Homage to Gaia: the life of an independent scientist James Lovelock Oxford University Press, 448pp, £19.99 ISBN 0192862138

James Lovelock's Gaia hypothesis proposes that life on earth acts as a whole in a way that keeps the planet habitable. Life is not just a passenger, but part of the system. The history of this theory illustrates in a richly entertaining manner how deeply science is entangled with the rest of our thought. Officially, scientists are supposed to judge each new theory impartially and on strictly objective criteria. In fact, when confronted with something new, they have just the same difficulty as the rest of us in deciding whether they like the smell of something enough to bother investigating it at all.

For a long time, scientists ignored Gaia theory because they were put off by its imagery. Now - helped by new imagery - they are readily absorbing, in a piecemeal way, many details of its science. But often, the larger and more significant shift of thought behind it still eludes them. As Lovelock writes: "In the 35 years of Gaia's existence as a theory, the view of the earth has changed profoundly. Yet, so far, only a tiny minority of scientists realise how much Gaia theory has helped to change their view. They have adopted my radical view of the earth without recognising where it came from, and they have forgotten the scorn with which most of them first greeted the idea of a self-regulating earth."

Why has this happened? For a start, modern specialisation does not easily cater for the idea of the earth as a whole; scientists, naturally, do not welcome the idea of having to read one another's journals. More seriously, however, holistic explanation, which works by looking outward, is the reverse of the reductionist kind, which looks inward for causation by particles. It seems obvious that both kinds are needed, yet recent fashion in science has leaned strongly towards using the second alone.

The imagery of Gaia, implying an enclosing, structured earth, counters atomistic, individualistic, reductionistic models, such as the selfish gene, which have dominated recent discussion. Thus, when serious discussion of Gaia theory began, biologists devoted to these models became its most excited opponents, arguing that the co-operation needed for a self-sustaining earth could not possibly be combined with the competition necessary for natural selection. Gaian thinkers have countered that these processes do not clash because they occur at different levels. Organisms that improve the growing conditions around them help themselves as well as others, so they can still compete with those others when they need to. Life is not a zero-sum game at the biological level, any more than it is in human society.

It will take a long time to work out the scientific consequences of this new "systems perspective". But the imaginative issues are already emerging. The neo-Darwinian biologists who established the strident imagery of selfish genes 20 years ago did not think of it as ideological. They see their metaphors as a kind of ornamental paint added to natural science, and they regard it as pure chance that their books came out at the height of the Reagan-Thatcher era, when selfishness had come back into fashion and celebrations of it were particularly welcome. Since then, however, it has emerged that the faults which their critics found with the science of these books are indeed linked to the faulty ideology of that epoch. In both spheres, individualism was out of control, forging an unbalanced, one-sided myth. Both Gaian science and the holistic Gaian myth work to correct that imbalance.

This new approach in no way undermines Darwin's true message. It is simply the next necessary development beyond it, a further horizon, which (as Lovelock points out) Darwin himself would have welcomed. In Homage to Gaia, a clear, sharp and often very funny book, Lovelock describes how he came to conceive this genuinely revolutionary idea, and how he has carried it through - from the time when, working for Nasa, he first noticed how the earth's lively atmosphere differs from the dead, static envelopes of the other planets, right up to the present day.

An independent scientist, not just in funding himself, but in his way of judging, Lovelock played a crucial part in launching the "green" movement by inventing the instruments that made atmospheric pollution detectable, and he has deeply influenced its thinking. Yet he has constantly told the movement's leaders how badly they are getting things wrong, and how they had better get them right. At the age of 80, Lovelock writes as well as ever, and shows little sign of slowing down. It is to be hoped that he will stay with us as long as we need him.

Mary Midgley is a writer and philosopher

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