Science of Discworld III

that came to mind, began to hint at the ‘mutability of species’. The hints were incomplete and ill-assorted. Deformed babies resembled new species. The beaks of Galápagos finches were of different shapes and sizes. Rheas were a puzzle, though: two distinct species of the giant birds had overlapping ranges in Patagonia. Why didn’t they merge into a single species?
    By July, he had secretly started a new notebook, his B Notebook.
    It was on the transmutation of species.
    By 1839 Darwin was building up a complete picture, and he wrote a 35-page summary of his thinking. A crucial influence was Thomas Malthus, whose 1826 Essay on the Principle of Population pointed out that the unchecked growth of organisms is exponential (or ‘geometric’, in the old-fashioned phrase of the time), whereas that ofresources is linear (‘arithmetic’). Exponential growth occurs when each step multiplies the size by some fixed amount, for example 1, 2, 4, 8, 16, 32, where each number is twice the previous one. Linear growth adds some fixed amount at each step, for instance 2, 4, 6, 8, 10, where each number exceeds the previous one by 2. However small the multiplier is in exponential growth, provided it is bigger than 1, and however large the number added in linear growth may be, it turns out that in the long run exponential growth always beats linear. Though it does take some time if the multiplier is close to 1 and the number being added is huge.
    Darwin had taken on board Malthus’s argument, and he had realised that in practice what keeps populations down is competition for resources, such as food and a place to live. This competition, he wrote, leads to ‘natural selection’, in which those creatures that are victorious in the ‘war of nature’ are the ones that produce the next generation. Individual creatures within a species are not exactly identical; those differences make it possible for the force of natural selection to produce slow, gradual changes. How far might such changes go? In Darwin’s view, very far indeed. Far enough to lead to entirely new species, given enough time. And thanks to geology, scientists now knew that the Earth was very, very old.
    Darwin, following family tradition, was a Unitarian. This particular branch of Christianity has been aptly described as ‘people who believe in at most one God’. As a sound Unitarian, he believed that the Deity must work on the grandest of scales. So he finished his summary with a powerful appeal to the Unitarian view of the Deity:
    It is derogatory that the Creator of countless systems of worlds should have created each of the myriads of creeping parasites and slimy worms which have swarmed each day of life on land and water on this one globe. We cease being astonished, however much we may deplore, that a group of animals should have been directly created to lay their eggs in bowels and flesh of others – that some organismsshould delight in cruelty … From death, famine, rapine, and the concealed war of nature we can see that the highest good, which we can conceive, the creation of the higher animals has directly come.
    God surely has better taste than to create nasty parasites directly . They exist only because they are a necessary step along the path that leads to cats, dogs, and us.
    Darwin had his hypothesis.
    Now he began to agonise about how to bring it to the waiting world.
    1 In 1865 FitzRoy did exactly the same, having been turned down for a promotion. Narrativium at work?
    2 Now considered to be thirteen, plus a fourteenth on the Cocos Islands. (Look, people write and complain if we don’t point this kind of thing out.)

ELEVEN
WIZARDS ON THE WARPATH
    I N THE GLOOM OF THE High Energy Magic building, Hex wrote. Every minute another page slid off the writing table.
    ‘“Boat sunk by collision with Spanish fishing vessel”,’ Ponder Stibbons read out, a tremor in his voice. ‘“Boat shipwrecked on uncharted reef near Madeira. Boat found drifting minus all crew, with the table laid for a meal. Boat catches fire, all lost. Boat struck by meteorite. Darwin accidentally shot by ship’s surgeon and naturalist during a collecting expedition on the island of St Jago. Darwin accidentally shot by ship’s captain. Darwin accidentally shot by himself. Darwin loses place on boat. Darwin leaves boat because of seasickness. Darwin loses notebooks. Darwin stung to death by wasps! Darwin bangs head on underside of table and loses memory …”’ He put