The Science of Discworld III - Darwin's Watch

12. THE WRONG BOOK

OUR FICTIONAL DARWIN HAS A lot more in common with the `real' one - the Darwin of the particular timeline that you inhabit, the one who wrote The Origin and not The Ology - than might at first be apparent. Or plausible. The irresistible force of narrativium induces us to imagine Charles Darwin as an old man with a beard, a stick, and a faint but definite hint of gorilla. And so he was, in his old age. But as a young man he was vigorous, athletic, and engaged in the kind of exuberant and not always politically correct activities that we expect of young men.

We've already learned of the real Darwin's amazing fortune in getting on board the Beagle and remaining there, culminating in his boundless delight at the geology of the coral island of St Jago. But there are other crucial nodalities, points of intervention, and thaumic occlusions in that version of Roundworld's historical record, and the wizards are exercising extreme care and attention in the hope of steering history through, past, and around these causal singularities.

For example, the Beagle really did come under fire from a cannon. When the ship tried to enter the harbour at Buenos Aires in 1832, one of the local guard ships fired at it. Darwin was convinced that he heard a cannonball whistle over his head, but it turned out that the shot was a blank, intended as a warning. FitzRoy, muttering angrily about insults to the British flag, sailed on, but was stopped by a quarantine boat: the harbour authorities were worried about cholera. Incensed, FitzRoy loaded all of the cannons on one side of his ship. As he sailed back out of the harbour he aimed them all at the guard ship, informing its crew that if they ever fired at the Beagle again, he would send their `rotten hulk' to the seabed.

Darwin really did learn to throw a bolas, too, on the pampas of Patagonia. He enjoyed hunting rheas, and watching the gauchos bring them down by entangling a bolas in their legs. But when he tried to do the same, all he managed was to trip up his own horse. The Origin might have vanished from history's timeline then and there, but Darwin survived, with only his pride hurt. The gauchos found the whole thing hugely amusing.

Charles even took part in suppressing an insurrection. When the Beagle reached Montevideo, shortly after the cannonball incident, FitzRoy complained to the local representative of Her Majesty's Royal Navy, who promptly set sail for Buenos Aires in his frigate HMS Druid to secure an apology. No sooner had the warship disappeared from view than there was a rebellion, with black soldiers taking over the town's central fort. The chief of police asked FitzRoy for help, and he dispatched a squad of fifty sailors, armed to the teeth ... with Darwin happily bringing up the rear. The mutineers immediately surrendered, and Darwin expressed disappointment that not a shot had been fired.

No expense, then, has been spared to bring you historical truth, inasmuch as so weighty a characteristic as truth can be attributed to something as ethereal as history. Except for the giant squid, of course. That happened in a different timeline, when the malign forces were getting extremely desperate and strayed into Twenty Thousand Leagues Under the Sea through some obscure warp in L-space.

The most important similarity between the two Darwins is less exciting, but essential to our tale. The real Charles Darwin, like his fictional counterpart, began by writing the wrong book. In fact, he wrote eight wrong books. They were very nice books, very worthy ... of great scientific value ... and they did his reputation no harm at all... . but they weren't about natural selection, his term for what later scientists would call `evolution'. Still, that book was brewing merrily away in the back of his mind, and until he was ready to bring it off the back burner, he had plenty of other things to write about.

It had been FitzRoy who had put the idea of authorship into his head. The Beagle's captain had signed himself up to write the story of his round-the-world voyage, based on the ship's log. He had also agreed to edit an accompanying book about a previous survey by the same vessel - the one where Stokes had shot himself. As the Beagle headed north-west from Cape town, stopped briefly at Bahia in Brazil, and turned north-east across the Atlantic towards its final destination in Falmouth, FitzRoy suggested to Darwin that the latter's diary might form the basis of a third volume on the natural history of the voyage, completing the trilogy.

Darwin was nervous but excited at the prospect of becoming an author. He had another book in mind, too, on geology. He'd been thinking about it ever since his revelation on the island of St Jago.

As soon as the ship had returned to England, FitzRoy got married and went on honeymoon, but he also made an impressive start to his book. Darwin began to worry that his own slow writing would delay the whole project, but FitzRoy's early enthusiasm soon ground to a halt. Between January and September 1837 Charles worked flat out, overtook the captain, and towards the year's end he sent his finished manuscript to the printer's. It took FitzRoy more than a year to catch up, so Darwin's contribution was held back, finally seeing the light of day in 1839 as volume 3 of the Narrative of the Surveying Voyages of HMS. Adventure and Beagle, Between the Years 1826 and 1836, with the subtitle Volume 3: Journal and Remarks, 1832-1836. After a few months the publishers reissued it on its own as journal of Researches into the Geology and Natural History of the Various Countries Visited by H.M.S. Beagle. It may have been the wrong book, but writing it had one very useful effect on Darwin's thinking. It forced him to try to make sense of all the things he had seen. Was there some overarching principle that could explain it all?

Next came his geology book, which eventually turned into three: one on coral reefs, one on volcanic islands, and one on the geology of South America. These established his scientific credentials and led to him winning a major Royal Society prize. Darwin was now recognised as one of the leading scientists in the land.

He was also making ever more extensive notes on the transmutation of species, but he still was in no hurry to publish. Quite the contrary. Elsewhere, political forces were at work aiming to destroy the influence of the Church, and one of their key points was that living creatures could easily have arisen without the intervention of a creator. Darwin, being (at that point in his life) a good Christian, was totally averse to anything that might seem to ally him with such people. He could not publicly espouse transmutation without risking serious damage to the Anglican Church, and nothing in the world would induce him to contemplate that. But his deep insight about natural selection wouldn't go away, so he continued developing it as a kind of hobby.

He did mention the insight to various scientific friends and acquaintances, among them Lyell, and also Joseph Dalton Hooker, who didn't dismiss the idea out of hand. But he did tell Darwin, `I shall be delighted to hear how you think this change may have taken place, as no presently conceived opinions satisfy me on this subject.' And he later said, rather acerbically, that `No one has hardly a right to examine the question of species who has not minutely examined many.' Darwin took this advice to heart and cast around for new species to become an expert on. In 1846 he sent the final proofs of his geology books back to the printer and celebrated by collecting the last bottle of preserved specimens from the Beagle voyage. At the top of the bottle he noticed a crustacean from the Chonos Archipelago - a barnacle.

That would do. It was as good as anything else.

Hooker helped Darwin set up his microscope and make some preliminary anatomical observations. Darwin asked Hooker to name the new beast, and together they decided on Arthrobalanus[29]. `Mr Arthrobalanus', as they privately called it, turned out to be somewhat unusual. `I believe Arthrobalanus has no ovisac at all!' Charles wrote. `The appearance of one is entirely owing to the splitting & tucking up of the posterior penis.' To resolve the mystery he took other barnacles from the bottle and looked at them, too. Now he was doing comparative anatomy of barnacles, and enjoying the hands-on experience immensely. This was better than writing.

By Christmas he had decided to study every barnacle known to humanity - the entire order of Cirripedia. Which turned out to be rather a lot, so he settled for the British ones. Even these were rather a lot, and in the end the task took eight years.

He might have finished earlier, but in 1848 he got interested in barnacle sex, and that was very peculiar indeed. Most barnacles were hermaphrodites, able to assume either sex. But some species had good old-fashioned males and females. Except that the males spent much of their lives embedded in the females.

Not only that: some supposedly hermaphrodite species also had tiny males that somehow assisted in the reproductive process.

Now Darwin became very excited, because he had convinced himself that what he was observing was a relic of evolution, as a hermaphrodite ancestor gradually developed separate sexes. A `missing link' for barnacle sex. He could reconstruct the barnacles' family tree, and what he thought he saw reinforced his views on natural selection. So even when he tried to do respectable science, and become a taxonomist, transmutation insisted in getting in on the act. In fact, if anything convinced Darwin he was right about transmutation, it was barnacles.

He became ill, but continued working on barnacles. In 1851 he published two books about them - one on fossil barnacles for the Palaeontographical Society, the other on the living ones for the Royal Society. By 1854 he had produced a sequel to each of them.

These were Darwin's eight wrong books:

1839 Journal of Researches into the Geology and Natural History of the Various Countries Visited by H.M.S. Beagle

1842 The Structure and Distribution of Coral Reefs

1844 Geological Observations on the Volcanic Islands Visited During the Voyage of H.M.S. Beagle

1846 Geological Observations of South America

1851 A Monograph on the Fossil Lepadidae, or, Pedunculated Cirripedes of Great Britain

1851 A Monograph on the Sub-class Cirripedia volume 1

1854 A Monograph on the Fossil Balanidae and Verrucidae of Great Britain

1854 A Monograph on the Sub-class Cirripedia volume 2

Not a hint of transmutation of species, the struggle for life, or natural selection.

Yet, in a strange way, all of his books - even the geological ones - were crucial steps towards the work that was now putting itself together inside his head. Darwin's ninth book would be pure dynamite. He wanted desperately to write it, but he had already decided that it would be far too dangerous to be published.

It is a common dilemma in science: whether to publish and be damned, or not to publish and be pre-erupted. You can have the credit for a truly revolutionary idea, or a quiet life, but not both.

Darwin was wary of publicity, and he was scared that putting his views into print might damage the Church. But there is nothing that more effectively galvanises a scientist than the fear that somebody else will pip them to the winning post. In this case, that somebody was Alfred Russel Wallace.

Wallace was another Victorian explorer, equally keen on natural history. Mostly because he could sell it. Unlike Darwin, he was not `gentry', and had no independent income. He was the son of an impecunious lawyer[30] and had been taken on at age fourteen as a builder's apprentice. He spent his evenings drinking free coffee in the Hall of Science off Tottenham Court Road in London. This was a socialist organisation, dedicated to the overthrow of private property and the downfall of the Church. Wallace's experiences as a youth reinforced a left-wing view of politics. He financed his own travels, and made a living by selling the specimens he collected - butterflies, beetles (a thousand labelled specimens per box, the dealers demanded)[31], even bird skins. He went on a collecting expedition to the Amazon in 1848, and again to the Malay Archipelago in 1854. There, in Borneo, he sought orang-utans. The idea that humans were somehow related to the great apes was simmering away in the collective subconscious, and Wallace wanted to investigate a potential human ancestor[32].

One miserable Borneo day, when a tropical monsoon raged outside and Wallace was stuck indoors, he put together a little scientific paper outlining some modest ideas that had just popped into his head. It eventually appeared in the Annals and Magazine of Natural History, a rather ordinary publication, and it was about the 'introduction' of species. Lyell, aware of Darwin's secret interest in such matters, pointed the paper out to him, and Charles began to read it. Then another of Charles's regular correspondents, Edward Blyth, wrote from Calcutta with the same recommendation. `What do you think of Wallace's paper in the Ann M.N.H.? Good! Upon the whole!' Darwin had met Wallace shortly before one of the latter's expeditions - he couldn't remember which - and he could see that the Ann M.N.H. paper had useful things to say about relationships between similar species. Especially the role of geography. But apart from that, he felt that the paper contained nothing new, and made an entry to that effect in one of his notebooks. Anyway, it seemed to Darwin that Wallace was talking about creation, not evolution. Nevertheless, he wrote to Wallace, encouraging him to continue developing his theory.

This was a Really Bad Idea.

Encouraged by Lyell and others, who were now warning him that if he delayed too long, others might snatch the prize, Darwin was putting together ever more elaborate essays on natural selection, but he continued to dither about publication. All that changed in an instant in June 1858, when the postman dropped a bombshell through Charles's letterbox. It was a package from Wallace, containing a twenty-page letter, sent from the Moluccas. Wallace had taken Darwin's advice to heart. And he had come up with a very similar theory. Very similar indeed.

Calamity. Darwin declared that his life's work was `smashed'. `Your words have come true with a vengeance,' he wrote to Lyell. The more he read Wallace's notes, the closer the ideas seemed to his own. `If Wallace had my MS manuscript sketch written out in 1842, he could not have made a better short abstract!' Darwin moaned in a letter to Lyell.

Staid Victorians would soon consider both Wallace and Darwin to be out of their minds, and Wallace certainly came close, for he was suffering from malaria when he composed his letter to Darwin. As a good socialist, Wallace had been taught not to trust the reasoning of Malthus, who had argued that the world's ability to feed itself grew linearly, while the population grew exponentially - implying that eventually the population would win and there would be too little food to go round. Socialists believed that human ingenuity could postpone such an event indefinitely. But by the 1850s even socialists were beginning to view Malthus in a more favourable light; after all, the threat of overpopulation was a very good reason to promote contraception, which made excellent sense to every good socialist. Half-delirious with fever, Wallace thought about the rich variety of species he had encountered, wondered how that fitted in with Malthus, put two and two together, and realised that you could have selective breeding without the need for a breeder.

As it turned out, he didn't have quite the same view as Darwin. Wallace thought that the main selective pressure came from the struggle to survive in a hostile environment - drought, storm, flood, whatever. It was this struggle that removed unfit creatures from the breeding pool. Darwin had a rather blunter view of the selection mechanism: competition among the organisms themselves. It wasn't quite `Nature red in tooth and claw' as Tennyson had written in his In Memoriam of 1850, but the claws were unsheathed and there was a certain pinkness to the teeth. To Darwin, the environment set a background of limited resources, but it was the creatures themselves that selected each other for the chop when they competed for those resources. Wallace's political leanings made him detect a purpose in natural selection: to `realise the ideal of a perfect man'. Darwin refused even to contemplate this kind of utopian hogwash.

Wallace hadn't mentioned publishing his theory, but Darwin now felt obliged to recommend it to him. At that point it looked as if Charles had compounded his Really Bad idea, but for once the universe was kind. Lyell, searching for a compromise, suggested that the two men might agree to publish their discoveries simultaneously. Darwin was concerned that this might make it look as if he'd pinched Wallace's theory, worried himself to distraction, and finally handed the negotiating over to Lyell and Hooker and washed his hands of it.

Fortunately, Wallace was a true gentleman (the accident of breeding notwithstanding) and he agreed that it would be unfair to Darwin to do anything else. He hadn't realised that Darwin had been working on exactly the same theory for many years, and he had no wish to steal such an eminent scientist's thunder, perish the very thought. Darwin quickly put together a short version of his own work, and Hooker and Lyell got the two papers inserted into the schedule of the Linnaean Society, a relatively new association for natural history. The Society was about to shut up shop for the summer, but the council fitted in an extra meeting at the last minute, and the two papers were duly read to an audience of about thirty fellows.

What did the fellows make of them? The President reported later that 1858 had been a rather dull year, not `marked by any of those striking discoveries which at once revolutionise, so to speak, our department of science'.

No matter. Darwin's fear of controversy was now irrelevant, because the cat was out of the bag, and there was no chance whatsoever that the beast could be stuffed back in. Yet, as it happened, the anticipated controversy didn't quite materialise. The meeting of the Linnaean Society had been rushed, and the fellows had departed muttering vaguely under their breaths, feeling that they ought to be outraged by such blasphemous ideas ... yet puzzled because the enormously respected (and respectable) Hooker and Lyell clearly felt that both papers had some merit.

And the ideas struck home with some. In particular, the VicePresident promptly removed all mention of the fixity of species from a paper he was working on.

Now that Darwin had been forced to put his head above the parapet, he would lose nothing by publishing the book that he had previously decided not to write, but had constantly been thinking about anyway. He had intended it to be a vast, multi-volume treatise with extensive references to scientific literature, examining every aspect of his theory. It was going to be called Natural Selection (a conscious or subconscious reference to Paley's Natural Theology?). But time was pressing. He polished up his existing essay, changing the title to On the Origin of Species and Varieties by Means of Natural Selection. Then, on the insistent advice of his publisher John Murray, he cut out the `and Varieties'. The first print run of 1250 copies went on sale in November 1859. Darwin sent Wallace a complimentary copy, with a note: `God knows what the public will think.'

In the event, the book sold out before publication. Over 1500 advance orders came in for those 1250 copies, and Darwin promptly started working on revisions for a second edition. Charles Kingsley, author of The Water-Babies, country rector, and Christian socialist, loved it, and wrote a lavish letter: `It is just as noble a conception of Deity, to believe that He created primal forms capable of selfdevelopment ... as to believe that He required a fresh act of intervention to supply the lacunas[33] which He himself had made.' Kingsley was something of a maverick, because of his socialist views, so praise from this source was something of a poisoned chalice.

The reviews, steadfast in their Christian orthodoxy, were distinctly less favourable. Even though Origin hardly mentions humanity, all the usual complaints about men and monkeys, and insults to God and His Church, were trotted out. What particularly galled the reviewers was that ordinary people were buying the thing. It was all right for the upper classes to toy with radical views, it had an attractive frisson of naughtiness and was perfectly harmless among gentlemen of breeding, though not ladies of course; but those same views might put ideas into the common folk's heads, if they were exposed to them, and upset the established order. For Heaven's sake, the book was even selling to commuters outside Waterloo railway station! It must be suppressed!

Too late. Murray geared up to print 3000 copies of the second edition, whose likely sales were not going to suffer from public controversy. And the people who mattered most to Darwin - Lyell, Hooker, and the anti-religious `evangelist' Thomas Henry Huxley - were impressed, and pretty much convinced. While Charles stayed out of the public debate, Huxley set to with a will. He was determined to advance the cause of atheism, and Origin gave him a point of leverage. The radical atheists loved the book, of course: its overall message and scientific weightiness were enough for them, and they weren't too concerned about fine points. Hewett Watson declared Darwin to be `the greatest revolutionist in natural history of this century'.

In the introduction to Origin, Darwin begins by telling his readers the background to his discovery: When on board H.M.S. Beagle, as naturalist, I was much struck with certain facts, in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species - that mystery of mysteries, as it has been called by one of our greatest philosophers. On my return home, it occurred to me, in 1837, that something might perhaps be made out of this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it.

Apologising profusely for lack of space, and time, to write something more comprehensive than his 150,000-word tome, Darwin then moves towards a short summary of his main idea. Writers on science generally appreciate that it is seldom enough to discuss the answer to a question: it is also necessary to explain the question. And that, of course, should be done first. Otherwise your readers will not appreciate the context into which the answer fits. Darwin was clearly aware of this principle, so he begins by pointing out that: It is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that each species had not been independently created, but had descended, like varieties, from other species. Nevertheless such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how innumerable species inhabiting this world have been modified, so as to acquire those perfections of structure and coadaptation which most justly excites our admiration.

Already we see a gesture towards Paley - 'perfections of structure' is a clear reference to the watch/watchmaker argument, and `had not been independently created' shows that Darwin doesn't buy Paley's conclusion. But we also see something that characterises the whole of Origin: Darwin's willingness to acknowledge difficulties in his theory. Time and again he raises possible objections - not as straw men, to be knocked flat again, but as serious points to be considered. More than once he concludes that there is more to be learned, before the objection can be resolved. Paley, to his credit, did something similar, though he didn't go as far as admitting ignorance: he knew that he was right. Darwin, a real scientist, not only had his doubts - he shared them with his readers. He would not have arrived at his theory to begin with if he had failed to seek the weaknesses of the hypotheses upon which it was based.

He also, of course, makes it clear what his own work is adding to the speculations of earlier 'transmutationists'. Namely: he has come up with a mechanism for species change. There are advantages in being honest about ,your own limitations: you gain the right to talk about the limitations of others. And now he tells us what that mechanism is. Species, we know, are variable - the domestication of wild species like chickens, cows, and dogs is clear evidence of that. Although that is deliberate selection by humans, it opens the door to selection by nature without human aid: I will then pass on to the variability of species in a state of nature ... We shall, however, be enabled to discuss what circumstances are most favourable to variation. In the next chapter the Struggle for Existence amongst all organic beings throughout the world, which inevitably follows from their high geometrical powers of increase, will be treated of ... The fundamental subject of Natural Selection will be treated at some length in the fourth chapter; and we shall then see how Natural Selection almost inevitably causes much Extinction of the less improved forms of life, and induces what I have called Divergence of Character.

He then promises four chapters on `the most apparent and gravest difficulties of the theory', prominent among these being to understand how a simple organism or organ can change into a highly complex one - another nod to Paley. The introduction ends with a flourish: I can entertain no doubt ... that the view which most naturalists entertain, and which I formerly entertained - namely, that each species has been independently created - is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendent of some other and generally extinct species ... Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification.

In essence, Darwin's theory of natural selection, which soon became known as evolution[34], is straightforward. Most people think they understand it, but its simplicity is deceptive, and its subtleties are easily underestimated. Many of the standard criticisms of evolutionary theory stem from common misunderstandings, not from what the theory actually proposes. The ongoing scientific debate about details is often misrepresented as disagreement with the general outline, which is an error based on too simple-minded a view of how science develops and what `knowledge' is.

Briefly, Darwin's theory goes like this.

1. Organisms, even those in the same species, are variable. Some are bigger than others, or bolder than others, or prettier than others.

2. This variability is to some extent hereditary, passed on to offspring.

3. Unchecked population growth would quickly exhaust the capacity of the planet, so something checks it: competition for limited resources.

4. Therefore as time passes, the organisms that do survive long enough to breed will be modified in ways that improve their chance of surviving to breed, a process called natural selection.

5. Ongoing slow changes can lead, in the long run, to big differences.

6. The long run has been very long indeed - hundreds of millions of years, maybe more. So by now those differences can have become huge.

It's relatively simple to put these six ingredients together and deduce that new species can arise without divine intervention - provided we can justify each ingredient.

Even though different species seem to stay pretty much the same - think lions, tigers, elephants, hippos, whatever - it is actually rather obvious that, in general, species are not fixed for all time. The changes are relatively slow, which is why we don't notice them. But they do happen. We've already seen that in Darwin's finches, evolutionary changes can be and have been observed on a timescale of years, and in bacteria they occur on a timescale of days.

The most obvious evidence for the variability of species, in Darwin's day and ours, was the domestication of animals - sheep, cows, pigs, chickens, dogs, cats ...

... and pigeons. Darwin was rather knowledgeable about pigeons, he belonged to two London pigeon clubs. Every pigeon-fancier knows that by selectively breeding particular combinations of male and female pigeons, it is possible to produce `varieties' of pigeons with particular characteristics. `The diversity of the breeds is something astonishing,' says Darwin in the first chapter of Origin. The English carrier pigeon has a wide mouth, large nostrils, elongated eyelids, a long beak. The short-faced tumbler has a short stubby beak like a finch. The common tumbler flies high up in a tightly knit flock, and has an odd habit of falling about in the sky, whence its name. The runt (despite its name) is huge, with a long beak and large feet. The barb is like the carrier but with a short, broad beak. The pouter has an inflatable crop and can puff out its chest. The turbit has a short beak and a line of reversed feathers on its chest. The Jacobin has so many reversed feathers on its neck that they form a hood. Then there are the trumpeter, laugher, fantail ... These are not separate species: they can interbreed, to produce viable `hybrids' - cross-breeds.

The enormous variety of dogs is so well known that we don't even need to mention examples. It's not that the dog species is exceptionally malleable, just that dog-breeders have been unusually active and imaginative. There is a dog for every purpose that a dog can carry out. Again, they're all dogs, not new (albeit related) species. They can mostly (barring really big size differences) interbreed, and artificial insemination can take care of mere size. Dog sperm plus dog egg makes fertile dog zygote, and, eventually, dog - independently of breed. That's why pedigree pooches need a pedigree, to guarantee that their parentage is pure. If the different varieties of dog were different species, that wouldn't be necessary.

In modem times, it has become clear that cats are just as malleable, but the cat-breeders have only just got going on exotic cats. The same goes for cows, pigs, goats, sheep ... and what about flowers? The number of varieties of garden flowers is immense.

By avoiding the creation of hybrids, the breeder can maintain the individual varieties over many generations. Pouter pigeons breed with pouters to produce (a substantial proportion of) pouters. Carriers mated with carriers produce (mostly) carriers. The underlying genetics, about which Darwin and his contemporaries knew nothing, is complicated enough that apparent hybrids can sometimes arise from what seems to be pure stock, just as two brown-eyed parents can nonetheless have a blue-eyed child. So pigeon-breeders have to eliminate the hybrids.

The existence of these cross-bred varieties does not, of itself, explain how new species can arise of their own accord. Varieties are not species; moreover, the guiding hand of the breeder is evident. But varieties do make it clear that there must be plenty of variability within a species. In fact, the variability is so great that one can readily imagine selective breeding leading to entirely new species, given enough time. And the avoidance of hybrids can maintain varieties from one generation to the next, so their characters (biologese for the features that distinguish them) are heritable (biologese for `able to be passed from one generation to the next'). So Darwin has his first ingredient: heritable variability.

The next ingredient was easier (though still controversial in some quarters). It was time. Oodles and oodles of time, the Deep Time of geologists. Not a few thousand years, but millions, tens of millions ... billions, in fact, though that was further than the Victorians were willing to go. Deep Time, as we've previously observed, is contrary to the biblical chronology of Bishop Ussher, which is why the idea remains controversial among certain Christian fundamentalists, who have bizarrely chosen to fight their corner on the weakest of grounds, completely needlessly. Deep Time is supported by so much evidence that a truly committed fundamentalist has to believe that his God is deliberately trying to fool him. Worse, if we can't trust the evidence of our own eyes, then we can't trust the apparent element of `design' in living creatures either. We can't trust anything.

Lyell realised that the age of the Earth must be many millions of years, when he looked at sedimentary rocks. These are rocks like limestone or sandstone which form in layers, and have been deposited either underwater, as muddy sediments, or in deserts, as accumulating sand. (Independent evidence for these processes comes from the fossils found in such rocks.) By studying the rate at which modern sediments accumulate, and comparing that with the thickness of known beds of sedimentary rock, Lyell could estimate the time it had taken for the layers of rock to be deposited. Something in the range 1000-10,000 years would produce a layer about a metre thick. But the chalk cliffs of the south coast, around Dover, are hundreds of metres thick. So that's several hundred thousand years of deposition, and we've only dealt with one of the numerous layers of rock that make up the geological column - the historical sequence of different rocks.

We now have many other kinds of evidence for the great age of our planet. The rate of decay of radioactive elements, which we can measure today and extrapolate backwards, is in general agreement with the evidence of the rock layers. The rate of movement of the continents, when combined with the distances they have moved, is again consistent with other estimates. We've seen that India was once attached to Africa, but about 200 million years ago it broke off, and by 40 million years ago it had moved all the way to its current position, butting up against Asia and pushing up the Himalayas.

When continents move apart - as Africa and South America, or Europe and North America, are doing now - new material forms on the ocean floor, flowing out from the mantle beneath to form huge mid-ocean ridges. The rocks in the ridges contain a record of the changes in the Earth's magnetic field, `frozen in' as the rock cooled. They show a long series of repeated reversals of the field polarity. Sometimes the `north' magnetic pole is at the northern end of the Earth, as now, but every so often the polarity flips, so that the magnetic pole near the northern end is the `south' one. Mathematical models of the Earth's magnetic field predict that such reversals occur roughly once every five million years. Count the number of reversals in the ocean-ridge rocks, multiply by five million ... again, the numbers fit reasonably well, and careful comparisons and a lot of disputation by experts lead to revised numbers that fit even better.

The Grand Canyon is a deep gash through layers of rock one mile (1.6km) thick. You have a choice. You can understand what the record of the rocks is telling you here: it took a very long time to lay down those rocks, and quite a long time - though less - for flashflooding in the Colorado river to erode them again. Or you can follow one book that until recently was displayed in the `science' section of the Grand Canyon bookstore, until a lot of scientists complained, and assert that the Grand Canyon is evidence for Noah's flood. The first choice fits huge amounts of evidence and geological understanding. The second is an excellent test of faith, because it fits absolutely nothing. A flood that lasted only 40 days could never have produced that kind of geological formation. A miracle? In that case, the Sahara desert could equally well be hailed as evidence for Noah's flood, miraculously not forming a deep canyon. Once you admit miracles, you can't pursue a logical thread.

Anyway, that's the second ingredient - Deep Time. It takes huge amounts of time to change organisms into entirely new species, if all you can do - as Darwin believed - is make very gradual changes. But even Deep Time, when combined with heritable variation, is not enough to lead to the kind of organised, coherent changes that are needed to create new species. There has to be a reason for such changes to occur, as well as opportunity and time. Darwin, as we've seen, found his reason in Malthus's contention that the unchecked growth of organisms is exponential, whereas that of resources is linear. In the long run, exponential growth always wins.

The first assertion is pretty much correct, the second highly debatable. The qualifier `unchecked' is crucial, and real populations only grow exponentially if there are plenty of resources available. Typically, the growth starts exponentially with a small population and then levels off as the population size increases. But in most species, two parents (let's think sexual species here) produce some larger number of offspring. A breeding female starling lays about 16 eggs in her life, and with `unchecked' growth, the starling population would multiply by 8 every lifetime. It would not be long before the planet was knee-deep in starlings. So, of necessity, 14 of those 16 offspring (on average) fail to breed - usually because something eats them. Just two become parents in their turn. A female frog may lay 10,000 eggs in her life, and nearly all die in various grotesque ways to achieve each two parents; a female cod contributes forty million or thereabouts of her offspring to planktonic food chains, for each two that breed. Here the multiplier, with `unchecked' growth, would be 20 million per cod-lifetime. Unchecked growth simply doesn't bear thinking about as a realistic prospect.

We suspect that Malthus plumped for linear growth of resources for a slightly silly reason. Victorian school-textbook mathematics distinguished two main types of sequence: geometric (exponential) and arithmetic (linear). There were plenty of other possibilities, but they didn't get into the textbooks. Having already assigned geometric growth to organisms, Malthus was left with arithmetic growth for resources. His main point doesn't depend on the actual growth rate, in any case, as long as it is less than exponential. As the starling example shows, most offspring die before breeding, and that's the main point here.

Given that most young starlings cannot possibly become parents, the question arises: which ones will? Darwin felt that the ones that survived to breed would be the ones best suited for survival, which makes sense. If one starling is better at finding food, or hanging on to it, than another one, then it's clear which one is more likely to do best if food supplies become limited. The better one might be unlucky and get eaten by a hawk; but across the population, starlings that are better equipped to survive are generally the ones that do survive.

This process of `natural selection' in effect plays the role of an external breeder. It chooses certain organisms and eliminates the rest. The choice is not conscious - there is no consciousness to do the choosing, and no preconceived purpose - but the end result is very similar. The main difference is that natural selection makes sensible choices, whereas human selection can make ridiculous ones (like dogs with faces so flattened they can hardly breathe). Sensible choices lead to sensible animals and plants, ones that are beautifully adapted for survival in whatever environment they happened to be in when natural selection was moulding them.

It is just like breeding new varieties of pigeon, but without a human breeder. Natural selection exploits the same variability of organisms that pigeon-breeding does. It makes choices based on survival value (in some environment) rather than whim. It is typically much slower than human intervention, but the timescale is so vast that this slowness doesn't matter much. Heritable variation plus natural selection inevitably lead, over Deep Time, to the origination of species.

Nature does it all on her own. There is no need for a series of acts of special creation. That doesn't imply that special creation has not occurred. It just removes any logical imperative for it.

Paley was wrong.

The watches don't need a watchmaker.

They can make themselves.