The Science of Discworld II

historical pattern, which can sometimes be compelling. We can question how scientific such stories are, because it is always easy to be wise after the event. But in this case the story generalises: rich weak kings are asking to be invaded by mean, poor barbarians. And that’s a prediction, wisdom before the event, and as such it is scientifically testable. 3
    The stories that evolutionary biologists tell are of the same kind, and they become science when they stop being Just-So Stories, justifications after the event, and become general principles that make predictions. These predictions are of a limited kind: ‘in these circumstances expect this behaviour’. They are not predictions of the type ‘On Tuesday at 7.43pm the first elephant trunk will evolve’. But this is what ‘prediction’ means in science: saying ahead of time that under certain conditions, certain things will happen. You don’t have to predict the timing of the experiment.
    An evolutionary example of this kind of pattern can be found in the co-evolution of ‘creodonts’, big cats like sabretooth tigers, and their ‘titanothere’ prey – large-hoofed mammals, often with huge horns. When it comes to improving performance for the big cats, the line of least resistance is to develop bigger teeth. Faced with that, the best response for the prey is to develop thicker skins and bigger horns. An evolutionary arms race now becomes pretty much unavoidable: the cats get bigger and bigger teeth, and the prey respond with thicker and thicker skins … to which the cats’ only response is even bigger teeth … and so it goes. An evolutionary arms race sets in, with both species trapped in a single strategy. The end result is that the cats’ teeth get so enormous that the poor animals can hardly move their heads, while the titanotheres’ skins, and multiple horns on nose and brow, and associated musculature, get so heavy that they find trouble dragging themselves across the plains. Both species promptly die out.
    This creodont-titanothere arms race has happened at least five times in evolutionary history, taking about five million years to run its course on each occasion. It is a striking example of an emergent pattern, and the fact that it plays out in exactly the same way over and over again confirms that there really is an underlying dynamic. In all likelihood it would be happening again, now, except for the arrival of humans, who have clobbered both the big cats and their slow prey.
    Notice that we’ve been calling these system-level patterns ‘stories’, and so they are. They have a narrative, a consistent internal logic; they have a beginning and an end. They are stories because they cannot be ‘reduced’ to an entity-level description; that would be more like an interminable soap opera. ‘Well, this electron bumped into that electron and the two of them got together and emitted a photon …’ repeated, with slight variations, a truly inconceivable number of times.One of the central questions about emergent dynamics is: what would happen if we ran the system again, in slightly different circumstances? Would the same patterns emerge, or would we see something completely different? If European history in the early twentieth century was rerun, but without Adolf Hitler, would World War II have happened anyway, by a different route? Or would it all have been sweetness and light? Historically, this is a crucial question. There is no doubting that Hitler was instrumental in starting World War II; the deeper question here is whether he was a product of the politics of the time, and in his absence someone else would have done much the same, or whether it was Hitler who moulded history and created a war when otherwise nothing would have happened.
    At risk of being controversial, we are inclined to the view that World War II was a pretty much inevitable consequence of the political situation in the 1930s, with Germany saddled with huge reparations for World War I, the trains not running on time … and Hitler was merely the medium through which the national will to war was expressed. But it’s not the answer that concerns us here: it is the nature of the question. It is a ‘what if’ question, and it is about historical phase space. It does not ask what happened; it asks what might have happened instead.
    This point is well understood on Discworld. In