. It also seems to contradict the law of conservation of energy, the physicist's way of saying that you can't get something for nothing. But, as we've seen, you can, by exploiting the new resources that become available once you get your space elevator up and running.
There is an analogy between space elevators and life. Life seems to contradict the usual rules of chemistry and physics, especially the rule known as the second law of thermodynamics, which says that things can't spontaneously get more complicated. Life does this because, like the space elevator, it has lifted itself to a new level of operation, where it can gain access to things and processes that were previously out of the question. Reproduction, in particular, is a wonderful method of getting round the difficulties of manufac¬turing a really complicated thing. Just build one that manufactures more of itself. The first one may be incredibly difficult, but all the rest come with no added effort.
What is the elevator for life? Let's try to be general here, and look at the common features of all the different proposals for 'the' origin of life. The main one seems to be the novel chemistry that can occur in small volumes adjacent to active surfaces. This is a long way from today's complex organisms, it's even a long way from today's bacteria, which are distinctly more complicated than their ancient predecessors. They have to be, to survive in a more compli¬cated world. Those active surfaces could be in underwater volcanic vents. Or hot rocks deep underground. Or they could be seashores. Imagine layers of complicated (because that's easy) but disorgan¬ized (ditto) molecular gunge on rocks which are wetted by the tides and irradiated by the sun. Anything in there that happens to pro¬duce a tiny 'space elevator' establishes a new baseline for further change. For example, photosynthesis is a space elevator in this sense. Once some bit of gunge has got it, that gunge can make use of the sun's energy instead of its own, churning out sugars in a steady stream. So perhaps 'the' origin of life was a whole series of tiny 'space elevators' that led, step by step, to organized but ever more complex chemistry.



THE LIBRARIAN KNUCKLED SWIFTLY through the outer regions of the University's library, although terms like 'outer' were hardly relevant in a library so deeply immersed in L-space.
It is known that knowledge is power, and power is energy, and energy is matter, and matter is mass, and therefore large accumulations of knowledge distort time and space. This is why all bookshops look alike, and why all second-hand bookshops seem so much bigger on the inside, and why all libraries, every¬where, are connected. Only the innermost circle of librarians know this, and take care to guard the secret. Civilization would not sur¬vive for long if it was generally known that a wrong turn in the stacks would lead into the Library of Alexandria just as the invaders were looking for the matches, or that a tiny patch of floor in the ref¬erence section is shared with the library in Braseneck College where Dr Whitbury proved that gods cannot possibly exist, just before that rather unfortunate thunderstorm.
The Librarian was saying 'ook ook' to himself under his breath, in the same way that a slightly distracted person searches aimlessly around the room saying 'scissors, scissors' in the hope that this will cause them to re-materialize. In fact he was saying 'evolution, evo¬lution'. He'd been sent to find a good book on it.
He had a very complicated reference card in his mouth. The wizards of UU knew all about evolution. It was a self-evi¬dent fact. You took some wolves, and by careful unnatural selection over the generations you got dogs of all shapes and sizes. You took some sour crab-apple trees and, by means of a stepladder, a fine paintbrush and a lot of patience, you got huge juicy apples. You took some rather scruffy desert horses and, with effort and a good stock book, you got a winner. Evolution was a demonstration of narrativium in action. Things improved. Even the human race was evolv¬ing, by means of education and other benefits of civilization; it had began with rather bad-mannered people in caves, and it had now produced the Faculty of Unseen University, beyond which it was probably impossible to evolve further.
Of course, there were people who occasionally advanced more radical ideas, but they were like the people who thought the world really mas round or that aliens were interested in the contents of their underwear.
Unnatural selection was a fact, but the wizards knew, they knew, that you couldn't start off with bananas and get fish.
The Librarian glanced at the card, and took a few surprising turnings. There was the occasional burst of noise on the other side of the shelves, rapidly changing as though someone was playing with handfuls of sound, and a flickering in the air. Someone talking was replaced with the absorbent silence of empty rooms was replaced with the crackling of flame and displaced by laughter ...
Eventually, after much walking and climbing, the Librarian was faced with a blank wall of books. He stepped up to them with librarianic confidence and they melted away in front of him.
He was in some sort of study. It was book-lined, although with rather fewer than the Librarian would have expected to find in such an important node of L-space. Perhaps there was just the one book ... and there it was, giving out L-radiation at a strength the Librarian had seldom encountered outside the seriously magical books in the locked cellars of Unseen University. It was a book and father of books, the progenitor of a whole race that would flutter down the centuries ...
It was also, unfortunately, still being written.
The author, pen still in hand, was staring at the Librarian as if he'd seen a ghost.
With the exception of his bald head and a beard that even a wiz¬ard would envy, he looked very, very much like the Librarian.
'My goodness ...'
'Ook?' The Librarian had not expected to be seen. The writer must have something very pertinent on his mind.
'What manner of shade are you ... ?'
A hand reached out, tremulously. Feeling that something was expected of him, the Librarian reached out as well, and the tips of the fingers touched.
The author blinked.
'Tell me, then,' he said, 'is Man an ape, or is he an angel?'
The Librarian knew this one.
'Ook,' he said, which meant: ape is best, because you don't have to fly and you're allowed sex, unless you work at Unseen University, worst luck.
Then he backed away hurriedly, ooking apologetic noises about the minor error in the spacetime coordinates, and knuckled off through the interstices of L-space and grabbed the first book he found that had the word 'Evolution' in the title.
The bearded man went on to write an even more amazing book. If only he had thought to use the word 'Ascent' there might not have been all that unpleasantness.
But, there again, perhaps not.

HEX let itself absorb more of the future ... call it ... knowledge. Words were so difficult. Everything was context. There was too much to learn. It was like trying to understand a giant machine when you didn't understand a screwdriver.
Sometimes HEX thought it was picking up fragmentary instruc¬tions. And, further away, much further away, there were little disjointed phrases in the soup of concepts which made sense but did not seem to be sensible. Some of them arrived unbidden.
Even as HEX pondered this, another one arrived and offered an opportunity to make $$$$ While You Sit On Your Butt!!!!! He con¬sidered this unlikely.

The title brought back by the Librarian was The Young Person's Guide to Evolution.
The Archchancellor turned the pages carefully. They were well illustrated. The Librarian knew his wizards.
'And this is a good book on evolution?' said the Archchancellor.
'Well, it makes no sense to me,' said the Archchancellor. 'I mean t'say, what the hell is this picture all about?'
It showed, on the left, a rather hunched-up, ape-like figure. As it crossed the page, it gradually arose and grew considerably less hairy until it was striding confidently towards the edge of the page, per¬haps pleased that it had essayed this perilous journey without at any time showing its genitals.
'Looks like me when I'm getting up in the mornings,' said the Dean, who was reading over his shoulder.
'Where'd the hair go?' Ridcully demanded.
'Well, some people shave,' said the Dean.
'This is a very strange book,' said Ridcully, looking accusingly at the Librarian, who kept quiet because in fact he was a little worried. He rather suspected he might have altered history, or at least a his¬tory, and on his flight back to the safety of UU he'd seized the first book that looked as though it might be suitable for people with a very high IQ but a mental age of about ten. It had been in an empty byway, far off his usual planes of exploration, and there had been very small red chairs in it.
'Oh, I get it. This is a fairy story,' said Ridcully. 'Frogs turnin' into princes, that kind of thing. See here ... there's something like our blobs, and then these fishes, and then it's a ... a newt, and then it's a big dragony type of thing and, hah, then it's a mouse, then here's an ape, and then it's a man. This sort of thing happens all the time out in the really rural areas, you know, where some of the 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, gentle¬men ... 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. 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 HEX'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 uni¬verse 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 nat¬ural 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.

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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 interven¬tion?
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. The wizards know this, and so did the Victorians. Until the nine¬teenth century, though, nobody seems to have realized that a very similar process might explain the remarkable diversity of life on Earth, from bacteria to bactrians, from oranges to orangutans.
They didn't appreciate that possibility for two reasons. When you bred dogs, what you got was a different kind of dog, not a banana or a fish. And breeding animals was the purest kind of magic: if a human being wanted a long thin dog, and if they started from short fat ones, and if they knew how the trick worked (if, so to speak, they cast the right 'spells') then they would get a long thin dog. Bananas, long and thin though they might be, were not a good starting point. Organisms couldn't change species, and they only changed form within their own species because people wanted them to.
Around 1850, two people independently began to wonder whether nature might play a similar game, but on a much longer timescale and in a much grander manner, and without any sense of purpose or goal (which had been the flaw in previous musings along similar lines). They considered a self-propelled magic: 'natural' selection as opposed to selection by people. One of them was Alfred Wallace; the other, far better known today, was Charles Darwin. Darwin spent years travelling the world. From 1831 to 1836 he was hired as ship's naturalist aboard HMS Beagle, and his job was to observe plants and animals and note down what he saw. In a letter of 1877 he says that while on the Beagle he believed in 'the permanence of species', but on his return home in 1836 he began to think about the deeper meaning of what he had seen, and realized that 'many facts indicated the common descent of species'. By this he meant that species that are different now probably came from ancestors that once belonged to the same species. Species must be able to change. That wasn't an entirely new idea, but he also came up with an effective mechanism for such changes, and that was new. Meanwhile Wallace was studying the flora and fauna of Brazil and the East Indies, and comparing what he saw in the two regions, and was coming to similar conclusions, and much the same expla¬nation. By 1858 Darwin was still mulling over his ideas, contemplating a grand publication of everything he wanted to say about the subject, while Wallace was getting ready to publish a short article containing the main idea. Being a true English gentleman, Wallace warned Darwin of his intentions so that Darwin could pub¬lish something first, and Darwin rapidly penned a short paper for the Linnaean Society, followed a year later by a book, The Origin of Species, a big book, but still not on the majestic scale that Darwin had originally intended. Wallace's paper appeared in the same jour¬nal shortly afterwards, but both papers were officially 'presented' to the Society at the same meeting.
What was the initial reaction to these two Earth-shattering arti¬cles? In his annual report for that year, the President of the Society, Thomas Bell, wrote that 'The year has not, indeed, been marked by any of those striking discoveries which at once revolutionize, so to speak, the department of science in which they occur.' However, this perception quickly changed as the sheer enormity of Darwin's and Wallace's theory began to sink in, and they took a lot of stick from Mustrum Ridcully's spiritual brethren for daring to come up with a plausible alternative to Biblical creation. What was this epoch-making alternative? An idea so simple that everybody else had missed it. Thomas Huxley is said to have remarked, on reading Origin: 'How extremely stupid not to have thought of that.'
This is the idea. You don't need a human being to push animals into new forms; they can do it to themselves, more precisely: to each other. This was the mechanism of natural selection. Herbert Spencer, who did the important journalistic job of interpreting Darwin's theory to the masses, coined the phrase, 'survival of the fittest' to describe it. The phrase had the advantage of convincing everybody that they understood what Darwin was saying, and it had the disadvantage of convincing everybody that they understood what Darwin was saying. It was a classic lie-to-children, and it deceives many critics of evolution to this day, causing them to aim at a long-disowned target, besides giving a spurious 'scientific' background to some extremely stupid and unpleasant political the¬ories.
Starting from an enormous range of observations of many species of plants and animals, Darwin had become convinced that organisms could change of their own accord, so much so that they could even, over very long periods, change so much that they gave rise to new species.
Imagine a lot of creatures of the same species. They are in com¬petition for resources, such as food, competing with each other, and with animals of other species. Now suppose that by random chance, one or more of these animals has offspring that are better at winning the competition. Then those animals are more likely to survive for long enough to produce the next generation, and the next generation is also better at winning. In contrast, if one or more of these animals has offspring that are worse at winning the com¬petition, then those animals are less likely to produce a succeeding generation, and even if they somehow do, that next generation is still worse at winning. Qearly even a tiny advantage will, over many generations, lead to a population composed almost entirely of the new high-powered winners. In fact, the effect of any advantage grows like compound interest, so it doesn't take all that long. Natural selection sounds like a very straightforward idea, but words like 'competition' and 'win' are loaded. It's easy to get the wrong impression of just how subtle evolution must be. When a baby bird falls out of the nest and gets gobbled up by a passing cat, it is easy to see the battle for survival as being fought between bird and cat. But if that is the competition, then cats are clear winners, so why haven't birds evolved away altogether? Why aren't there just cats?
Because cats and birds long ago came, unwittingly, to a mutual accommodation in which both can survive. If birds could breed unchecked, there would soon be far too many birds for their food supply to support them. A female starling, for instance, lays about 16 eggs in her life. If they all survived, and this continued, the star¬ling population would multiply by eight every generation, 16 babies for every two parents. Such 'exponential' growth is amaz¬ingly rapid: by the 70th generation a sphere the size of the solar system would be occupied entirely by starlings (instead of by pigeons, which appears to be its natural destiny).
The only 'growth rate' for the population that works is for each breeding pair of adult starlings to produce, on average, exactly one breeding pair of adult starlings. Replacement, but no more, and no less. Anything more than replacement, and the population explodes; anything less, and it eventually dies out. So of those 16 eggs, an average of 14 must not survive to breed. And that's where the cat comes in, along with all the other things that make it tough to be a bird, especially a young one. In a way, the cats are doing the birds a favour, collectively, though maybe not as individuals. (It depends if you're one of the two that survive to breed or the 14 that don't.)
Rather more obviously, the birds are doing the cats a favour, cat food literally drops out of the skies, manna from heaven. So what stops it getting out of hand is that if a group of greedy cats happens to evolve somewhere, they rapidly eat themselves out of existence again. The more restrained cats next door survive to breed, and quickly take over the vacated territory. So those cats that eat just enough birds to maintain their food supply will win a competition against the greedy cats. Cats and birds aren't competing because they're not playing the same game. The real competitions are between cats and other cats, and between birds and other birds. This may seem a wasteful process, but it isn't. A female starling has no trouble laying her 16 eggs. Life is reproductive, it makes rea¬sonably close, though not exact, copies of itself, in quantity, and 'cheaply'. Evolution can easily 'try out' many different possibilities, and discard those that don't work. And that's an astonishingly effective way to home in on what does work.
As Huxley said, it's such an obvious idea. It caused so much trouble from religionists because it takes the gloss off one of their favourite arguments, the argument from design. Living creatures seem so perfectly put together that surely they must have been designed, and if so, there must have been a Designer. Darwinism made it clear that a process of random, purposeless variation trimmed by self-induced selection can achieve equally impressive results, so there can be the semblance of design without any Designer.
There are plenty of details to Darwinism that still aren't under¬stood, as with all science, but most of the obvious ways of trying to shoot it down have been answered effectively. The classic example -still routinely trotted out by creationists and others even though Darwin himself had a pretty good answer, is the evolution of the eye. The human eye is a complex structure, and all of its compo¬nents have to fit together to a high degree of accuracy, or it won't work. If we claim that such a complex structure has evolved, we must accept that it evolved gradually. It can't all have come into being at once. But if so, then at every stage along the evolutionary track the still-evolving proto-eye must offer some kind of survival advantage to the creature that possesses it. How can this happen? The question is often asked in the form 'What use is half an eye?', to which you are expected to conclude 'nothing', followed by a rapid conversion to some religion or other. 'Nothing' is a reasonable answer, but to the wrong question. There are lots of ways to get to an eye gradually that do not require it to be assembled piece by piece like a jigsaw puzzle. Evolution does not build creatures piece by piece like the God of Evolution in The Last Continent. Darwin himself pointed out that in creatures alive in his day you could find all kinds of light-sensitive organs, starting with patches of skin, then increasing in complexity, light-gathering power, and ability to detect fine detail, right up to structures as sophisticated as the human eye. There is a continuum of eyelike organs in the living world, and every creature gains an advantage by having its own type of light-sensing device, in comparison to similar creatures that have a slightly less effective device of a similar kind.
In 1994 Daniel Nilsson and Susanne Pelger used a computer to see what would happen to a mathematical model of a light-sensing surface if it was allowed to change in small, random, biologically feasible ways, with only those changes that improved its sensitivity to light being retained. They found that within 400,000 generations, an evolutionary blink of an eye, that flat surface gradually changed into a recognizable eye, complete with a lens. The lens even bent light differently in different places, just like our eye and unlike normal spectacle lenses. At every tiny step along the way, a creature with the improved 'eye' would be better than those with the old version.
At no stage was there ever 'half an eye'. There were just light-sensing things that got better at it.

Since the 1950s, we have been in possession of a new and central piece of the evolutionary jigsaw, one that Darwin would have given his right arm to know about. This is the physical, more precisely, chemical, nature of whatever it is that ensures that characteristics of organisms can change and be passed from one generation to the next.
You know the word: gene.
You know the molecule: DNA.
You even know how it works: DNA carries the genetic code, a kind of chemical 'blueprint' for an organism.
And, probably, a lot of what you know is lies-to-children.
Just as 'survival of the fittest' captured the imaginations of the Victorians, so 'DNA' has captured the imaginations of today's pub¬lic. However, imaginations thrive best if they are left free to roam: they grow tired and feeble in captivity. Captive imaginations do breed quite effectively, because they are protected from the terrible predator known as Thought.
DNA has two striking properties, which play a significant role in the complex chemistry of life: it can encode information, and that information can be copied. (Other molecules process the DNA information, for example by making proteins according to recipes encoded in DNA.) From this point of view a living organism is a kind of molecular computer. Of course there's much more to life than that, but DNA is central to any discussion of life on Earth. DNA is life's most important molecular-level 'space elevator', a platform from which life can launch itself into higher realms.
The complexity of living creatures arises not because they are made from some special kind of matter- the now-discredited 'vitalist' theory, but because their matter is organized in an exceedingly intricate fashion. DNA does a lot of the routine 'bookkeeping' that keeps living creatures organized. Every cell of (nearly) every living organism contains its 'genome', a kind of code message written in DNA, which gives that organism a lot of hints about how to behave at the molecular level. (Exceptions are various viruses, on the boundary between life and non-life, which use a slightly different code.)
This is why it was possible to clone Dolly the Sheep, to take an ordinary cell from an adult sheep and make it grow into another sheep. The trick actually requires three adult sheep. First, there's the one from which you take the cell: call her 'Dolly's Mum'. Then you persuade the cell's nucleus to forget that it came from an adult and to think that it's back in the egg, and then you implant it into an egg from a second sheep ('Egg Donor'). Then you put the egg into the uterus of the third sheep ('Surrogate Mum') so that it can grow into a normal lamb.
Dolly is often said to be a perfect copy of Dolly's Mum, but that's not completely true. For a start, certain parts of Dolly's DNA come not from Dolly's Mum, but from Egg Donor. And even if that slight difference had been fixed, Dolly could still differ in many ways from her 'mother', because sheep DNA is not a complete list of instructions for 'how to build a sheep'. DNA is more like a recipe, and it assumes you already know how to set up your kitchen. So the recipe doesn't say 'put the mixture in a greased pan and place in an oven set to 400°F,' for instance: it says 'put the mixture in the oven' and assumes that you know it needs to go in a pan and that the oven should be set to a standard temperature. In particular, sheep DNA leaves out the vital instruction 'put the mixture inside a sheep', but that's the only place (as yet) where you can turn a fertil¬ized sheep egg into a lamb. So even Surrogate Mum played a considerable role in determining what happened when the DNA recipe for Dolly was 'obeyed'.
Many biologists think that this is a minor objection, after all, Egg Donor and Surrogate Mum work the way they do because their DNA contains the information that makes them do it. But things that aren't in any organism's DNA may be essential for the repro¬ductive cycle. A good example occurs in yeast, a plant that can turn sugar into alcohol and give off carbon dioxide. The entire DNA code for one species of yeast is now known. Thousands of experi¬mentalists have played genetic games with yeast, then spun the beasties in a centrifuge to separate the DNA, from which they can work out the code. When you do this, you leave a scummy residue in the bottom of the test tube, but since it's not DNA, you know it can't be important for genetics, and you throw it away. And so they all did, until in 1997 one geneticist asked a stupid question. If it's not DNA, what's it for? What's in that scummy residue, anyway?
The answer was simple, and baffling. Prions. Lots and lots of them.
A prion is a smallish protein molecule that can act as a catalyst for the formation of more protein molecules just like itself. Unlike DNA, it doesn't do this by replication. Instead, it needs a supply of proteins that are almost like itself, but not quite, the right atoms, in the right order, but folded into the wrong shape. The prion attaches itself to such a protein, jiggles it around a bit, and nudges it into the same shape as the prion. So now you've got more prions, and the process speeds up.
Prions are molecular preachers: they make more of themselves by converting the heathen, not by splitting into identical twins. The most notorious prion is the one that is believed to be the cause of BSE, 'mad cow disease'. The protein that gets converted happens to be a key component of the cow's brain, which is why infected cows lose coordination, stagger around, foam at the mouth, and look crazy. What does yeast want prions for? Without prions, yeast can't reproduce. The protein-making instructions in its DNA sometimes make a protein that is folded into the wrong shape. When a yeast cell divides, it copies its DNA to each half, but it shares the prions (which can be topped up by converting other pro¬teins). So here's a case where, even on the molecular level, an organism's DNA does not specify everything about that organism. There's a lot about the DNA code system that we don't under¬stand, but one part that we do is the 'genetic code'. Some segments of DNA are recipes for proteins. In fact, they come very close to being exact blueprints for proteins, because they list the precise components of the protein and they list them in exactly the right order. Proteins are made from a catalogue of fairly tiny molecules known as amino acids. For most organisms, humans included, the catalogue contains exactly 22 amino acids. If you string lots of amino acids together in a row, and let them fold up into a relatively compact tangle, you get a protein. The one thing the DNA doesn't list is how to fold the resulting molecule up, but usually it folds the right way of its own accord. Occasionally, when it doesn't, there are more servant molecules to nudge it into the right shape. Just such a servant molecule, rejoicing in the name HSP90, is turning molecu¬lar genetics upside down even as we write. HSP90 'insists' that proteins fold into the orthodox shape, even if there are a few mutations in the DNA that codes for those proteins. When the organism is 'stressed', diverting HSP90 to other functions, these cryptic mutations suddenly get expressed, the proteins acquire the unorthodox shape that goes along with their mutated DNA codes. In effect, this says that you can trigger a genetic change by non-genetic means.
Segments of DNA that code for working proteins are called genes. Segments that don't rejoice in a variety of names. Some of them code for proteins that control when a given gene 'switches on', that is, starts to make proteins: these are known as regulatory (or homeotic) genes. Some bits are colloquially called 'junk DNA', a scientific term meaning 'we don't know what these bits are for'. Some literally minded scientists read this as 'they're not for any¬thing', thereby getting the horse of nature neatly aligned with the rear end of the cart of human understanding. Most likely they are a mix of different things: DNA that used to have some function way back in evolution but currently does not (and might possibly be revived if, say, an ancient parasite reappeared), DNA that controls how genes switch their protein manufacturing on and off, DNA that controls those, and so on. Some may actually be genuine junk. And some (so the joke goes) may encode a message like 'It was me, I'm God, I existed all along, ha ha.'

Evolutionary processes do not always direct themselves along paths that are neatly comprehensible to humans. This doesn't mean Darwin was wrong: it means that even when he's right, there may be a surprising absence of narrativium, so that a 'story' that makes perfect sense to evolution may not make sense to humans. We sus¬pect that a lot of what you find in living organisms is like that -offering a small advantage at every stage of its evolution, but an advantage in such a complex game is that we can't tell a convincing story about why it's an advantage. To show just how bizarre evolu¬tionary processes can be, even in comparatively simple circumstances, we must look not to animals or plants, but to elec¬tronic circuits.
Since 1993 an engineer named Adrian Thompson has been evolving circuits. The basic technique, known as 'genetic algo¬rithms', is quite widely used in computer science. An algorithm is a specific program, or recipe, to solve a given problem. One way to find algorithms for really tough problems is to 'cross-breed' them and apply natural selection. By 'cross breed' we mean 'mix parts of one algorithm with parts of the other'. Biologists call this 'recom¬bination' and each sexual organism, like you, recombines its parents' chromosomes in just this manner. Such a technique, or its result, is called a genetic algorithm. When the method works, it works brilliantly; its main disadvantage is that you can't always give a sensible explanation of how the resulting algorithm accomplishes whatever it does. More of that in a moment: first we must discuss the electronics.
Thompson wondered what would happen if you used the genetic algorithm approach on an electronic circuit. Decide on some task, randomly cross-breed circuits that might or might not solve it, keep the ones that do better than the rest, and repeat for as many generations as it takes.
Most electronic engineers, thinking about such a project, will quickly realize that it's silly to use genuine circuits. Instead, you can simulate the circuits on a computer (since you know exactly how a circuit behaves) and do the whole job more quickly and more cheaply in simulation. Thompson mistrusted this line of argument, though: maybe real circuits 'knew' something that a simulation would miss.
He decided on a task: to distinguish between two input signals of different frequencies, 1 kilohertz and 10 kilohertz, that is, sig¬nals that made 1000 vibrations per second and 10,000 vibrations per second. Think of them as sound: a low tone and a high tone. The circuit should accept the tone as input signal, process it in some manner to be determined by its eventual structure, and pro¬duce an output signal. For the high tone, the circuit should output a steady zero volts, that is, no output at all, and for the low tone, the circuit should output a steady five volts. (Actually, these prop¬erties were not specified at the start: any two different steady signals would have been acceptable. But that's how it ended up.)
It would take forever to build thousands of trial circuits by hand, so he employed a 'field-programmable gate array'. This is a microchip that contains a number of very tiny transistorized 'logic cells', mildly intelligent switches, so to speak, whose connections can be changed by loading new instructions into the chip's config¬uration memory.
Those instructions are analogous to an organism's DNA code, and can be cross-bred. That's what Thompson did. He started with an array of one hundred logic cells, and used a computer to ran¬domly generate a population of fifty instruction codes. The computer loaded each set into the array, fed in the two tones, looked at the outputs, and tried to find some feature that might help in evolving a decent circuit. To begin with, that feature was anything that didn't look totally random. The 'fittest' individual in the first generation produced a steady five-volt output no matter which tone it heard. The least fit instruction codes were then killed off (deleted), the fit ones were bred (copied and recombined), and the process was repeated.
What's most interesting about the experiment is not the details, but how the system homed in on a solution, and the remarkable nature of that solution. By the 220th generation, the fittest circuit produced outputs that were pretty much the same as the inputs, two waveforms of different frequencies. The same effect could have been obtained with no circuit at all, just a bare wire! The desired steady output signals were not yet in prospect.
By the 650th generation, the output for the low tone was steady, but the high tone still produced a variable output signal. It took until generation 2800 for the circuit to give approximately steady, and different, signals for the two tones; only by generation 4100 did the odd glitch get ironed out, after which point little further evolu¬tion occurred.
The strangest thing about the eventual solution was its struc¬ture. No human engineer would ever have invented it. Indeed no human engineer would have been able to find a solution with a mere 100 logic cells. The human engineer's solution, though, would have been comprehensible, we would be able to tell a convincing 'story' about why it worked. For example, it would include a 'clock', a cir¬cuit that ticks at a constant rate. That would give a baseline to compare the other frequencies against. But you can't make a clock with 100 logic cells. The evolutionary solution didn't bother with a clock. Instead, it routed the input signal through a complicated series of loops. These presumably generated time-delayed and oth¬erwise processed versions of the signals, which eventually were combined to produce the steady outputs. Presumably. Thompson described how it functioned like this: 'Really, I don't have the faintest idea how it works.'
Amazingly, further study of the final solution showed that only 32 of its 100 logic cells were actually needed. The rest could be removed from the circuit without affecting its behaviour. At first it looked as if five other logic cells could be removed, they were not connected electrically to the rest, nor to the input or output. However, if these were removed, the circuit ceased to work. Presumably these cells reacted to physical properties of the rest of the circuit other than electrical current, magnetic fields, say. Whatever the reason, Thompson's hunch that a real silicon circuit would have more tricks up its sleeve than a computer simulation turned out to be absolutely right.
The technological justification for Thompson's work is the pos¬sibility of evolving highly efficient circuits. But the message for basic evolutionary theory is also important. In effect, it tells us that evolution has no need for narrativium. An evolved solution may 'work' without it being at all clear how it does whatever it does. It may not follow any 'design principle' that makes sense to human beings. Instead, it can follow the emergent logic of Ant Country, which can't be captured in a simple story.
Of course, evolution may sometimes hit on 'designed' solutions, as happens for the eye. Sometimes it hits on solutions that do have a narrative, but we fail to appreciate the story. Stick insects look like sticks, and their eggs look like seeds. There is a kind of Discworld logic to this, since seeds are the 'eggs' of sticks, and prior to the the¬ory of evolution taking hold the Victorians approved of this 'logic' because it looked like God being consistent. The early evolutionists didn't see it that way, and they worried about it; but they worried a lot more when they found that some stick insect eggs looked like lit¬tle snails. It seemed silly for anything to resemble the favourite food of nearly everything else. In fact, it seemed to be a flat contradiction to the evolutionary story. The puzzle was solved only in 1994, after forest fires in Australia. When new plant shoots came up out of the ashes, they were covered in baby stick insects. Ants had carried the 'seeds', and the 'baby snails', down into their subterranean nests, thinking they were the real thing. Being safely underground, the stick insect eggs escaped the fires. In fact, baby stick insects look, and run, just like ants: this should have been a clue, but nobody made the connection.
And sometimes evolution's solution has no narrative structure. To test Darwin's theories thoroughly, we should be looking for evolved systems that don't conform to a simple narrative descrip¬tion, as well as for ones that do. Many of the brain's sensory systems may well be like this. The first few layers of the visual cortex, for example, perform generalized functions like detecting edges, but we have no idea how lower layers work, and that may well be because they don't conform to any design principles that we cur¬rently can recognize. Our sense of smell seems to be 'organized' along very strange lines, not at all as clearly structured as the visual cortex, and it too may be lacking any element of design.
More importantly, genes may well be like this. Biologists habit¬ually talk of 'the function of a gene', what it does. The unspoken assumption is that it does only one thing, or a small list of things. This is pure magic: the gene as a spell. It is conceived as being a spell in the same sense that 'Cold Start' in a car is. But a lot of genes may not do anything that can be summed up in a simple story. The job they evolved to do is 'build an organism', and they evolved as a team, like Thompson's circuits. When evolution turns up solutions of this kind, conventional reductionism is not much help in under¬standing those solutions. You can list neural connections till the cows come home, but you won't understand how the cows' visual systems distinguish a cowshed from a bull.



RINCEWIND WAS FINDING, now that he was back at what appeared to be his real size, that he was com¬ing to enjoy this world after all. It was so marvellously dull.
Every so often he'd be moved forward a few tens of millions of years. The sea levels would change. There seemed to be more land around, speckled with volcanoes. Sand was turning up on the edge of the sea. Yet the sheer vast ringing silence dominated everything. Oh, there'd be storms, and at night there were brilliant meteor showers that practically hissed across the sky, but these only underlined the absent symphony of life. He was rather pleased with 'symphony of life'. 'Mr Stibbons?' he said. 'Yes?' said Ponder 's voice in his helmet. 'There seem to be a lot of comets about.' 'Yes, they seem to go with roundworld systems. Is this a prob¬lem?'
'Aren't they going to crash into this world?' Rincewind heard the muted sounds of debate in the back¬ground, and then Ponder said: 'The Archchancellor says snowballs don't hurt.' 'Oh. Good.' 'We're going to move you on a few million years now. Ready?'
'Millions and millions of years of dullness,' said the Senior Wrangler.
'There are more blobs today,' said Ponder.
'Oh, good. We need more blobs.'
There was a yell from Rincewind. The wizards rushed to the omniscope.
'Good heavens,' said the Dean. 'Is that a higher lifeform?' 'I think? said Ponder, 'that seat cushions have inherited the world.'

They lay in the warm shallow water. They were dark green. They were reassuringly dull.
But the other things weren't.
Blobs drifted over the sea like giant eyeballs, black, purple, and green. The water itself was covered with them. A scum of them rolled in the surf. The aerial ones bobbed only a few inches above the waves, thick as fog, overshadowing one another in their fight for height.
'Have you ever seen anything like that?' said the Senior Wranger.
'Not legally,' said the Dean. A blob burst. Audio reception on the omniscope was not good, but the sound was, in short, phut. The stricken thing disappeared into the sea, and the floating blobs closed in over it.
'Get Rincewind to try to communicate with them,' said Ridcully.
'What have blobs got to talk about, sir?' said Ponder 'Besides, they're not making any noise. I don't think phut counts.'
'They're various colours,' said the Lecturer in Recent Runes. 'Perhaps they communicate by changing colour? Like those sea creatures...’ He snapped his fingers as an aid to memory.
'Lobsters?' the Dean supplied.
'Really?' said the Senior Wrangler. 'I didn't know they did that.'
'Oh yes,' said Ridcully. 'Red means "help!"'
'No, I think the Lecturer in Recent Runes is referring to squid,' said Ponder, who knew that this sort of thing could go on for a long time. He added hurriedly, 'I'll tell Rincewind to give it a try.'

Rincewind, apparently knee deep in blobs, said: 'What do you mean?'
'Well ... could you get embarrassed, perhaps?'
'No, but I'm getting angry!'
'That might work, if you get red enough. They'll think you want help.'
'Do you know there's something else here besides blobs?'
Some of the blobs trailed strands in the faint breeze blowing across the beach. When they tangled up on a blob gasbag, which put some stress on the line, the little blob on the end let go its grip on a rock, the line gradually shortened, and the gasbag bobbed onwards with its new passenger.
Rincewind saw them on a number of blobs. The blobs did not look healthy.
'Predators,' Ponder told him.
'I'm on a beach with predators?'
'If it really worries you, try not to look blobby. We'll keep an eye on them. Er ... the Faculty is of the opinion that intelligence is most likely to arise in creatures that eat lots of things.'
'Probably because they eat lots of things. We'll try a few big jumps in time, all right?'
'I suppose so.'
The world flickered ...
'The sea's a lot further away. There's a few floating blobs. More black blobs this time.'
... flickered ...
'Well out at sea, great rafts of purple blobs, some blobs in the air.'
...flickered ...
'Great steaming piles of onions!' 'What?' said Ponder.
'I knew it! I just knew it! This whole damn place was just lulling me into a false sense of security!' What's happening?' 'It's a snowball. The whole world's a giant snowball!'

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THE EARTH HAS BEEN A GIANT SNOWBALL on many occasions. It was a snowball 2.7 billion years ago, 2.2 billion years ago, and 2 billion years ago. It was a really cold snowball 800 million years ago, and this was followed by a series of global cold snaps that lasted until 600 million years ago. It reverted to snowball mode 300 million years ago, and has been that way on and off for most of the last 50 million years. Ice has played a significant part in the story of life. Just how significant a part, we are now beginning to appreci¬ate.
We first began to realize this when we found evidence of the most recent snowball. About one and a half million years ago, round about the time that humans began to become the dominant species on Earth, the planet got very cold. The old name for this period was the Ice Age. We don't call it that any more because it wasn't one Age: we talk of 'glacial-interglacial cycles'. Is there a connection? Did the cold climate drive the naked ape to evolve enough intelligence to kill other animals and use their fur to keep warm? To discover and use fire?
This used to be a popular theory. It's possible. Probably not, though: there are too many holes in the logic. But a much earlier, and much more severe, Ice Age very nearly put a stop to the whole of that 'life' nonsense. And, ironically, its failure to do so may have unleashed the full diversity of life as we now know it.

Thanks to the pioneering insights of Louis Agassiz, Victorian sci¬entists knew that the Earth had once been a lot colder than it is now, because they could see the evidence all around them, in the form of the shapes of valleys. In many parts of the world today you can find glaciers, huge 'rivers' of ice, which flow, very slowly, under the pressure of new ice forming further uphill. Glaciers carry large quantities of rock, and they gouge and grind their way along, form¬ing valleys whose cross-section is shaped like a smooth U. All over Europe, indeed over much of the world, there are identical valleys, but no sign of ice for hundreds or thousands of miles. The Victorian geologists pieced together a picture that was a bit worry¬ing in some ways, but reassuring overall. About 1.6 million years back, at the start of the Pleistocene era, the Earth suddenly became colder. The ice caps at the poles advanced, thanks to a rapid build¬up of snow, and gouged out those U-shaped valleys. Then the ice retreated again. Four times in all, it was thought, the ice had advanced and retreated, with much of Europe being buried under a layer of ice several miles thick.
Still, there was no need to worry, the geologists said. We seemed to be safe and snug in the middle of a warm period, with no prospect of being buried under miles of ice for quite some time ...
The picture is no longer so comfortable. Indeed, some people think that the greatest threat to humanity is not global warming, but an incipient ice age. How ironic, and how undeserved, if our pollution of the planet cancels out a natural disaster!
As usual, the main reason we now know a lot more is that new kinds of observation became possible, propped up by new theories to explain what it is that they measure and why we can be reason¬ably sure that they do. These new methods range from clever methods for dating old rocks to studies of the proportions of dif¬ferent isotopes in cores drilled from ancient ice, backed up by ocean-drilling to study the layers of sediment deposited on the sea floor. Warm seas sustain different living creatures, whose death deposits different sediment, so there is a link from sediments to cli¬mate.
All of these methods reinforce each other, and lead to very much the same picture. Every so often the Earth begins to cool, becom¬ing 10°-15°C colder near the poles and 5°C colder elsewhere. Then it suddenly warms up, possibly becoming 5°C warmer than the cur¬rent norm. In between big fluctuations, there are smaller ones: 'mini ice ages'. The typical gap between a decent-sized ice age and the next is around 75,000 years, often less, nothing like the com¬fortable 400,000 years of 'interglacial' expected by the Victorians. The most worrying finding of all is that periods of high tempera¬tures, that is, like we get now, seldom lasted more than 20,000 years.
The last major glaciation ended 18,000 years ago.
Wrap up well, folks.

What caused the ice ages? It turns out that the Earth isn't quite as nice a planet as we like to think, and its orbit round the sun isn't quite as stable and repetitive as we usually assume. The currently accepted theory was devised in 1920 by a Serbian called Milutin Milankovitch. In broad terms, the Earth goes round the sun in an ellipse, almost a circle, but there are three features of the Earth's motion that change. One is the amount through which the Earth's axis tilts, about 23° at the moment, but varying slightly in a cycle that lasts roughly 41,000 years. Another is a change in the position of Earth's closest approach to the sun, which varies in a 20,000-year cycle. The third is a variation in the eccentricity of the Earth's orbit, how oval it is, whose period is around 100,000 years. Putting all three cycles together, it is possible to calculate the changes in heat received from the sun. These calculations agree with the known variations in the Earth's temperature, and it seems particularly likely that the Earth's warming up after ice ages is due to increased warmth from the sun, thanks to these three astronomical cycles.
It may seem unsurprising that when the Earth receives more heat from the sun, it warms up, and when it doesn't, it cools down, but not all of the heat that reaches the upper atmosphere gets down to the ground. It can be reflected by clouds, and even if it gets to ground level it can be reflected from the oceans and from snow and ice. It is thought that during ice ages, this reflection causes the Earth to lose more heat than it would otherwise do, so ice ages auto¬matically make themselves morse. We get kicked out of them when the incoming heat from the sun is so great that the ice starts to melt despite the lost heat. Or maybe the ice gets dirty, or ... It's not so clear that we get kicked into an ice age when less of the sun's warmth reaches the Earth, indeed the slide into an ice age is usu¬ally more gradual than the climb back out of it.
All of which makes one wonder whether global warming caused by gases excreted from animals might be partly responsible. When gases such as carbon dioxide and methane build up in the atmos¬phere, they cause the famous 'greenhouse effect', trapping more sunlight than usual, hence more heat. Right now, most scientists have become convinced, the Earth's supply of 'greenhouse gases' is growing faster than it would otherwise do thanks to human activi¬ties such as farming (burning rainforests to clear land), driving cars, burning coal and oil for electricity, and farming again (cows pro¬duce a lot of methane: grass goes in one end and methane emerges at the other). And how could we forget the carbon dioxide breathed out by people? One person is equivalent to half a car, maybe more.
Maybe in the past there were vast civilizations of which we now know nothing, all traces having vanished, except for their effect on the global temperature. Maybe the Earth seethed with vast herds of cattle, buffalo, elephants busily excreting methane. But most scien¬tists think that climate change results from variations in five different factors: the sun's output of radiant heat, the Earth's orbit, the composition of the atmosphere, the amount of dust produced by volcanoes, and levels of land and oceans resulting from move¬ment of the Earth's crust. We can't yet put together a really coherent picture in which the measurements match the theory as closely as we'd like, but one thing that is becoming clear is that the Earth's climate has more than one 'equilibrium' state. It stays in or near one such state for a while, then switches comparatively rapidly to another, and so on.
The original idea was that one state was a warm climate, like the one we have now, and the other was a cold 'ice age' one. In 1998 Didier Paillard refined this idea to a three-state model: interglacial (warm), mild glacial (coldish), and glacial (very cold). A drop in heat received from the sun below some critical threshold, caused by those astronomical cycles, triggers a switch from warm to coldish. When the resulting ice builds up sufficiently, it reflects so much of the sun's heat that this triggers another switch from coldish to very cold. But when the sun's heat finally builds up again to another threshold value, thanks once more to the three astronomical cycles, then the climate switches back to warm. This model fits observa¬tions deduced from the amount of oxygen-18 (a radioactive isotope of oxygen) in geological deposits.

Finally, some drama. About 800 million years ago there was an ice age so severe that it very nearly killed off all of the surface life on Earth. This 'big freeze' lasted for between 10 and 20 million years, the ice reached the equator, and it seems that the seas froze to a depth of half a mile (1 km) or more. According to the 'snowball Earth' theory, ice covered the entire Earth at this time. However, if ice really covered the whole Earth, it should have done more dam¬age than the fossil record indicates. So maybe the Earth's axis tilted a lot more than astronomers are willing to concede, and the poles lost their ice while equatorial regions gained it. Or perhaps conti¬nental drift was more rapid at that time than we think, and we've mapped out the extent of the ice incorrectly. Whatever the details, though, it was a spectacularly icy world.
Although the big freeze came close to wiping out ail surface life, it may indirectly have created a lot of today's biodiversity. The big shift from single-celled creatures to multi-celled ones also hap¬pened 800 million years ago. It is plausible that the big freeze cleared away a lot of the single-celled lifeforms and opened up new possibilities for multi-celled life, culminating in the Cambrian Explosion 540 million years ago. Mass extinctions are typically suc¬ceeded by sudden bursts of diversity, in which life reverts from being a 'professional' at the evolutionary game to being an 'ama¬teur'. It then takes a while for the less able amateurs to be eliminated, and until they are, all sorts of strange strategies for making a living can temporarily thrive. The succession of icy peri¬ods that followed the big freeze could only have assisted this process.
However, it may have been the other way round. The invention of the anus by triplobiasts may have changed the ecology of the seas. Faeces would have dropped to the sea-bed, where bacteria could specialize in breaking them down. Other organisms could then become filter feeders, living on those bacteria, perhaps sending their larvae up into the plankton for dispersal, as modern filter-feeders do. Several new ways of life depended on this primeval composting system. And it's possible that the successful return of phosphorus and nitrogen into the marine cycles led to an explosion of algae, which reduced atmospheric carbon dioxide, cut back on the greenhouse effect, and triggered the big freeze.
Fortunately for us, the big freeze wasn't quite long enough, or cold enough, to kill off everything. (Bacteria in volcanic vents on the ocean floor and in the Earth's crust would have survived no matter what, but evolution would have been set back a long, long way.) So when the Earth warmed, life exploded into a fresh, com¬petition-free world. Paradoxically, a major reason why we are here today may be that we very nearly weren't. Our entire evolutionary history is full of these good news-bad news scenarios, where life leaps forward joyously over the bodies of the fallen ...
Rincewind can be forgiven for feeling that Roundworld has it in for him. Life has suffered from many different kinds of natural dis¬aster. Here are two more. In the Permian/Triassic extinction of 250 million years ago, 96% of all species died within the space of a few hundred thousand years. William Hobster and Mordeckai Magaritz think this happened because they suffocated. Carbon iso¬topes show that a lot of coal and shale oxidized in the run-up to the extinction, probably because of a fall in sea level, which exposed more land. The result was a lot more carbon dioxide and a lot less oxygen, which was reduced to half today's level. Land species were especially badly affected.
Another global extinction, though less severe, occurred 55 mil¬lion years ago: the Palaeocene/Eocene boundary. In cores of sediment drilled from the Antarctic, James Kennett and Lowell Stott discovered evidence of the sudden death of a lot of marine species. It seemed that trillions of tons (tonnes) of methane had burst from the ocean, sending temperatures through the roof, methane being a powerful greenhouse gas. Jenny Dickens suggested that the methane was released from deposits of methane hydrates in permafrost and on the seabed. Methane hydrates are a crystal lat¬tice of water enclosing methane gas: they are created when bacteria in mud release the gas and it becomes trapped.
Coincidentally, one of the main results of the Palaeocene/Eocene extinction was a burst of evolutionary diversity, leading in particular to the higher primates, and us. Whether something is a disaster depends on your point of view. Rocks may not have a point of view, as Ponder Stibbons pointed out, but we certainly do.



I   THINK   IT  LOOKS  MORE   LIKE A   HOGSWATCHNIGHT ORNAMENT,' said the Senior Wrangler later, as the wizards took a pre-dinner drink and stared into the omniscope at the glittering white world. 'Quite pretty, really.'
'Bang go the blobs,' said Ponder Stibbons.
'Phut,' said the Dean, cheerfully. 'More sherry, Archchancellor?'
'Perhaps some instability in the sun ...' Ponder mused.
'Made by unskilled labour,' said Archchancellor Ridcully. 'Bound to happen sooner or later. And then it's nothing but frozen death, the tea-time of the gods and an eternity of cold.'
'Sniffleheim,' said the Dean, who'd got to the sherry ahead of everyone else.
'According to HEX, the air of the planet has changed,' said Ponder.
'A bit academic now, isn't it?' said the Senior Wrangler.
'Ah, I've got an idea!' said the Dean, beaming. 'We can get HEX to reverse the thaumic flow in the cthonic matrix of the optimized bi-direction octagonate, can't we?'
'Well, that's the opinion of four glasses of sherry,' said the Archchancellor briskly, to break the ensuing silence. 'However, if I may express a preference, something that isn't complete gibberish would be more welcome next time, please. So, Mister Stibbons, is this the end of the world?'
'And if it is,' said the Senior Wrangler, 'are we going to have a lot of heroes turning up?'
'What are you talking about, man?' said Ridcully.
'Well, the Dean seems to think we're like gods, and a great many mythologies suggest that when heroes die they go to feast forever in the halls of the gods,' said the Senior Wrangler. 'I just need to know if I should alert the kitchens, that's all'
'They're only blobs,' said Ridcully. 'What can they do that's heroic?'
'I don't know ... stealing something from the gods is a very clas¬sical way,' the Senior Wrangler mused.
'Are you saying we should check our pockets?' said the Archchancellor.
'Well, I haven't seen my penknife lately,' said the Senior Wranger. 'It was just a thought, anyway.'
Ridcully slapped the despondent Stibbons on the back.
'Chin up, lad!' he roared. 'It was a wonderful effort! Admittedly the outcome was a lot of blobs with the intelligence of pea soup, but you shouldn't let utter hopeless failure get you down.'
'We never do,' said the Dean.

It was after breakfast next day when Ponder Stibbons wandered into the High Energy Magic building. A scene of desolation met his eye. There were cups and plates everywhere. Paper littered the floor. Forgotten cigarettes had etched their charred trails on the edge of desks. A half-eaten sardine, cheese and blackcurrant pizza, untouched for days, was inching its way to safety.
Sighing, he picked up a broom, and went over the tray contain¬ing HEX's overnight write-out.
It seemed a lot fuller than he would have expected.

'Not just blobs, there's all sorts of stuff! Some of it's wiggling ...'
'Is that a plant or is it an animal?'
'I'msure it's a plant.'
'Isn't it... walking ... rather fast?'
'I don't know I've never seen a plant walking before.'
The wizardery of UU was filtering back in the building as the news got around. The senior members of the faculty were clustered around the omniscope, explaining to one another, now that the impossible had happened, that of course it had been inevitable.
'All those cracks under the sea,' said the Dean. 'And the volca¬noes, of course. Heat's bound to build up over time.'
'That doesn't explain all the different shapes, though,' said the Senior Wrangler. 'I mean, the whole sea looks like somebody had just turned over a very big stone.'
'I suppose the blobs had time to consider their future when they were under the ice,' said the Dean. 'It suppose you could think of it as a very long winter evening.'
'I vote for lavatories,' said the Lecturer in Recent Runes.
'Well, I'm sure we all do,' said Ridcully. 'But why at this point?'
'I mean that the blobs were ... you know ... excusing themselves for millions and millions of years, then you're get a lot of, er, manure ...'the Lecturer ventured.
'A shitload,' said the Dean.
'Dean! Really!'
'Sorry, Archchancellor.'
'... and we know dunghills absolutely teem with life ...' the Lecturer went on.
'They used to think that rubbish heaps actually generated rats,' said Ridcully. 'Of course, that was just a superstition. It's really seagulls. But you saying life is, as it were, advancing by eating dead men's shoes? Or blobs, in this case. Not shoes, of course, because they didn't have any feet. And wouldn't have been bright enough to invent shoes even if they did. And even if they had been, they couldn't have done. Because there was, at that time, nothing from which shoes might be made. But apart from that, the metaphor stands.'
'There still are blobs in there,' said the Dean. 'There's just lots of other things, too.'
'Any of it lookin' intelligent?' said Ridcully.
'I'mnot certain how we'd spot that at this stage ...'
'Simple. Is anything killing something it doesn't intend to eat?'
They stared into the teeming broth.
'Bit hard to define intentions, really,' said the Dean, after a while.
'Well, does anything look as if it is about to become intelligent?'
They watched again.
'That thing like two spiders joined together?' said the Senior Wrangler after a while. 'It looks very thoughtful.'
'I think it looks very dead.'
'Look, I can see how we can settle this whole evolution business once and for all,' said Ridcully, turning away. 'Mister Stibbons, can HEX use the omniscope to see if anything changes into anything else?'
'Over a moderately sized area, I think he probably can, sir.'
'Get it to pay attention to the land,' said the Dean. 'Is there any¬thing happening on the land?'
'There's a certain greenishness, sir. Seaweed with attitude, really.'
'That's where the interesting stuff will happen, mark my words. I don't know what this universe is using for narrativium, but land's where we'll see any intelligent life.'
'How do you define intelligence?' said Ridcully. 'In the long term, I mean.'
'Universities are a good sign,' said the Dean, to general approval.
'You don't think that perhaps fire and the wheel might be more universally indicative?' said Ponder carefully.
'Not if you live in the water,' said the Senior Wrangler. 'The sea's the place here, I'll be bound. On this world practically noth¬ing happens on the land.'
'But in the water everything's eating each other!'
'Then I'll look forward to seeing what happens to the last one served,' said the Senior Wrangler.
'No, when it comes to universities, the land's the place,' said the Dean. 'Paper won't last five minutes under water. Wouldn't you say so, Librarian?'
The Librarian was still staring into the omniscope.
'Ook,' he said.
'What's that he said?' said Ridcully.
'He said "I think the Senior Wrangler might be right",' said Ponder, going over to the omniscope. 'Oh ... look at this ...'

The creature had at least four eyes and ten tentacles. It was using some of the tentacles to manoeuvre a slab of rock against another slab.
'It's building a bookcase?' said Ridcully.
'Or possibly a crude rock shelter,' said Ponder Stibbons.
'There we are, then,' said the Senior Wrangler. 'Personal prop¬erty. Once something is yours, of course you want to improve it. The first step on the road to progress.'
'I'm not sure it's got actual legs,' said Ponder.
'The first slither, then,' said the Senior Wrangler, as the rock slipped from the creatures tentacles. 'We should help it,' he added firmly. 'After all, it wouldn't be here if it wasn't for us.'
'Hold on, hold on,' said the Lecturer in Recent Runes. 'It's only making a shelter. I mean, the Bower Bird builds intricate nests, doesn't it? And the Clock Cuckoo even builds a clock for its mate, and no one says they're intelligent as such.'
'Obviously not,' said the Dean. 'They never get the numerals right, the clocks fall apart after a few months, and they generally lose two hours a day. That doesn't sound like intelligence to me.'
'What are you suggesting, Runes?' said Ridcully.
'Why don't we send young Rincewind down again in that virtu-ally-there suit? With a trowel, perhaps, and an illustrated manual on basic construction?'
'Would they be able to see him?'
'Er ... gentlemen ...' said Ponder, who had been letting the eye of the omniscope drift further into the shallows.
'I don't see why not,' said Ridcully.
'Er ... there's a ... there's ...'
'It's one thing to push planets around over millions of years, but at this level we couldn't even give our builder down there a heavy pat on the back,' said the Dean. 'Even if we knew which part of him was his back.'
'Er ... something's paddling, sir! Something's going for a paddle, sir!'
It was probably the strangest cry of warning since the famous 'Should the reactor have gone that colour?' The wizards clustered around the omniscope.
Something had gone for a paddle. It had hundreds of little legs.

Rincewind was in his new office, filing rocks. He'd worked out quite a good system, based on size, shape, colour and twenty-seven other qualities including whether or not he felt that it was a friendly sort of rock.
With careful attention to cross-referencing, he reckoned that dealing with just those rocks in this room would take him at least three quiet, blessed years.
And he was therefore surprised to find himself picked up bodily and virtually carried towards the High Energy Magic building holding, in one hand, a hard square light grey rock and, in the other hand, a rock that appeared to be well disposed to people.
'Is this yours?' roared Ridcully, stepping side to reveal the omniscope.
The Luggage was now bobbing contently a few metres offshore.
'Er ...' said Rincewind. 'Sort of mine.'
'So how did it get there?’
'Er ... it's probably looking for me,' said Rincewind. 'Sometimes it loses track.'
'But that's another universe!' said the Dean.
'Can you call it back?'
'Good heavens, no. If I could call it back, I'd send it away.'
'Sapient pearwood is meta-magical and will track its owner absolutely anywhere in time and space,' said Ponder.
'Yes, but not this bit!' said Ridcully.
'I don't recall "not this bit" ever being recorded as a valid sub¬set of "time and space", sir,' said Ponder. 'In fact, "not this bit" has never even been accepted as a valid part of any magical invocation, ever since the late Funnit the Foregetful tried to use it as a last-minute addition to his famously successful spell to destroy the entire tree he was sitting in.'
'The Luggage may consist of a subset of at least n dimensions which may co-exist with any other set of >n dimensions,' said the Bursar.
'Don't pay any attention, Stibbons,' said Ridcully wearily. 'He's been spouting this stuff ever since he tried to understand HEX's write-out. It's completely gibberish. What's 'V, then, old chap?'
'Umpt,' said the Bursar.
'Ah, imaginary numbers again,' said the Dean. 'That's the one he says should come between three and four.'
'There isn't a number between three and four,' said Ridcully.
'He imagines there is,' said the Dean.
'Could we get inside the Luggage in order to physically go into the project universe?' said Ponder.
'You could try,' said Rincewind. 'I personally would rather saw my own nose off.'
'Ah. Really?'
'But the thought occurs,' said Ridcully, 'that we can use it to bring things back. Eh?'
Down under the warm water, the strange creature's stone struc¬ture collapsed for the umpteenth time.

A week went past. On Tuesday a left-over snowball collided with the planet, causing considerable vexation to the wizards and destroying an entire species of net-weaving jellyfish of which the Senior Wrangler had professed great hopes. But at least the Luggage could be used to bring back any specimens stupid enough to swim into something sitting underwater with its lid open, and this included practically everything in the sea at the moment.
Life in the round world seemed to possess a quality so prevalent that the wizards even discussed the idea that it was some conceptual element, which was perhaps trying to fill the gap left by the non¬existent deitygen.
'However,' Ridcully announced, 'Bloodimindium is not a good name.'
'Perhaps if we change the accent slightly,' said the Lecturer in Recent Runes. 'Blod-di-min-dium, do you think?'
'They've certainly got a lot of it, whatever we call it,' said the Dean. 'It's not a world to let a complete catastrophe get it down.'
Things turned up. Shellfish suddenly seemed very popular, A theory gaining ground was that the world itself was generating them in some sort of automatic way.
'Obviously, if you have too many rabbits, you need to invent foxes,' said the Dean, at one of the regular meetings. 'If you've got fish, and you want phosphates, you need seabirds.'
'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 dis¬appointment. 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 con¬tender 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 promis¬ing, 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 mar¬ble 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 min¬utes 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.'

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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 blun¬dered 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 increas¬ing 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 sim¬ple 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 'prim¬itive' 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 cock¬roach may be primitive in the sense that it is simpler than an elephant, it is not primitive in the sense of being an ancient ances¬tral 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 cock¬roach today. In particular, the DNA of a Cretaceous cockroach was probably significantly different from the DNA of a modern cock¬roach. 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' pic¬ture 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.
However, the tree image is misleading in several respects. There is, for instance, no relation between the thickness of a branch and the size of the corresponding population, the thick trunk at the bottom may represent fewer organisms, or less total organic mass, than the twig at the top. (Think about the human twig ...) The way branches split may also be misleading: it implies a kind of long-term continuity of species, even when new ones appear, because on a tree the new branches grow gradually out of the old ones. Darwin thought that speciation, the formation of new species, is gener¬ally gradual, but he may have been wrong. The theory of 'punctuated equilibrium' of Stephen Jay Gould and Niles Eldredge maintains the contrary: speciation is sudden. In fact there are excel¬lent mathematical reasons for expecting speciation to have elements of both, sometimes sudden, sometimes gradual.
Another problem with the Tree of Life image is that many of its branches are missing, many species go unrepresented in the fossil record. The most misleading feature of all is the way humans get placed right at the top. For psychological reasons we equate height with importance (as in the phrase 'your royal highness'), and we rather like the idea that we're the most important creature on the planet. However, the height of a species in the Tree of Life indicates when it flourished, so every modern organism, be it a cockroach, a bee, a tapeworm, or a cow, is just as exalted as we are.
Gould, in Wonderful Life, objected to the 'tree' image for other reasons, and he based his objections on a remarkable series of fos¬sils preserved in a layer of rock known as the Burgess Shale. These fossils, which date from the start of the Cambrian era, are the remains of soft-bodied creatures living on mud-banks at the base of an algal reef, which became trapped under a mudslide. Very few fos¬sils of soft-bodied creatures exist, because normally only the harder parts survive fossilization. However, the significance of the Burgess Shale fossils went unrecognized from their discovery by Charles Walcott in 1909, until Harry Whittington took a closer look at them in 1971. The organisms were all squashed flat, and it was virtually impossible to recognize what shape they'd been while alive. Then Simon Conway Morris teased the squished layers apart, and reconstructed the original forms using a computer, and the strange secret of the Burgess Shale was revealed to the world.
Until that point, palaeontologists had classified the Burgess Shale organisms into various conventional types, worms, arthro¬pods, whatever. But now it became clear that most of those assignments were mistaken. We knew, for example, just four con¬ventional types of arthropod: trilobites (now extinct), chelicerates (spiders, scorpions), crustaceans (crabs, shrimp), and uniramians (insects and others). The Burgess Shale contains representatives of all of these, but it also contains twenty other radically different types. In that one mudslide, preserved in layers of shale like pressed flowers in the pages of a book, we find more diversity than in the whole of life today.
Musing on this amazing discovery, Gould realized that most branches of the Tree of Life that grew from the Burgess beasts must have 'snapped off' by way of extinction. Long ago, 20 of those 24 arthropod body plans disappeared from the face of the Earth. The Grim Reaper was pruning the Tree of Life, and being heavy-handed with the shears. So Gould suggested that a better image than a tree would be something like scrubland. Here and there 'bushes' of species sprouted from the primal ground level. Most, however, ceased to grow, and were pruned to a standstill hundreds of millions of years ago. Other bushes grew to tall shrubs before stopping ... and one tall tree made it right up to the present day. Or maybe we've reconstructed it incorrectly, amalgamating several dif¬ferent trees into one.
This new image changes our view of human evolution. One aniT mal in the Burgess Shale, named Pikaia, is a chordate. This is the group that evolved into all of today's animals that have a spinal cord, including fishes, amphibians, reptiles, birds, and mammals. Pikaia is our distant ancestor. Another creature in the Burgess Shale, Nectocaris, has an arthropod-like front end but a chordate back, and it has left no surviving progeny. Yet they both shared the same environment, and neither is more obviously 'fit' to survive than the other. Indeed, if one had been less evolutionarily fit, it would almost certainly have died out long before the fossils were formed. So what determined which branch survived and which didn't? Gould's suggestion was: chance.
The Burgess Shale formed on a major geological boundary: at the end of the Precambrian era and the start of the Palaeozoic. The early part of the Palaeozoic is known as the Cambrian period, and it is a time of enormous biological diversity, the 'Cambrian explo¬sion'. The Earth's creatures were recovering from the mass extinction of the Ediacarans, and evolution took the opportunity to play new games, because for a while it didn't matter much if it played them badly. The 'selection pressure' on new body-plans was small because life hadn't fully recovered from the big die-back. In these circumstances, said Gould, what survives and what does not is mostly a matter of luck, mudslide or no mudslide, dry climate or wet. If you were to re-run evolution past this point, it's quite likely that totally different organisms would survive, different branches of the Tree of Life would be snipped off.
Second time round, it could easily be our branch that got pruned.

This vision of evolution as a 'contingent' process, one with a lot of random chance involved, has a certain appeal. It is a very strong way to make the point that humans are not the pinnacle of creation, not the purpose of the whole enterprise. How could we be, if a few random glitches could have swept us from the board altogether? However, Gould rather overplayed his hand (and he backed off a bit in subsequent writings). One minor problem is that more recent reconstructions of the Burgess Shale beasts suggests that their diversity may have been somewhat overrated, though they were still very diverse.
But the main hole in the argument is convergence. Evolution settles on solutions to problems of survival, and often the range of solutions is small. Our present world is littered with examples of 'convergent evolution', in which creatures have very similar forms but very different evolutionary histories. The shark and the dol¬phin, for instance, have the same streamlined shape, pointed snout, and triangular dorsal fin. But the shark is a fish and the dolphin is a mammal.
We can divide features of organisms into two broad classes: universals and parochials. Universals are general solutions to survival problems, methods that are widely applicable and which evolved independently on several occasions. Wings, for instance, are universals for flight: they evolved separately in insects, birds, bats, even flying fish. Parochials happen by accident, and there's no reason for them to be repeated. Our foodway crosses our airway, leading to lots of coughs and splutters when 'something goes down the wrong way'. This isn't a universal: we have it because it so happened that our distant ancestor who first crawled out of the ocean had it. It's not even a terribly sensible arrangement, it just works well enough for its flaws not to count against us when combined with everything else that makes us human. Its deficiencies were tolerated from the first fish-out-of-water, through amphibians and dinosaurs, to mod¬ern birds, and from amphibians through mammal-like reptiles to mammals like us. Because evolution can't easily 'un-evolve' major features of body-plans, we're stuck with it.
If our distant ancestors had got themselves killed off by acci¬dent, would anything like us still be around? It seems very unlikely that creatures exactly like us would have turned up, because a lot of what makes us tick is parochials. But intelligence looks like a clear case of a universal, cephalopods evolved intelligence independ¬ently of mammals, and anyway, intelligence is such a generic trick. So it seems likely that some other form of intelligent life would have evolved instead, though not necessarily adhering to the same timetable. On an alternative Earth, intelligent crabs might invent a fantasy world shaped like a shallow bowl that rides on six sponges on the back of a giant sea urchin. Three of them could at this very moment be writing The Science of Dishworld.
Sorry. But it is true. But for a fall of rock here, a tidal pattern there, we wouldn't have been us. The interesting thing is that we almost certainly would have been something else.



HEX WAS THINKING HARD AGAIN. Running the little universe was taking much less time than k had expected. It more or less ran itself now, in fact. The gravity operated without much attention, rainclouds formed with no major interference and rained every day. Balls went around one another.
HEX didn't think it was a shame about the crabs going. HEX had¬n't thought it was marvellous that the crabs had turned up. HEX thought about the crabs as something that had happened. But it had been interesting to eavesdrop on Crabbity, the way the crabs named themselves, thought about the universe (in terms of crabs), had legends of the Great Crab clearly visible in the Moon, passed on in curious marks the thoughts of great crabs, and wrote down poetry about the nobility and frailty of crab life, being totally accu¬rate, as it turned out, on this last point.
HEX wondered: if you have life, then intelligence will arise some¬where. If you have intelligence, then extelligence will arise somewhere. If it doesn't, intelligence hasn't got much to be intelli¬gent about. It was the difference between one little oceanic crustacean and an entire wall of chalk.
The machine also wondered if it should pass on these insights to the wizards, especially since they actually lived in one of the world's more interesting outcrops of extelligence. But HEX knew that its creators were infinitely cleverer than it was. And great masters of disguise, obviously.

The Lecturer in Recent Runes had designed a creature.
'Really, all we need is a basic limpet or whelk, to begin with,' he said, as they looked at the blackboard. 'We bring it back here where proper magic works, try a few growth spells, and then let Nature take its course. And, since these extinctions seem to be wiping out everything, it'll gradually become the dominant feature.'
'What's the scale again?' said Ridcully, critically.
'About two miles to the tip of the cone,' said the Lecturer. 'About four miles across the base.'
'Not very mobile, then,' said the Dean.
'The weight of the shell will certainly hamper it, but I imagine it should be able to move its own length in a year, perhaps two.'
'What'll it eat, then?'
'Everything else.'
'Such as...'
'Everything. I'd advise suction holes around the base here so that it can filter seawater for useful things like plankton.'
'Plankton being…?'
'Oh, whales, shoals offish and so on.'
The wizards looked long and hard at the huge cone-shaped object.
'Intelligence?' said Ridcully.
'What for?' said the Lecturer in Recent Runes.
'It will withstand anything except a direct hit with a comet, and I estimate it'll have a lifespan of about 500,000 years.'
'And then it'll die?' said Ridcully.
'Yes. I estimate it will, by then, take it twenty-four hours and one second to absorb enough food to last it for twenty-four hours.'
'So after that it will be dead?'
'Will it know?'
'Probably not.'
'Back to the drawing board, Senior Lecturer.'

Ponder sighed.
'It's no good ducking,' he said. 'That won't help. We're paying special attention to comets. We'll let you know in plenty of time.'
'You've got no idea what it was like!' said Rincewind, creeping along the beach. 'And the noise!'
'Have you seen the Luggage?'
'It certainly made my ears ring, I can tell you!'
'And the Luggage?'
'What? Oh ... gone. Have you looked at that side of the planet? There's a whole new set of mountain ranges!'
The wizards had let time run forward for a while after the strike. It made such a depressing mess of everything. Now, drawing on its bottomless reserves of bloodimindium, life was returning in strength. Crabs were already back although, here, at least, they did¬n't seem inclined to make even simple structures. Perhaps something in their souls told them it'd be a waste of time in the long run.
Rincewind mentally crossed them off the list. Look for signs of intelligence, the Archchancellor had said. As far as Rincewind was concerned, anything really intelligent would be keeping out of the way of the wizards. If you saw a wizard looking at you, Rincewind would advise, then you should walk into a tree or say 'dur?'.
All along the beaches, and out below the surf, everything was acting with commendable stupidity.
A soft sound made him look down. He'd almost stepped on a fish.
It was some way from the water line, and squirming across the mud towards a pool of brackish water.
A kind man by nature, Rincewind picked it up gingerly and car¬ried it back to the sea. It flopped around in the shallows for a while and then, to his amazement, inched its way back on to the mud.
He put it back again, in deeper water this time.
Thirty seconds kter, it was back on the beach.
Rincewind crouched down, as the thing wiggled determinedly onwards.
'Would it help to talk to someone?' he said. 'I mean, you've got a good life out there in the sea, no sense in throwing it all away, is there? There's always a silver lining if you know where to look. Okay, okay, life's a beach. And you're a pretty ugly fish. But, you know, beauty is only sk- scale deep, and...’
'What's happening?' said Ponder's voice in his ear.
'I was talking to this fish,' said Rincewind.
'It keeps coming out of the water. It seems to want to go for whatever is the opposite of a paddle.'
'You told me to keep a look out for anything interesting.'
'The consensus here is that fish aren't interesting,' said Ponder. 'Fish are dull.'
'I can see bigger fish in the shallows,' said Rincewind. Terhaps it's trying to keep away from them?'
'Rincewind, fish are designed for living in water That's why they're fish. Go and find some crabs. And put the poor freakish thing back in the sea, for goodness' sake.'

'Perhaps a rethink is in order here,' said Ridcully.
'About the newts,' said Ponder.
'Newts is going far too far,' said the Dean. 'I've seen more shapely things in the privy.'
'I want the person who put the newts on this continent to own up right now,' said Ridcully.
'No one could,' said the Senior Wrangler. 'No one's seen the Luggage since the last comet. We couldn't get anything in there.'
'I know, because I had a tank of thaumically treated whelks all ready to go,' said the Lecturer in Recent Runes. 'And what, pray, am I supposed to do with them?'
'Some sort of chowder would appear to be in order,' said the Dean.
'Evolution makes things better,' said Ridcully. 'It can't make them different. All right, some rather dull amphibians seem to have turned up. But, and this is important, those fish Rincewind reported are still around. Now, if they were going to turn into things with legs, why are they still here?'
'Tadpoles are fish,' said the Bursar.
'But a tadpole knows it's going to be a frog,' said Ridcully patiently. 'There's no narrativium on this world. That fish couldn't be saying to itself "Ah, a new life beckons on dry land, walking around on things I haven't yet got a name for."' No, either the planet is somehow generating new life, or we're back to the old "hidden gods" theory.'
'It's all gone wrong, you know,' said the Dean. 'It's the bloodi-mindium. Even gods couldn't control this place. Once there's life, there's complete and utter chaos. Remember that book the Librarian brought back? It's a complete fantasy! Nothing seems to happen like that at all! Everything just does what it likes!'
'Progress is being made,' said Ponder.
'Big amphibians?' sneered the Senior Wrangler. 'And things were going so well in the sea. Remember those jellyfish that made nets? And the crabs even had a flourishing land civilization! They had practically got a culture!'
'They ate captured enemies alive' said the Lecturer in Recent Runes, patiently.
'Well... yes. But with a certain amount of etiquette, at least,' the Senior Wrangler admitted. 'And in front of their sand statue of the Great Big Crab. They were obviously attempting to control their world. And what good did it do them? A million tons of white hot ice smack between the eyestalks. It's so upsetting.'
Terhaps they should have eaten more enemies,' said the Dean.
Terhaps sooner or later the pknet will get the message,' said Ridcully.
'Time for the giant whelks, perhaps?' said the Lecturer in Recent Runes, hopefully.
'Big newts is what we've got right now,' said Ridcully. He glanced at the Dean and Senior Wrangler. Ridcully hadn't main¬tained his position atop the boiling heap of UU wizardry without a little political savvy. 'And newts, gentlemen, might be the way to go. Amphibians? At home in the water and on land? The best of both worlds, I fancy.'
The two wizards exchanged sheepish glances.
'Well ... I suppose ...' said the Senior Wrangler.
'Could be,' the Dean said grudgingly. 'Could be.'
'There we are then,' said Ridcully happily. 'The future is newt.'

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Let's take a step away from the unfolding ancestral tale of The Fish That Came Out From The Sea and look at a more philosophical issue.
The wizards are puzzled. On Discworld, things happen because narrative imperative makes them happen. There is no choice about ends, only about means. The Lecturer in Recent Runes is trying to make a sustainable lifeform happen. He thinks that the obstacle to sustainability is the fragility of life, so the only way he can see to achieve this is the two-mile limpet, proof against everything the sky can drop on it.
It never occurs to him that lifeforms might achieve sustainabil¬ity by other, less direct methods, despite the evidence of his eyes that suggests that a dogged tenacity appears to allow life to arise in the most inhospitable environment, effectively re-creating itself over and over again. The wizards are torn between evidence that a planet is the last place you'd choose to create life, and evidence that life doesn't agree.
On Discworld, it is clearly recognized that million-to-one chances happen nine times out of ten. The reason is that every Discworld character lives out a story, and the demands of the story determine how their lives unfold. If a million to one chance is required to keep that story on track, then that's what will happen, appalling odds notwithstanding. On Discworld, abstractions gener¬ally show up as things, so there is even a thing, narrativium, that ensures that everybody obeys the narrative imperative. Another personification of the abstract, Death, also makes sure that each individual's story comes to an end exactly when it's supposed to. Even if a character tries to behave contrary to the story in which they find themselves, narrativium makes sure that the end result is consistent with the story anyway.
What's puzzling the wizards is that our world isn't like that ...
Or is it?
After all, people live on our world too, and it's people that drive stories.
As case in point, a story about people who drive. The setting is Jerez Grand Prix circuit, last race of the 1997-98 Formula One motor racing season ... Ace driver Michael Schumacher is one Championship point ahead of arch-rival Jacques Villeneuve. Villeneuve's team-mate Heinz-Harold Frentzen may well play a crucial tactical role. The drivers are competing for 'pole position' on the starting grid, which goes to whoever produces the fastest lap in the qualifying sessions. So what happens? Unprecedentedly, Villeneuve, Schumacher, and Frentzen all lap in 1 minute 21.072 seconds, the same time to a thousandth of a second. An amazing coincidence.
Well: 'coincidence' it surely was, the lap times coincided. But was it truly amazing?
Questions like this arise in science, too, and they're important. How significant is a statistical cluster of leukaemia cases near a nuclear installation? Does a strong correlation between lung cancer and having a smoker in the family really indicate that secondary smoking is dangerous? Are sexually abnormal fish a sign of oestro¬gen-like chemicals in our water supply?
A case in point. It is said that 84% of the children of Israeli fighter pilots are girls. What is it about the life of a fighter pilot that produces such a predominance of daughters? Could an answer lead to a breakthrough in choosing the sex of your children? Or is it just a statistical freak? It's not so easy to decide. Gut feelings are worse than useless, because human beings have a rather poor intuition for random events. Many people believe that lottery numbers that have so far been neglected are more likely to come up in future. But the lottery machine has no 'memory', its future is independent of its past. Those coloured plastic balls do not know how often they have come up in previous draws, and they have no tendency to compen¬sate for past imbalances.
Our intuition goes even further astray when it comes to coinci¬dences. You go to the swimming baths, and the guy behind the counter pulls a key at random from a drawer. You arrive in the changing room and are relieved to find that very few lockers are in use ... and then it turns out that three people have been given lock¬ers next to yours, and it's all 'sorry!' and banging locker doors together Or you are in Hawaii, for the only time in your life ... and you bump into the Hungarian you worked with at Harvard. Or you're on honeymoon camping in a remote part of Ireland ... and you and your new wife meet your Head of Department and his new wife, walking the other way along an otherwise deserted beach. All of these have happened to Jack.

Why do we find coincidences so striking? Because we expect ran¬dom events to be evenly distributed, so statistical clumps surprise us. We think that a 'typical' lottery draw is something like 5, 14, 27, 36, 39,45, but that 1,2, 3,19,20,21 is far less likely. Actually, these two sets of numbers have exactly the same probability: 1 in 13,983,816. A typical lottery draw often includes several numbers close together, because sequences of six random numbers between 1 and 49 are more likely to be clumpy than not.
How do we know this? Probability theorists tackle such ques¬tions using 'sample spaces', their name for what we earlier called a 'phase space', a conceptual 'space' that organizes all the possibil¬ities. A sample space contains not just the event that concerns us, but all possible alternatives. If we are rolling a die, for instance, then the sample space is 1, 2, 3, 4, 5, 6. For the lottery, the sample space is the set of all sequences of six different numbers between 1 and 49. A numerical value is assigned to each event in the sample space, called its 'probability', and this corresponds to how likely that event is to happen. For fair dice each value is equally likely, with a proba¬bility of 1/6. Ditto for the lottery, but now with a probability of 1/13,983,816.
We can use a sample space approach to get a ball-park estimate of how amazing the Formula One coincidence was. Top drivers all lap at very nearly the same speed, so the three fastest times can eas¬ily fall inside the same tenth-of-a-second period. At intervals of a thousandth of a second, there are one hundred possible lap times for each to 'choose' from: this list determines the sample space, The probability of the coincidence turns out to be one chance in ten thousand. Unlikely enough to be striking, but not so unlikely that we ought to feel amazed.
Estimates like this help to explain astounding coincidences reported in newspapers, such as a bridge player getting a 'perfect hand', all thirteen cards in one suit. The number of games of bridge played every week worldwide is huge, so huge that every few weeks the actual events explore the entire sample space. So occa¬sionally a perfect hand actually does turn up, with the frequency that its small but non-zero probability predicts. The probability of all four players getting a perfect hand at the same time, though, is so micoscopic that even if every planet in the galaxy had a billion inhabitants, all playing bridge every day for a billion years, you wouldn't expect it to happen.
Nevertheless, every so often the newspapers report a four-way perfect hand. The sensible conclusion is not that a miracle hap¬pened, but that something changed the odds. Possibly the players got close to a four-way perfect hand, and the tale grew in the telling, so that when the journalist arrived with a photographer, another kind of narrative imperative ensured that their story fitted what the jour¬nalist had been told. Possibly they deliberately cheated to get their names in the papers. Scientists, especially, tend to underestimate the propensity of people to lie. More than one scientist has been fooled into accepting apparent evidence of extrasensory perception or other 'supernatural' events, which can actually be traced to delib¬erate trickery.
Many other apparent coincidences, on close investigation, slither into a grey area in which trickery is strongly suspected, but may never be proved, either because sufficient evidence is unob¬tainable, or because it's not worth the trouble. Another way to be fooled about a coincidence is to be unaware of hidden constraints that limit the sample space. That 'perfect hand' could perhaps be explained by the way bridge players often shuffle cards for the next deal, which can be summed up as: poorly. If a pack of cards is arranged so that the top four cards consist of one from each suit, and thereafter every fourth card is in the same suit, then you can cut (but not shuffle, admittedly) the pack as many times as you like, and it will deal out a four-way perfect hand. At the end of a game, the cards lie on the table in a fairly ordered manner, not a random one, so it's not so surprising if they possess a degree of structure after they've been picked up.
So even with a mathematically tidy example like bridge, the choice of the 'right' sample space is not entirely straightforward. The actual sample space is 'packs of cards of the kind that bridge players habitually assemble after concluding a game', not 'all possi¬ble packs of cards'. That changes the odds.
Unfortunately, statisticians tend to work with the 'obvious' sam¬ple space. For that question about Israeli fighter pilots, for instance, they would naturally take the sample space to be all children of Israeli fighter pilots. But that might well be the wrong choice, as the next tale illustrates.
According to Scandinavian folklore, King Olaf of Norway was in dispute with the King of Sweden about ownership of an island, and they agreed to throw dice for it: two dice, highest total wins. The Swedish king threw a double-six. 'You may as well give up now,' he declared in triumph. Undeterred, Olaf threw the dice ... One turned up six ... the other split in half, one face showing a six and the other a one. 'Thirteen, I win,' said Olaf.'
Something similar occurs in The Colour ofMagic, where several gods are playing dice to decide certain events on the Discworld:

The Lady nodded slightly. She picked up the dice-cup and held it steady as a rock, yet all the Gods could hear the three cubes rattling about inside. And then she sent them bouncing across the table.
A six. A three. A five.
Something was happening to the five, however. Battered by the chance collision of several billion molecules, the die flipped onto a point, spun gently and came down a seven.
Blind lo picked up the cube and counted the sides.
'Come on,' he said wearily. 'Play fair.'

Nature's sample space is often bigger than a conventional statisti¬cian would expect. Sample spaces are a human way to model reality: they do not capture all of it. And when it comes to estimating sig¬nificance, a different choice of sample space can completely change our estimates of probabilities. The reason for this is an extremely important factor, 'selective reporting', which is a type of narra-tivium in action. This factor tends to be ignored in most conventional statistics. That perfect hand at bridge, for instance, is far more likely to make it to the local or even national press than an imperfect one. How often do you see the headline BRIDGE PLAYER GETS ENTIRELY ORDINARY HAND, for instance? The human brain is an irrepressible pattern-seeking device, and it seizes on certain events that it considers significant, whether or not they really are. In so doing, it ignores all the 'neighbouring' events that would help it judge how likely or unlikely the perceived coincidence actually is.
Selective reporting affects the significance of those Formula One times. If it hadn't been them, maybe the tennis scores in the US Open would have contained some unusual pattern, or the football results, or the golf ... Any one of those would have been reported, too, but none of the failed coincidences, the ones that didn't hap¬pen, would have hit the headlines. FORMULA ONE DRIVERS RECORD DIFFERENT LAP TIMES ... If we include just ten major sporting events in our list of would-be's that weren't, that one in ten thousand chance comes down to only one in a thousand.

Having understood this, let's go back to the Israeli fighter pilots. Conventional statistics would set up the obvious sample space, assign probabilities to boy and girl children, and calculate the chance of getting 84% girls in a purely random trial. If this were less than one in a hundred, say, then the data would be declared 'significant at the 99% level'. But this analysis ignores selective reporting. Why did we look at the sexes of Israeli fighter pilots' chil¬dren in the first place? Because our attention had already been drawn to a clump. If instead the clump had been the heights of the children of Israeli aircraft manufacturers, or the musical abilities of the wives of Israeli air traffic controllers, then our clump-seeking brains would again have drawn the fact to our attention. So our computation of the significance level tacitly excludes many other factors that didn 't clump, making it fallacious.
The human brain filters vast quantities of data, seeking things that appear unusual, and only then does it send out a conscious sig¬nal: Wow! Look at that! The wider we cast our pattern-seeking net, the more likely it is to catch a clump. For this reason, it's illegitimate to include the data that brought the clump to our attention as part of the evidence that the same clump is unusual. It would be like sorting through a pack of cards until you found the ace of spades, putting it on the table, and then claiming miraculous powers that unerringly accomplish a feat whose probability is one in 52.
Exactly this error was made in early experiments on extra-sen¬sory perception. Thousands of subjects were asked to guess cards from a special pack of five symbols. Anyone whose success rate was above average was invited back, while the others were sent home. After this had gone on for several weeks, the survivors all had an amazing record of success! Then these 'good guessers' were tested some more. Strangely, as time went on, their success rate slowly dropped back towards the average, as if their powers were 'running down'. Actually, that effect wasn't strange at all. It happened because the initial high scores were included in the running total. If they had been omitted, then the scoring rate would have dropped, immediately, to near average.
So it is with the fighter pilots. The curious figures that drew researchers' attention to these particular effects may well have been the result of selective reporting, or selective attention. If so, then we can make a simple prediction: 'From now on, the figures will revert to fifty-fifty.' If this prediction fails, and if the results instead con¬firm the bias that revealed the clump, then the new data can be considered significant, and a significance level can sensibly be assigned by the usual methods. But the smart money is on a fifty-fifty split.

The alleged decline in the human sperm count may be an example of selective reporting. The story, widely repeated in the press, is that over the past 50 years the human sperm count for 'normal' men has halved. We don't mean selective reporting by the people who published the first evidence, they took pains to avoid all the sources of bias that they could think of. The 'selective reporting' was done by researchers who had contrary evidence but didn't pub¬lish it because they thought it must be wrong, by journal referees who accepted papers that confirmed a decline more often than they accepted those that didn't, and by the press, who strung together a whole pile of sex-related defects in various parts of the animal kingdom into a single seamless story, unaware that each individual instance has an entirely reasonable explanation that has nothing to do with falling sperm-counts and often nothing to do with sex.
Sexual abnormalities in fish near sewer outlets, for instance, are probably due to excess nitrites, which all fish-breeders know cause abnormalities of all kinds, and not to oestrogen-like compounds in the water, which would bolster the 'sperm count' story. Current data from fertility clinics, by the way, show no signs of a decline.
Humans add narrativium to their world. They insist in inter¬preting the universe as if it's telling a story. This leads them to focus on facts that fit the story, while ignoring those that don't. But we mustn't let the coincidence, the clump, choose the sample space -when we do that, we're ignoring the surrounding space of near-coincidences.
Jack and Ian managed to test this theory on a trip to Sweden. On the plane, Jack predicted that a coincidence would happen at Stockholm airport, for reasons of selective reporting. If they looked hard enough, they'd find one. They got to the bus stop out¬side the terminal, and no coincidences had occurred. But they couldn't find the right bus, so Jack went back to the enquiries desk. As he waited, someone came up next to him, Stefano, a mathe¬matician who normally occupied the office next door to Jack's. Prediction confirmed. But what was really needed was evidence of a near-coincidence, one that hadn't happened, but could have been selectively reported if it had. For instance, if some other acquain¬tance had shown up at exactly the same time, but on the wrong day, or at the wrong airport, they'd never have noticed. Near coinci¬dences, by definition, are hard to observe ... but not impossible. Ianhappened to mention all of the above to his friend Ted, who was vis¬iting soon after. 'Stockholm?' said Ted: 'When?' Iantold him. 'Which hotel?' Iantold him 'Funny, I was staying there one day later than your Had the trip been one day later, the 'coincidental' encounter with Stefano wouldn't have happened, but the one with Ted would.
What we must not do, then, is to look back at past events and find significance in the inevitable few that look odd. That is the way of the pyramidologists and the tea-leaf readers. Every pattern of raindrops on the pavement is unique. We're not saying that if one such patterns happens to spell your name, this is not to be won¬dered at, but if your name had been written on the pavement in Beijing during the Ming dynasty, at midnight, nobody would have noticed. We should not look at past history when assessing signifi¬cance: we should look at all the other things that might have happened instead.
Every event is unique. Until we place that event in a category, we can't work out which background to view it against. Until we choose a background, we can't estimate the event's probability. If we consider the sample space of all possible DNA codes, for instance, then we can calculate the probability of a human being having exactly your DNA code, which is vanishingly small. But it would be silly to conclude that it is impossible for you to exist.



'THE FUTURE IS LIZARD,' said Ridcully. 'Obviously.' It was a few days later. The omniscope was focused on a mound of leaves and rotting vegeta¬tion a little way from the banks of the river. There was a large depression hanging over the Senior Wrangler, and the Dean had a black eye. The war between land and sea had just entered a terminal stage.
'Little portable seas,' said Ponder. 'You know, I never thought of them like that.'
'An egg is an egg, however you look at it,' said Ridcully. 'Look, you two, I don't want to see a scuffle like that again, d'you hear?' The Senior Wrangler dabbed at his bleeding nose. 'He goaded be,' he said. 'Id's still osuns, howeber you look at id.' 'A private ocean full of food,' said Ponder, still entranced. 'Hidden in a heap of... well, compost. Which heats up. That's like having private sunshine.'
The little lizard-like creatures that had hatched from the eggs in the mound slithered and slid down the bank into the water, bright-eyed and hopeful. The first few were instantly snapped up by a large male lying in wait among the weeds.
'However, the mothers still have something to learn about post¬natal care,' said Ridcully. 'I wonder if they'll have time to learn? And how did they know how to do this? Who's telling them?'

The wizards were depressed again. Most days started that way now. Creatures seemed to turn up in the world randomly, and certainly not according to any pictures in a book. If things were changing into other things, and no one had seen that happen yet, why were the original things still the original things? If the land was so great, why were any fish left in the sea?
The air-breathing fishes that Rincewind had seen still seemed to be around, lurking in swamps and muddy beaches. Things changed, but still stayed the same.
And if there was any truth at all in Ponder's tentative theory that things did change into other things, it led to the depressing thought that, well, the world was filling up with quitters, creatures which -instead of staying where they were, and really making a go of life in the ocean or the swamp or wherever, were running away to lurk in some niche and grow legs. The kind offish that'd come out of water was, frankly, a disgrace to the species. It kept coughing all the time, like someone who'd just given up smoking.
And there was no purpose, Ridcully kept saying. Life was on land. According to the book, there should be some big lizards. But nothing seemed to be making much of an effort. The moment any¬thing felt safe, it stopped bothering.
Rincewind, currently relaxing on a rock, rather liked it. There were large animals snuffling around in the greenery near the rock he was sitting on; in general shape and appearance, they looked like a small skinny hippopotamus designed in the dark by a complete amateur. They were hairy. They coughed, too.
Things that were doing sufficiently beetle-like things for him to think of them as beetles ambled across the ground.
Ponder had told him the continents were moving again, so he kept a firm grip on his rock just in case.
Best of all, nothing seemed to be thinking. Rincewind was con¬vinced that no good came of that sort of thing.
The last few weeks of Discworld time had been instructive. The wizards had tentatively identified several dozen embryo civiliza¬tions, or at least creatures that seemed to be concerned about more than simply where their next meal was coming from. And where were they now? There was a squid one, HEX said, out in the really deep cold water. Apart from that, ice or fire or both at once came to the thinkers and the stupid alike. There was probably some kind of moral involved.
The air shimmered, and half a dozen ghostly figures appeared in front of him.
There were, in pale shadowy colours, the wizards. Silvery lines flickered across their bodies and, periodically, they flickered.
'Now, remember,' said Ponder Stibbons, and his voice sounded muffled, 'You are in fact still in the High Energy Magic building. If you walk slowly HEX will try to adjust your feet to local ground level. You'll have a limited ability to move things, although HEX will do the actual work...’
'Can we eat?' said the Senior Wrangler.
'No, sir. Your mouth isn't here.'
'Well, then, what am I talking out of?'
'Could be anyone's guess, sir,' said Ponder diplomatically. 'We can hear you because our ears are in the HEM, and you can hear the sounds made here because HEX is presenting you with an analogue of them. Don't worry about it. It'll seem quite natural after a while.'
The ghost of the Dean kicked at the soil. A fraction of a second later, a little heap of earth splashed up.
'Amazing!' he said, happily.
'Excuse me?' said Rincewind.
They turned.
'Oh, Rincewind,' said Ridcully, as one might say 'oh dear, it's raining'. 'It's you.'
'Yes, sir.'
'Mister Stibbons here's found a way of getting HEX to operate more than one virtually-there suit, d'you see? So we thought we'd come down and smell the roses.'
'Not for several hundred million years, sir,' said Ponder.
'Dull, isn't it,' said the Lecturer in Recent Runes, looking around. 'Not a lot going on. Lots of life, but it's just hanging around.'
Ridcully rubbed his hands together.
'Well, we're going to liven it up,' he said. 'We're going to move things forward fast while we're here. A few prods in the right place, that's what these creatures need.'
'The time travelling is not much fun,' said Rincewind. 'You tend to end up under a volcano or at the bottom of the sea.'
'We shall see,' said Ridcully firmly. 'I've had enough of this.
Look at those damn sloppy things over there. 'He cupped his hands and shouted, 'Life in the sea not good enough for you, eh? Skiving off, eh? Got a note from your mother, have you?' He lowered his hands. 'All right, Mister Stibbons ... tell HEX to take us forward, oh, fifty million years, hang on, what was that?'
Thunder rolled around the horizon.
'Probably just another snowball landing,' said Rincewind morosely. 'There's generally one around just when things are set¬tled. It was in the sea, I expect. Stand by for the tidal wave.' He nodded at the browsing creatures, who had glanced up briefly.
'The Dean thinks all this hammering from rocks is making the life on this world very resilient,' said Ridcully.
'Well, that's certainly a point of view,' said Rincewind. 'But in a little while a wave the size of the University is going to wash this beach on to the top of those mountains over there. Then I expect the local volcanoes will all let go ... again ... so stand by for a coun¬try-sized sea of lava coming the other way. After that there'll probably be outbreaks of rain that you could use to etch copper, fol¬lowed by a bit of a cold spell for a few years and some fog you could cut up in lumps.' He sniffed. 'That which does not kill you can give you a really bad headache.'
He glanced at the sky. Strange lightning was flickering between the clouds, and now there was a glow on the horizon.
'Damn,' he said, in the same tone of voice. 'This is going to be one of the times when the atmosphere catches fire. I hate it when that happens.'
Ridcully gave him a long blank look, and then said, 'Mister Stibbons?'
'Make that seventy thousand years, will you? And, er ... right now, if you would be so good.'
The wizard vanished.
All the insects stopped buzzing in the bushes.
The hairy lizards carried on placidly eating the leaves. Then, something made them look up…
The sun jerked across the sky, became very briefly a reddish-yellow band across a twilight hemisphere, and then the world was simply a grey mist. Below Rincewind's feet it was quite dark, and above him it was almost white. Around him, the greyness flickered.
'Is this what it always looks like?' said the Dean.
'Something has to stand still for a couple of thousand years before you see it at all,' said Rincewind.
'I thought it would be more exciting...’
The light flickered, and sun exploded into the sky, the wizards saw waves around them for a moment, and then there was darkness.
'I told you,' said Rincewind. 'We're under water.'
'The land sank under all the volcanoes?' said Ridcully.
'Probably just moved away,' said Rincewind. 'There's a lot of that sort of thing down here.'
They rose above the surface as HEX adjusted to the new condi¬tions. A landmass was smeared on the horizon, under a bank of cloud.
'See?' said Rincewind. 'It's a pain. Time travel always means you end up walking.'
'hex, move us to the nearest land, please. Inland about ten miles,' said Ponder.
'You mean I could have just asked?' said Rincewind. 'All this time, I needn't have been walking?'
'Oh, yes.'
The landscape blurred for a second.
'You could have said,' said Rincewind accusingly, as they were rushed past, and sometimes through, a forest of giant ferns.
The view stabilized. The wizard had been through to the edge of the forest. Low-growing shrubs stretched away towards more ferns.
'Not much about,' said Ridcully, leaning against a trunk. 'Can I smoke my pipe here, Stibbons?'
'Since technically you'll be smoking in the High Energy Building, yes, sir.'
Rincewind apparently struck a match on the tree trunk. 'Amazing,' he said.
'That's odd, sir,' said Ponder. 'I didn't think there would be any proper trees yet.'
'Well, here they are,' said Ridcully. 'And I can see at least another three more ...'
Rincewind had already started to run. The fact that nothing can harm you is no reason for not being scared. An expert can always find a reason for being scared.
The fact that the nearest trunk had toenails was a good one.
From among the ferns above, a large head appeared on the end of too much neck.
'Ah,' said Ridcully calmly. 'Still bloody lizards, I see.'

Ponder was working the Rules again. Now they read:

1 Things fall apart, but centres hold
2 Everything moves in curves
3 You get balls
4 Big balls tell space to bend
5 There are no turtles anywhere
(after this one he'd added Except ordinary ones)
6 Life turns up everywhere it can
7 Life turns up everywhere it can't
8 There is something like narrativium
9 There may be something called bloodimindium (see rule 7)

He stopped to think. Behind him, a very large lizard killed and ate a slightly smaller one. Ponder didn't bother to turn around. They'd been watching lizards for more than a hundred million years, all day, in fact, and even the Dean was giving up on them.
'Too well adapted,' he said. 'Nopressure on them, you see,'
'They're certainly very dull,' said Ridcully. 'Interesting colours, though.'
'Brain the size of a walnut and some of them think with their backsides,' said the Senior Wrangler.
'Your type of people, Dean,' said Ridcully.
'I shall choose to ignore that, Archchancellor,' said the Dean coldly.
'You've been interfering again, haven't you,' Ridcully went on. 'I saw you pushing some of the small lizards out of that tree.'
'Well, you've got to admit that they look a bit like birds,' said the Dean.
'And did they learn to fly?'
'Not in so many words, no. Not horizontally.'
'Eat, fight, mate and die,' said the Lecturer in Recent Runes. 'Even the crabs were better than this. Even the blobs made an effort. When they come to write the history of this world, this is the page everyone will skip. Terribly dull lizards, they'll be called. You mark my words.'
'They have stayed around for a hundred million years, sir,' said Rincewind, who felt he had to stand up for non-achievers.
'And what have they done? Is there a single line of poetry? A building of any sort? A piece of simple artwork?'
'They've just not died, sir.'
'Not dying out is some kind of achievement, is it?' said the Lecturer in Recent Runes.
'Best kind there is, sir.'
'Pah!' said the Dean. 'All they prove is that species go soft when there's nothing happening! It's nice and warm, there's plenty to eat ... it's just the sea without water. A few periods of vulcanism or a medium-sized comet would soon have them sitting up straight and paying attention.'
The air shimmered and Ponder Stibbons appeared.
'We have intelligence, gentlemen,' he said.
'I know,' said the Dean.
'I mean, the omniscope has found signs of developing intelli¬gence. Twice, sir.'

The herd was big. It was made up of large, almost hemispherical creatures, with faces that had all the incisive cogitation of a cow.
Much smaller creatures were trotting along at the edges. They were dark, scrawny and warbled to one another almost without cease.
They also carried pointed sticks.
'Well ...' Ridcully began, dismissively.
'They're herding them, sir!' said Ponder.
'But wolves chase sheep ...'
'Not with pointy sticks, sir. And look there ...'
One of the beasts was towing a crude travois, covered with leaves. Several herders were lying on it. They were pale around the muzzles.
'Are they sick, d'you think?' said the Dean.
'Just old, sir.'
'Why'd they want to slow themselves down with a lot of old peo¬ple?'
Ponder dared a short pause before answering.
'They're the library, sir. I suppose. They can remember things. Places to hunt, good waterholes, that sort of thing. And that means they must have some sort of language.'
'It's a start, I suppose,' said Ridcully.
'Start, sir? They've nearly done it all!' Ponder put his hand to his ear. 'Oh ... and HEX says there's more, sir. Er ... different.'
'How different?'
'In the sea again, sir,'
'Aha,' said the Senior Wrangler.
In fact on the sea was more accurate, he had to admit. The colony they found stretched for miles, linking a chain of small rocky islands and sandbanks as beads on a chain of tethered driftwood and rafts of floating seaweed.
The creatures inhabiting it were another type of lizard. Still extremely dull, the wizards considered, compared to some of the others. They weren't even an interesting colour and they had hardly any spikes. But they were ... busy creatures.
'That seaweed ... does it look sort of regular to you?' said the Lecturer in Recent Runes, as they drifted over a crude wall. They're not farming, are they?'
'I think ...' Ponder looked down. The water washed over the wall of rocks. 'It's a big cage for fish. The whole lagoon. Er ... I think they've built the walls like that so the tide lets the fish come in and then they're stuck when it goes down.'
Lizards turned their heads as the semi-transparent men floated past, but seemed to treat them as no more than passing shadows.
'They're harnessing the power of the sea?' said Ridcully. 'That's clever.'
Lizards were diving at the far side of the lagoon. Some were busy around rock pools on one of the lower islands. Small lizards swam in the shallows. Along one stretch of driftwood walkways, strips of seaweed were drying in the breeze. And over everything was a yip-yipping of conversation. And it was conversation, Ponder decided. Animals didn't wait for other animals to finish. Nor did wizards, of course, but they were a breed apart.
A little way away, a lizard was carefully painting the skin of another lizard, using a twig and some pigments in half-shells. The one doing the painting was wearing a necklace of different shells, Ponder realized.
'Tools,' he murmured. 'Symbols. Abstract thought. Things of value ... is this a civilization, or are we merely tribal at the moment?'

'Where's the sun?' said the Senior Wrangler. 'It's always so hazy, and it's hard to get used to directions here. Wherever you point, it's at the back of your own head.'
Rincewind pointed towards the horizon, where there was a red glow behind the clouds.
'I call it Widdershins,' he said. 'Just like at home.'
'Ah. The sun sets Widdershins.'
'No. It doesn't do anything,' said Rincewind. 'It stays where it is. The horizon comes up.'
'But it doesn't fall on us?'
'It tries to, but the other horizon drags us away before it hap¬pens.'
'The more time I spend on this globe, the more I feel I should be holding on to something,' the Dean muttered.
'And the light isn't reflected around the world?' said the Senior Wrangler. 'It is at home. It's always very beautiful, the glow that comes up through the waterfall.'
'No,' said Rincewind. 'It just gets dark, unless the moon is up.'
'And there's still just the one sun, isn't there?' said the Senior Wrangler, a man with something on his mind.
'We didn't add another one?'
'So ... er ... what is that light over there?'
As one wizard, they turned towards the opposite horizon.
'Whoops,' said the Dean, as the distant thunder died away and lights streamed high across the sky.
The lizards had heard it too. Ponder looked around. They were lining the walkways, watching the horizon with all the intelligent interest of a thinking creature wondering what the future may hold...
'Let's get back to the High Energy Magic building before the boiling rain, shall we?' said Ridcully. 'This really is too depressing.'

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Poruke 18761
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Life turns up everywhere it can't.
And just when it seems to have got itself going really comfortably, with a sustainable lifestyle and gradual progress towards higher things, along comes a major catastrophe and sets it back twenty million years. Yet, paradoxically, those same disasters also pave the way to radically new lifeforms ...
It's all rather confusing.
Life is resilient, but any particular species may not be. Life is constantly devising new tricks. The one with eggs is brilliant: pro¬vide the developing embryo with its own personal life-support machine. Inside, the environment is tailored to the needs of that species, and what's outside doesn't matter much, because there's a barrier to keep it out.
Life is adaptable. It changes the rules of its own game. As soon as eggs make their appearance, the stage is set for the evolution of egg-eaters ...
Life is diverse. The more players there are, the more ways there are to make a living by taking in each others' washing.
Life is repetitious. When it finds a trick that works, it churns out thousands of variations on the same basic theme. The great biolo¬gist John (J.B.S.) Haldane was once asked what question he would like to pose to God, and replied that he'd like to know why He has such an inordinate fondness for beetles.
There are a third of a million beetle species today, far more than in any other plant or animal group. In 1998 Brian Farrell came up with a possible answer to Haldane's query. Beetles appeared about 250 million years ago, but the number of species didn't explode until about 100 million years ago. That happens to be just when flowering plants came into existence. The 'phase space' avail¬able for organisms suddenly acquired a new dimension, a new resource became available for exploitation. The beetles were beauti¬fully poised to take advantage by eating the new plants, especially their leaves. It used to be thought that flowering plants and polli¬nating insects drove each other to wilder and wilder diversity, but that's not true. However, it is true for beetles. Nearly half of today's beetle species are leaf-eaters. It's still an effective tactic.

Sometimes natural disasters don't just eliminate a species or two. The fossil record contains a number of 'mass extinctions' in which a substantial proportion of all life on Earth disappeared. The best-known mass extinction is the death of the dinosaurs, 65 million years ago.
In order not to mislead you, we should point out at once that there is no scientific evidence for the existence of any dinosaur civ¬ilization, no matter what events are going on in the Roundworld Project. But... whenever a scientist says 'there is no scientific evi¬dence for', there are three important questions you should ask -especially if it's a government scientist. These are: 'Is there any evi¬dence against?, 'Has anyone looked?', and 'If they did, would they expect to find anything?'
The answers here are 'no,' 'no', and 'no'. Deep Time hides a lot, especially when it's assisted by continental movement, the bulldoz¬ing ice sheets, volcanic action and the occasional doomed asteroid. There are few surviving human artefacts more than ten thousand years old, and if we died out today, the only evidence of our civi¬lization that might survive for a million years would be a few dead probes in deep space and various bits of debris on the Moon. Sixty-five million? Not a chance. So although a dinosaurian civilization is pure fantasy, or, rather, pure speculation, we can't rule it out absolutely. As for dinosaurs who were sufficiently advanced to use tools, herd other dinosaurs ... well, Deep Time would wash over them without a ripple.
Dinosaurs are always among the most popular exhibits at muse¬ums. They remind us that the world wasn't always like it is now; and they remind us that humans have been on this planet for a very short time, geologically speaking. Basically, dinosaurs are ancient lizards. The ones whose bones we all go to gawp at in museums are rather big lizards, but many were much smaller. The name means 'terrible lizard', and anyone who watched Jurassic Park will under¬stand why.
An Italian fossil collector who watched the Spielberg movie sud¬denly realized that a perplexing fossil, filed away for years in his basement, might well be a bit of a dinosaur. He then sent it to a nearby university, where it was found not just to be a dinosaur, but a new species. It was a young therapod, small flesh-eating dinosaurs that are the closest relatives of birds. Interestingly, it did¬n't have any feathers. A story straight out of the movies: narrative imperative at work in our own world ... traceable, as always, to selective reporting. How many fossil hunters owned a bit of dinosaur bone but didn't make the connection after seeing the movie?
In the human mind, dinosaurs resonate with myths about drag¬ons, common to many cultures and many times; and many miles of suggestions have appeared to explain how the dragon-thoughts in our minds have come down to us, over millions of years of evolu¬tion, from real dinosaur images and fears in the minds of our ancient ancestors. However, those ancestors must have been very ancient, for those of our ancestors that overlapped the dinosaurs were probably tiny shrewlike creatures that lived in holes and ate insects. After more than a hundred million years of success, the dinosaurs all died out, 65 million years ago, and the evidence is that their demise was sudden. Did proto-shrews have nightmares about dinosaurs, all that time ago? Could such nightmares have survived 65 million years of natural selection? In particular, do shrews today have nightmares about fire-breathing dragons, or is it just us? It seems likely that the dragon myth comes from other, less lit¬eral, tendencies of that dark, history-laden organ that we call the human mind.
Dinosaurs exert a timeless fascination, especially for children. Dinosaurs are genuine monsters, they actually existed, and some of them, the ones we all know about, were gigantic. They are also safely dead.
Many small children, even if they are resistant to the standard reading materials in school, can reel off a long list of dinosaur names. 'Velociraptor' was not notable among them before Jurassic Park, but it is now. Those of us who still have an affection for the brontosaur often need to be reminded that for silly reasons science has deemed that henceforth that sinuous swamp-dwelling giant must be renamed the apatosaur. So attuned are we to the dinosaurs that the drama of their sudden disappearance has captured our imaginations more than any other bit of pakeontology. Even our own origins attract less media attention.
What about the sudden demise?
For a start, quite a few scientists have disputed that it ever was sudden. The fossil record implicates the end of the Cretaceous period, 65 million years ago, as 'D-Day'. This was also the start of the so-called Tertiary period, or Age of Mammals, so the end of the dinosaurs is usually called the K/T boundary, 'K' because Germans spell Cretaceous with a K. But if we assume that the end of the Cretaceous was 'when it happened', then many species seemed to have anticipated their end by vanishing from the fossil record five to ten million years earlier. Did amorous dinosaurs, per¬haps, say to each other 'It's just not worth going through with this reproduction business, dear, we're all going to be wiped out in ten million years.'? No. So why the fuzzy fade-out over millions of years? There are good statistical reasons why we might not be able to locate fossils right up to the end, even if the species concerned were still alive.
To set the  remark in context:  how many specimens of Tyrannosaurus rex, the most famous dinosaur of all, do you think that the world's universities and museums have between them? Not copies, but originals, dug from the rock by palaeontologists? Hundreds ... surely?
No. Until Jurassic Park, there were precisely three, and the times when those particular animals lived have a spread of five million years. Three more fossilized T. rexes have been found since, because Jurassic Park gave dinosaurs a lot of favourable publicity, making it possible to drum up enough money to go out and find some more. With that rate of success, the chance of a future race finding any fossil humanoids, over the whole period of our and our ancestors' existence, would be negligible. So if some species had survived on Earth for a five million year period, it is entirely likely that no fos¬sils of it will have been found, especially if it lived on dry land, where fossils seldom form. This may suggest that the fossil record isn't much use, but quite the contrary applies. Every fossil that we find is proof positive that the corresponding species did actually exist; moreover, we can get a pretty accurate impression of the grand flow of Life from an incomplete sample. One lizard fossil is enough to establish the presence of lizards, even if we've found only one species out of the ten thousand that were around.
Bearing this in mind, though, we can easily see that even if the death of the dinosaurs was extremely sudden, then the fossil record might easily give a different impression. Suppose that fossils of a given species turn up randomly about every five million years. Sometimes they're like buses, and three come along at once, that is, within a million years of each other. Other times, they're also like buses: you wait all day (ten million years) and don't see any at ail. During the ten million year run-up to the K/T boundary, you find random fossils. For some species, the last one you find is from 75 million years ago; for others it's from 70 million years ago. For a few, by chance, it's from 65 million years ago. So you seem to see a grad¬ual fade-out.
Unfortunately, you'd see much the same if there really had been a gradual fade-out. How can you tell the difference? You should look at species whose fossils are far more common. If the demise was a sudden one, those ought to show a sharper cut-off. Species that live wholly or partially in water get fossilized more often, so the best way to time the K/T mass extinction is to look at fossils of marine species. Wise scientists therefore mostly ignore the dinosaur drama and fiddle around with tiny snails and other undrarnatic species instead. When they do, they find that ichthyosaurs also died out about then, as did the last of the ammonites and many other marine groups. So something sudden and dramatic really did hap¬pen at the actual boundary, but there may well have been a succession of other events just before it too.

What kind of drama? An important clue comes from deposits of iridium, a rare metal in the Earth's crust. Iridium is distinctly more common in some meteorites, particularly those from the asteroid belt between Mars and Jupiter So if you find an unusually rich deposit of iridium on Earth, then it may well have come from an impacting meteorite.
In 1979 the Nobel-winning physicist Luis Alvarez was musing along such lines, and he and his geologist son Walter Alvarez discovered a layer of clay that contains a hundred times as much irid¬ium as normal. It was laid down right at the K/T boundary, and it can be found over the whole of the Earth's land mass. The Alvarezes interpreted this discovery as a strong hint that a meteorite impact caused the K/T extinction. The total amount of iridium in the layer is estimated to be around 200,000 tons (tonnes), which is about the amount you'd expect to find in a meteorite 6 miles (10 km) across. If a meteorite that size were to hit the Earth, travelling at a typical 10 miles per second (16 kps), it would leave an impact crater 40 miles (65 km) in diameter. The blast would have been equivalent to thousands of hydrogen bombs, it would have thrown enormous quantities of dust into the atmosphere, blanking out sun¬light for years, and if it happened to hit the ocean, a better than 50/50 chance, it would cause huge tidal waves and a short-lived burst of superheated steam. Plants would die, large plant-eating dinosaurs would run out of food and die too, carnivorous dinosaurs would quickly follow. Insects would on the whole fare a little better, as would insect-eaters.
Much evidence has accumulated that the Chicxulub crater, a buried rock formation in the Yucatan region of southern Mexico, is the remnant of this impact. Crystals of Shocked' quartz were spread far and wide from the impact site: the biggest ones are found near the crater, and smaller ones are found half way round the world. In 1998 a piece of the actual meteorite, a tenth of an inch (2.5 mm) across, was found in the north Pacific Ocean by Frank Kyte. The fragment looks like part of an asteroid, ruling out a possible alternative, a comet, which might also create a similar crater. According to A. Shukolyukov and G.W. Lugmair, the proportions of chromium isotopes in K/T sediment confirm that view. And Andrew Smith and Charlotte Jeffery have found that mass die-backs of sea urchins which occurred at the K/T boundary were worst in the regions around central America, where we think the meteorite came down.
Although the evidence for an impact is strong, and has grown considerably over the twenty years since the Alvarezes first advanced their meteorite-strike theory, a strongly dissenting group of palaeontologists has looked to terrestrial events, not dramatic astronomical interference, to explain the K/T extinction. There was certainly a rapid series of climatic changes at the end of the Cretaceous, with very drastic changes of sea level as ice caps grew or melted. There is also good evidence that some seas, perhaps all, lost their oxygen-based ecology to become vast, stinking, black, anaerobic sinks. The fossil evidence for this consists of black iron-and sulphur-rich lines in sediments. The most dramatic terrestrial events were undoubtedly associated with the vulcanism which resulted in the so-called Deccan Traps, huge geological deposits of lava. The whole of Asia seems to have been covered with volcanoes, and they produced enough lava that it would have formed a layer 50 yards (45 m) thick if it had been spread over the whole continent. Such extensive vulcanism would have had enormous effects on the atmosphere; carbon dioxide emissions that warmed the atmosphere by the greenhouse effect, sulphur compounds resulting in terrible acid rain and freshwater pollution over the entire planet, and tiny rock particles blocking sunlight and causing 'nuclear winters' for decades at a time. Could the volcanoes that formed the Deccan Traps have killed the dinosaurs, instead of a meteorite? Much depends on the timing.
Our preferred theory, not because there is good independent evidence for it but because it would explain so much, and because it has a moral, is that the two causes are linked. The Chicxulub crater is very nearly opposite the Deccan Traps, on the other side of the planet. Perhaps volcanic activity in Asia began some millions of years before the K/T boundary, causing occasional ecological crises for the larger animals but nothing really final Then the meteorite hit, causing shockwaves which passed right through the Earth and converged, focused as if by a lens on just that fragile region of the planet's crust. (A similar effect happened on Mercury, where a gigantic impact crater called the Caloris Basin is directly opposite 'weird terrain' caused by focused shockwaves.)
There would then have been a gigantic, synchronized burst of vulcanism, on top of all the events of the collision, which would have been pretty bad on their own. The combination could have polished off innumerable animal species. In support of this idea, it should be said that another geological deposit, the Siberian Traps, contains ten times as much lava as the Deccan system, and it so hap¬pens that the Siberian Traps were kid down at the time of another mass extinction, the great Permian extinction, which we mentioned earlier. To pile on further evidence: some geologists believe they have found another meteorite impact site in modern Australia, which in Permian times was opposite to Siberia.
The moral of this tale is that we should not look for 'the' cause of the dinosaur extinction. It is very rare for there to be just one cause of a natural event, unlike scientific experiments which are specially set up to reveal unique explanations.
On Discworld, not only does Death come for humans, scythe in hand, but diminutive sub-Deaths come for other animals, for example the Death of Rats in Soul Music, from whom a single, typ¬ical quote will suffice: 'SQUEAK.'
The Death of Dinosaurs would have been something to see, with volcanoes in one hand and an asteroid in the other, trailing a cloak of ice ...
They were wonderfully cinematic reptiles, weren't they? Trust the wizards to get it wrong.

There is another lesson to be learned from our emphasis on the demise of the dinosaurs. Many other large and/or dramatic reptiles died out at the end of the Cretaceous, notably the plesiosaurs (famous as a possible 'explanation' of the mythical Loch Ness mon¬ster), the ichthyosaurs (enormous fish-shaped predators, reptilian whales and dolphins), the pterosaurs (strange flying forms, of which the pterodactyls appear in all the dinosaur films and are labelled, wrongly, dinosaurs), and especially the mosasaurs .,.
What were they? They were as dramatic as the dinosaurs, but they weren't dinosaurs. They didn't have as good a PR firm, though, because few non-specialists have heard of them. They are popularly known as fish-lizards, not as good a name as 'terrible lizard', and it describes them well. Some were nearly as fish-like as ichthyosaurs, or dolphins, some were rather crocodile-like, some were fifty-foot predators like the great white shark, some were just a couple of feet long and fed on baby ammonites and other common molluscs. They lasted a good twenty million years, and for much of that time they seem to have been the dominant marine predators. Yet most people meet the word in stories about dinosaurs, assume that the mosasaur was a not-very-interesting kind of dinosaur, and promptly forget them.
The other really strange thing about the K/T extinction, prob¬ably not a 'thing' in any meaningful sense, because in this context a thing would be an equation of unknowns, whereas what we have is a diversity of related puzzles, is which creatures survived it. In the sea, the ammonites all died out, as did the other shelled forms like belemnites, unrolled ammonites, but the nautilus came through, as did the cuttlefish, squids, and octopuses. Amazingly the croco¬diles, which to our eyes are about as dinosaur-like as you can get without actually being one, survived the K/T event with little loss of diversity. And those little dinosaurs called 'birds' came through pretty well unscathed. (There's a story here that we need to tell, quickly. Not so long ago, the idea that birds are the living remnants of the dinosaurs was new, controversial, and therefore a hot topic. Then it rapidly turned into the prevailing wisdom. New fossil dis¬coveries, however, have shown conclusively that the major families of modern birds diverged, in an evolutionary sense, long before the K/T event. So they aren't remnants of the dinosaurs that otherwise died, they got out early by ceasing to be dinosaurs at all.)

Myths, not least Jurassic Park itself, have suggested that dinosaurs are not 'really' extinct at all. They survive, or so semi-fact semi-fic¬tion accounts lead us to believe, in Lost World South American valleys, on uninhabited islands, in the depths of Loch Ness, on other planets, or more mystically as DNA preserved inside blood¬sucking insects trapped and encased in amber. Alas, almost certainly not. In particular, 'ancient DNA' reportedly extracted from insects fossilized in amber comes from modern contaminants, not prehistoric organisms, at least if the amber is more than a hundred thousand years old.
Significantly, no one has made a film bringing back dodos, moas, pygmy elephants, or mosasaurs, only dinosaurs and Hitler are popular for the reawakening myth. Both at the same time would be a good trick.
Dinosaurs are the ultimate icon for an evolutionary fact which we generally ignore, and definitely find uncomfortable to think about: nearly all species that have ever existed are extinct. As soon as we realize that, we are forced to look at conservation of animal species in new ways. Does it really matter that the lesser spotted pogo-bird is down to its last hundred specimens, or that a hundred species of tree-snail on a Pacific island have been eaten out of exis¬tence by predators introduced by human activity? Some issues, like the importation of Nile Perch into Lake Victoria in order to improve the game fishing, which has resulted in the loss of many hundreds of fascinating 'cichlid' fish species, are regretted even by the people responsible, if only because the new lake ecosystem seems to be much less productive. Everyone (except purveyors of bizarre ancient 'medicines', their even more foolish customers, and some unreconstructed barbarians) seems to agree that the loss of magnificent creatures like the great whales, elephants, rhinos and of course plants like ginkgoes and sequoias would be a tragedy. Nevertheless, we persist in reducing the diversity of species in ecosystems all around the planet, losing many species of beetles and bacteria with hardly any regrets.
From the point of view of the majority of humans, there are 'good' species, unimportant species, and 'bad' species like smallpox and mosquitoes, which we would clearly be better off without. Unless you take an extreme view on the 'rights' of all living crea¬tures to a continued existence, you find yourself having to pass judgment about which species should be conserved. And if you do take such an extreme view, you've got a real problem trying to pre¬serve the rights of cheetahs and those of their prey, such as gazelles. On the other hand, if you take the task of passing judgment seri¬ously, you can't just assume that, say, mosquitoes are bad and should be eliminated. Ecosystems are dynamic, and the loss of a species in one place may cause unexpected trouble elsewhere. You have to examine the unintended consequences of your methods as well as the intended ones. When worldwide efforts were made to eradicate mosquitoes, with the aim of getting rid of malaria, the preferred route was mass sprayings of the insecticide DDT. For a time this appeared to be working, but the result in the medium term was to destroy all manner of beneficial insects and other creatures, and to produce resistant strains of mosquitoes which if anything were worse than their predecessors. DDT is now banned world¬wide, which unfortunately doesn't stop some people continuing to use it.
In the past, the environment was a context for us, we evolved to suit it. Now we've become a context for the environment, we change it to suit us. We need to learn how to do that, but going back to some imaginary golden age in which primitive humans allegedly lived in harmony with nature isn't the answer. It may not be politi¬cally correct to say so, but most primitive humans did as much environmental damage as their puny technology would allow. When humans came to the Americas from Siberia, by way of Alaska, they slaughtered their way right down to the tip of South America in a few tens of thousands of years, wiping out dozens of species, giant tree sloths and mastodons (ancient elephants, like mammoths but different), for example. The Anasazi Indians in the southern part of today's USA cut down forests to build their cliff dwellings, creating some of the most arid areas of the United States. The Maoris killed off the moas. Modern humans may be even more destructive, but there are more of us and technology can amplify our actions. Nevertheless, by the time humans were able to articulate the term 'natural environment', there wasn't one. We had changed the face of continents, in ways big and small.
To live in harmony with nature, we must know how to sing the same song as nature. To do that, we must understand nature. Good intentions aren't enough. Science might be, if we use it wisely.



GLOOM HAD SETTLED OVER THE WIZARDS.  Some of them had even refused a third helping at dinner. 'It's not as if they were very advanced,' said the Dean, in an attempt to cheer everyone up. 'They weren't even using metal. And their writing was frankly nothing but pictograms.'
'Why doesn't that sort of this thing happen here?' said the Senior Wrangler, merely toying with his trifle.
'Well, there have been historical examples of mass extinction,' said Ponder.
'Yes, but only as a result of argumentative wizardry. That's quite different. You don't expect rocks to drop out of the sky.'
'You don't expect them to stay up?’ said Ridcully. 'In a proper universe, the turtle snaps up most of them and the elephants get the rest. Protects the world. Y'know, it seems to me that the most sen¬sible thing any intelligent lifeform could do on that little world would be to get off it.'
'Nowhere to go,' said Ponder.
'Nonsense! There's a big moon. And there's other balls floating around this star.'
'All too hot, too cold, or completely without atmosphere,' said Ponder.
'People would just have to make their own entertainment. Anyway ... there's plenty of other suns, isn't there?'
'All far too far away. It would take ... well, lifetimes to get there.'
'Yes, but being extinct takes forever.'
Ponder sighed. 'You'd set out not even knowing if there's a world you could live on, sir,' he said.
'Yes, but you'd be leavin' one that you'd know you couldn't,' said Ridcully calmly. 'Not for any length of time, anyway.'
'There are new lifeforms turning up, sir. I went and checked before dinner.'
'Tell that to the lizards,' sighed the Senior Wrangler. 'Any of the new ones any good?' said Ridcully. 'They're ... more fluffy, sir.' 'Doin' anything interesting?'
'Eating leaves, mainly,' said Ponder. 'There are some much more realistic trees now.'
'Billions of years of history and we've got a better tree,' sighed the Senior Wrangler.
'No, no, that's got to be a step in the right direction,' said Ridcully, thoughtfully.
'Oh? How so?'
'You can make paper out of trees.'

The wizards stared into the omniscope.
'Oh, how nice,' said the Lecturer in Recent Runes. 'Ice again. It's a long time since we've had a really big freeze.'
'Well, look at the universe,' said the Dean. 'It's mainly freezing cold with small patches of boiling hot. The planet's only doing what it knows.'
'You know, we're certainly learning a lot from this project,' said Ridcully. 'But it's mainly that we should be grateful we're living on a proper world.'

A few million years passed, as they do.
The Dean was on the beach and almost in tears. The other wiz¬ards appeared nearby and wandered over to see what the fuss was about.
Rincewind was waist deep in water, apparently struggling with a medium-sized dog.
'That's right,' the Dean shouted. 'Turn it round! Use a stick if you have to!'
'What the thunder is going on here?' said Ricully.
'Look at them!' said the Dean, beside himself with rage. 'Backsliders! Caught them trying to return to the ocean!'
Ridcully glanced at one of the creatures, which was lying in the shallows and chewing on a crab.
'Didn't catch them soon enough, did you,' he said. 'They've got webbed paws.'
'There's been too much of this sort of thing lately!' snapped the Dean. He waved his finger at one of the creatures, who watched it carefully in case it turned out to be a fish.
'What would your ancestors say, my friend, if they saw you rush¬ing into the water just because times are a bit tough on land?' he said.
'Er ... '"Welcome back"?' suggested Rincewind, trying to avoid the snapping jaws.
"Long time no sea'?' said the Senior Wrangler, cheerfully.
The creature begged, uncertainly.
'Oh, go on, if you must,' said the Dean. 'Fish, fish, fish ... you'll turn into a fish one of these days!'
'Y'know, going back to the sea might not be a bad idea,' said Ridcully, as they strolled away along the beach. 'Beaches are edges. You always get interestin' stuff on the edge. Look at those lizards we saw on the islands. Their world was all edges.'
'Yes, but giving up the land to just go swimming around in the water? I don't call that evolution.'
'But if you go on land where you have to grow a decent brain and some cunning and a bit of muscle in order to get anything done, and then you go back to see the sea where the fish have never had to think about anything very much, you could really, er, kick butt.'
'Do fish have...?'
'All right, all right. I meant, in a manner of speaking. It was just a thought, anyway.' Uncharacteristically, the Archchancellor frowned.
'Back to the sea,' he said. 'Well, you can't blame them.'

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