Science of Discworld III

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 thatmechanism 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, 6 is straightforward. Most people think theyunderstand 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.
Organisms, even those in the same species, are variable. Some are bigger than others, or bolder than others, or prettier than others.
This variability is to some extent hereditary, passed on to offspring.
Unchecked population growth would quickly exhaust the capacity of the planet, so something checks it: competition for limited resources.
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.
Ongoing slow changes can lead, in the long run, to big differences.
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