Science of Discworld III

to a sharp focus, forming an image. The eye has a retina to receive that image, just as a telescope has an observer, or a screen on to which the image is projected.
    The lens of the eye is useless without the retina; the retina is useless without the lens. You can’t put an eye together piecemeal – you need all of it, at once, or it can’t work. Later supporters of theist explanations of life turned Paley’s subtle arguments into a simplistic slogan: ‘What use is half an eye?’
    One reason to doubt Paley’s explanation of ‘design’ is that in science, you very seldom get what you see. Nature is far from obvious. The waves on the ocean may seem to be travelling, but the water is mainly going round and round in tiny circles. (If it wasn’t, the land would quickly be swamped.) The Sun may appear to orbit the Earth, but actually it’s the other way round. Mountains, apparently solid and stable, rise and fall over geological timescales. Continents move. Stars explode. So the explanation ‘it appears designed because it is designed’ is a bit too trite, a bit too obvious, a bit too shallow. That doesn’t prove it’s wrong, but it gives us pause.
    Darwin was one of a select group of people who realised that there might be an alternative. Instead of some cosmic designer creating the impressive organisation of organisms, that organisation might come into being of its own accord. Or, more accurately, as an inevitable consequence of the physical nature of life, and its interactions with its environment. Living creatures, Darwin suggested, are not the product of design, but of what we now call ‘evolution’ – a process of slow, incremental change, almost imperceptible from one generation to the next, but capable of accumulating over extensive periods of time. Evolution is a consequence of three things. One is the ability of living creatures to pass on some of their attributes to their offspring. The second is the slightly hit-and-miss nature of that ability: what they pass on is seldom a precise copy, though it usually comes close. The third is ‘natural selection’ – creatures that are better at survival are the ones that manage to breed, and pass on their survival attributes.
    Natural selection is slow.
    As an accomplished student of geology – Victorian-style field geology, where you traipse about the landscape trying to work out what rocks lie under your feet, or halfway up the next mountain, and how they got there – Darwin was well aware of the sheer abyssal depth of geological time. The record of the rocks offered compelling evidence that the Earth must be very, very old indeed: tens or hundreds of millions of years, maybe more. Today’s figure of 4.5 billion years is even longer than the Victorian geologists dared imagine, but probably would not have surprised them.
    Even a few million years is a very long time. Small changes can turn into huge ones over such a period of time. Imagine a species of worm four inches (10 cm) long, whose length increases by one thousandth of a per cent every year, so that even very accurate measurements would not detect any change on a yearly basis. In a hundred million years, the descendants of that worm would be 30 feet (10 m) long. From annelid to anaconda. The longest worm alive today sometimes reaches lengths of 150 feet (50m), but it is a marine worm: Lineus longissimus , which lives in the North Sea and can be found under boulders at low tide. Earthworms are a lot shorter, but the Megascolecid worms of Australia can grow to a length of 10 feet (3m), which is still impressive.
    We’re not suggesting that evolution happens with quite that degree of simplicity or regularity, but there’s no question that geological time allows huge changes to occur by imperceptible steps. In fact, most evolutionary changes are a lot faster. Observations of ‘Darwin’s finches’, 13 species of bird that inhabit the Galápagos Islands, reveal measurable changes from one year to the next – for example, in the average sizes of the birds’ beaks.
    If we want to explain the rich panoply of life on Earth, it is not enough to observe that living creatures can change as the generations pass. There must also be something that drives those changes in a ‘creative’ direction. The only driving force that Paley couldimagine was God, making conscious, intelligent choices and designing them in from the beginning. Darwin was more acutely aware that organisms can and do change from