There has been something strangely liberating about the global financial collapse of the past few months - particularly if you have a nice safe job in higher education. For most of my lifetime we have been raised on a series of cod-economic orthodoxies, at the heart of which was the prejudice that markets are usually the best way of organising human affairs of any kind. Of course, the reason we have believed this for so long is that it is very nearly true. Market economics is amazingly powerful and has, over the past 200 years or so, transformed the material circumstances of human life like nothing since the development of agriculture.

But precisely because it is such a good and powerful idea, market economics has a tendency to get out of control. It has spent much of the past quarter-century looking, not like an invaluable economic tool, but like the central principle around which our societies should be built, morally as well as pragmatically. It's not just that we've been prejudiced in favour of markets in all sorts of areas of public policy, sometimes wisely and sometimes not. We've let ourselves think of the market as an arbiter of value. Yes, if no one is willing to pay for something, then perhaps that something isn't very valuable. But that is a cramped and sometimes distorted concept of value. For 25 years we've gone along with it, with a vague sense of unease.

Now it turns out that the nagging sense that this was all a bit odd, that orthodoxies had become overbearing and too distant from reality, was right.

I'm not talking about an anti-market backlash. As Martin Luther said, human beings are the kind of creatures who, having fallen off their horse on the right, get back up and then carefully fall off on the left. In fact, I've now just about finished talking about economics. The point is that ideas tend to get out of control, and good ideas - such as market economics - are particularly liable to get out of control. We see that something works in one arena and we start trying to apply it elsewhere. We get excited and think it's a universal panacea. And the better the original idea, the more this process can go wrong.

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It's a kind of intellectual cancer. It's not bad ideas that are dangerous, any more than it's weak cells that cause cancer. It's the good ideas, the powerful ideas, the flexible and adaptable ideas that you've got to watch. They're the ones that can metastasise. They get into your bloodstream and start establishing themselves all over the place. They don't stay where they belong and do what they're good at; they try to take over.

As we collectively embark on a bout of economic chemotherapy, it's worth looking at what other ideas there might be out there waiting to turn malignant on us.

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We're looking for good ideas, powerful and necessary ideas, that are so powerful that they wriggle loose and start reinterpreting the whole world. We've had a few of those. Marxism doesn't really count because it wasn't that great an idea to begin with; it was just based on some brilliant economic and historical analysis. Anyway, it didn't mutate - it did more or less what it promised to, for better or (mostly) for worse. Quantum mechanics might have been another: it's certainly brilliant and pervasive, but it is so weird and so counter-intuitive that most laypeople haven't managed to extract any particularly compelling analogies or metaphors from it. Freudianism is a better example: a brilliant pattern of thinking that for a while seemed to be getting out of hand. It's certainly pervasive, but I don't think we quite believe in it enough for it to be that dangerous any more. Environmentalism could be the next one, as the long-term crises start to crest and as ends and means become blurred. (One useful thought experiment here: if we found a cheap and effective way to scrub greenhouse gases from the atmosphere, would that make it OK to go on emitting them? And if not, why not?)

Right now, this year's anniversaries give us a candidate: Darwinism.

A brilliant, simple, subtle idea. Indeed, an indispensable idea. But the best idea ever? That sort of talk makes me slightly queasy, and not just because it's much too early to say. Messianic adulation of a thinker, of the kind we've seen plenty of with Darwin this year, is an obvious early-warning sign that his or her ideas are at risk of breaching the levees. Personally, I think Shakespeare should have beaten Darwin in the race for top Briton, but maybe that's just rank arts-faculty prejudice. Still, there's no doubt that Darwinism has permanently changed the way we look at life, including ourselves. So it meets the first criterion: a very good, very powerful and very pervasive concept.

And we all know how easily it gets out of control. One problem - a problem Darwin was well aware of - is that we do tend to get our ises and our oughts mixed up, and therefore to assume that natural selection is a moral process. We know it isn't, but the prejudice is deep-rooted. That led to social Darwinism of all sorts of unpleasant kinds in the later 19th and early 20th centuries, prejudices that usually depended on pretty vague understandings of Darwin's thought and were all the more powerful for that. William Jennings Bryan, the great American populist who was defeated in the famous Scopes monkey trial, opposed Darwinism at root for political reasons, not scientific or religious ones. He had seen what a social-Darwinist mentality had done to pre-First World War Germany, and wanted no truck with it.

Nazism has cured us of that, or at least kept the problem under control. But Darwinism is still too powerful an idea to stay put and do what it's supposed to.

What it's supposed to do, what it does better than anything else we've yet come up with, is to explain how entities that compete with each other and self-replicate imperfectly can develop astonishing levels of complexity and diversity, and can do so much faster than intuition would suggest. And in addition to this theoretical explanation, Darwinism provides a historical explanation. It is not simply that this process can happen, but that it has happened and we are among its products.

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But there are some things it does less well and some things it cannot do at all. The historical explanation that Darwinism offers is particularly popular right now (not least in economics), as we use the realisation that we are evolved beings to analyse our own behaviour and make-up. There's some brilliant science going on here, but as a historian I worry that there's also some sloppy history. The risk is that we start telling evolutionary just-so stories, working backwards from where we are to deduce how we must have got here. When you hear another explanation of how a particular human characteristic (or alleged characteristic) derives from our origins as savannah-dwelling lion-fodder, ask a simple question: is this testable? If so (and it often is), it's a scientific hypothesis. Otherwise, it's an origin myth.

More seriously, Darwinism does not and cannot explain anything about entities that do not replicate themselves. Or at least it can do so only weakly, by analogy or as a metaphor - forms of explanation that aren't usually very respectable as science.

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This may seem obvious but it is worth pointing out. The Gaia hypothesis, as its founder James Lovelock has insisted roundly and repeatedly, does not state that the world has evolved. It states (as I understand it) that the world has many of the characteristics of a single self-regulating system, which is not the same thing. But that distinction has been lost on many of those who like the idea.

Or again: physics is not Darwinian. Attempts to explain how the laws of physics are emergent properties are fascinating but they have nothing to do with Darwinism. This is not a sphere in which competition, self-replication or imperfect copying apply. If Rupert Sheldrake wants to argue that the cosmos is alive, then good luck to him. But if he wants to argue that the universe is fundamentally Darwinist, I want to see the evidence that universes (or any non-living entities within them) reproduce and compete with each other. It's like saying that Kepler's laws of motion are Freudian or describing students as customers. You can do it, but only by a determined act of ideological will, and in the process you conceal more than you reveal.

A great deal of this, of course, comes back to Darwinism's most celebrated modern advocate and reinterpreter. Richard Dawkins' forays beyond biology have earned him plenty of brickbats, which he clearly enjoys, but I think those criticisms misread him. To describe his world view as atheist, as he himself does, is too negative. His beliefs are more positive than that: he's a Darwinist, or a trans-Darwinist. He is intoxicated by Darwinism, and has let it become the universal explanation. We all know nowadays that life evolved, but that is a historical fact that doesn't begin to settle any of the larger philosophical issues. What it does do, or what it allows us to do, is to paint a picture. It has given us, or more particularly it has given Dawkins, a vision of complexity spontaneously generating itself - a world of cranes, not of skyhooks, as he puts it; a vision that is so intellectually entrancing that he wants to use it to explain everything. It's aesthetically appealing, but it's an act of faith.

The most notorious example of this is his concept of the meme, a bald-faced claim that ideas can sensibly be treated as self-replicating entities. I wish. Teaching would be a lot easier. At the risk of stating the obvious, ideas do not breed like mice, nor do they spread like parasites. People learn them and pass them on. The point of the meme concept is a polemical one, of course: to point out that erroneous ideas can still be very popular if they appeal to people in some way. That's hardly news, but it's a worthwhile truth. It reminds us that there is no simple relationship between how true an idea is and how popular or unpopular, how fashionable or unfashionable it is. But errors don't reproduce spontaneously, any more than truths do, or any more than comets do. (If they did, the concept of the meme itself would be pointless.) It's a Darwinian process by only the weakest of analogies. Nor is it a particularly revealing analogy, since it tells us nothing about why particular ideas spread or do not spread.

In practice, "meme" is little more than a term of abuse, code for "an idea with which I disagree but which I do not have time to rebut right now", or merely "an idea whose popularity I wish to disparage". And we can all play that game, but we have enough terms of abuse already.

So what's the cure for this form of intellectual cancer? Same old same old, I'm afraid: early detection and regular self-examination. The symptom to watch for is that sense of unease that many of us suppressed about the hegemony of market economics; the protest of an intellect that is being bent into an uncomfortable shape and forced to stay there. If you find yourself thinking, stuff it, maybe nationalised banks might work better than this shower; or quietly protesting that the cigar really is a cigar; or having difficulties with the breezy claim that the universe is simultaneously meaningless and comprehensible - then you'll probably make a full recovery.

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Unless, of course, the idea that all ideas are carcinogenic turns cancerous itself, spreading dubious (and cod-Darwinist) concepts around and choking off perfectly sensible analogies and patterns of thought. You'll have to sort that one out for yourselves; excuse me, I seem to have trod in a paradox again. But that's what happens when you push a metaphor too far.