Watson was anything but happy, however. Indeed, according to most people at the event, he looked completely shattered. The scientist who, with Francis Crick, had discovered the structure of DNA and revolutionised modern biology, and who is revered as one of the greatest scientists of his day, was shaking badly. He rambled, paused and then rambled again as he talked to friends. Finally he produced a prepared statement and began reading it out to the assembled journalists.

He could only apologise 'unreservedly', the statement said, for his assertion, published last Sunday, that he thought black people were less intelligent than white. 'This is not what I meant.'

Then Watson hesitated, returned to his script and finally wandered off his topic completely to end up, bizarrely, describing the behaviour of the San bushmen of Botswana. 'It was a tragic sight,' said one guest.

It is not hard to understand why Watson was so distraught, of course. His remarks, which by Wednesday had made front-page headlines across Britain, had attracted a fusillade of abuse from scientists, politicians and equality campaigners. 'It is a shame that a man with a record of scientific distinction should see his work overshadowed by his own irrational prejudices,' said David Lammy, the Skills Minister. London mayor Ken Livingstone fulminated in a similar manner: 'Such ignorant comments...are utterly offensive and give succour to the most backward in our society.'

Worse was to follow. The Science Museum cancelled a sell-out meeting it had planned to hold to honour 79-year-old Watson on the grounds that his remarks had gone 'beyond the point of acceptable debate'. Several other centres scheduled to host his talks followed suit.

After the Royal Society meeting, Watson and his wife returned to their hotel, Claridge's, where they learned that the scientist's employers, the Cold Spring Harbor Laboratory in Long Island, had just disowned his remarks. The next morning, it emerged the laboratory had also suspended Watson as its chancellor. Enough was enough, the scientist announced. 'I am going home to try to save my job.' By Friday afternoon he was back in New York and was heading for Cold Spring Harbor. His chances of remaining chancellor there look slim.

It is, by any standards, a depressing story. One of the world's most distinguished scientists has fled from the country, his reputation and his academic future left in tatters. As a tale of intellectual hubris, it is hard to beat.

But the fall of Jim Watson goes beyond mere personal tragedy. It also raises key issues of immediate concern. The first is simple: is there any evidence there are major differences in the intellectual potential of races? The second is more complex: how should we react when a scientist of Watson's standing makes such provocative remarks?

In the first case, most scientists have quickly jumped into the furore to dismantle the idea that significant intellectual differences exist between Africans and others. 'Defining intelligence is complex and there are many forms of intelligence, not all of which are captured by IQ tests,' said the Oxford neurologist Colin Blakemore. 'In any case, it would be as unethical to organise society around some numerical indicator of difference as it would to do so on the basis of skin colour.'

Other scientists point out that our species is so young - Homo sapiens emerged from its African homeland only 100,000 years ago - that it simply has not had time to evolve any significant differences in intellectual capacity as its various groups of people have spread round the globe and settled in different regions. Only the most superficial differences - notably skin colour - separate the world's different population groupings. Underneath that skin, people are remarkably alike.

This argument does not reject the idea that notable variations in intellect exist between individuals, but it stresses that these differences exist within racial groups, not between them. Judging a man or woman by the colour of their skin will get you nowhere, in other words. As Craig Venter, who pioneered much of America's work in decoding the human genome, put it: 'There is no basis in scientific fact or in the human gene code for the notion that skin colour will be predictive of intelligence.'

The second issue raised by his claims is far more vexed. How should we react to claims such as those made by Watson? Lammy, Livingstone and other politicians urged he should be silenced on the grounds that his views would only give succour to Britain's racist fringe - as indeed they have. By Thursday, as Watson was making his mumbled Royal Society apology, he was being hailed as the New Galileo by the BNP. Those who objected so virulently to the science behind racism were 'simply denying the facts of science and stand in the same position as those Catholic theologians who offered Galileo the choice of recantation or the stake', claimed a BNP website article.

Certainly Watson was extraordinarily naive. But was it right to cancel public meetings at which he could be called to account for his views? Senior staff at the Science Museum in London clearly thought so, as they did at the Bristol Cultural Development Department Partnership, which was set to host a public meeting with Watson this week. They, too, decided not to hold their meeting on the grounds that the scientist's views were 'unacceptably provocative'.

Not every centre scheduled to host public meetings with Watson took this view, however. The Centre for Life in Newcastle said it would go ahead with its meeting, scheduled to have been held today, on the grounds that it would provide the public with the chance to question the scientist and then make up its own mind about his claims. 'We had some calls expressing misgivings about our decision to welcome Watson, but most people supported us. We were going to give him a robust but fair hearing and let people decide for themselves,' said a spokesman.

In the end, Watson's decided to return home, so no meetings occurred, a move that has dismayed many scientists who believed that it was vital Watson confront his critics and his public. 'What is ethically wrong is the hounding, by what can only be described as an illiberal and intolerant "thought police", of one of the most distinguished scientists of our time, out of the Science Museum, and maybe out of the laboratory that he has devoted much of his life to, building up a world-class reputation,' said Richard Dawkins, who been due to conduct a public interview with Watson this week in Oxford.

Dawkins's stance was supported by Blakemore. 'Jim Watson is well known for being provocative and politically incorrect. But it would be a sad world if such a distinguished scientist was silenced because of his more unpalatable views.'

This last remark goes to the heart of the issue. Watson is renowned for his controversial views. He sees himself as a free-thinker, though it must be admitted his ideas often simply seem eccentric. In 1997, he suggested it would be acceptable to terminate a foetus if it carried a gene that might mean the adult that grows from it was gay. He has also suggested a link between sunlight and libido. 'That is why you have Latin lovers. You've never heard of an English lover,' he said. And Watson has also proposed that a foetus destined to be 'stupid' should be aborted.

Such maverick remarks led the journal Science to conclude, in 1990, that 'to many in the scientific community, Watson has long been something of a wild man, and his colleagues tend to hold their collective breath whenever he veers from the script'. This track record explains one intriguing feature of the Watson affair. Although the Sunday Times carried the interview in which the scientist outlined his disparaging views about black people, the paper kept his remarks buried in its colour magazine.

A story was offered to the Sunday Times newsdesk by magazine staff, but was declined on the grounds that Watson had said such things in the past, as indeed he had. Thus it was left to Simon Kelner, editor of the Independent, to take Watson's claims and to run them as its lead story on Wednesday, under the banner: 'Africans are less intelligent that Westerners, says DNA pioneer'. In this way, Watson's fate was sealed.

But while it is obvious that the scientist holds some fairly illiberal views on certain issues, it is also clear he has acted in a courageous, progressive manner on other issues. When he was director of America's human genome project, he fought bitterly to prevent the US government from adopting a policy that would allow it to patent human genetic material discovered by project scientists in order to exploit them as sources of new drugs and medicines. The idea was 'lunacy', said Watson, the equivalent of applying for ownership of the laws of nature. When he realised he was losing the battle, he simply handed in his resignation.

Nor is it at all clear that Watson is a racist, a point stressed last week by the Pulitzer-winning biologist E O Wilson, of Harvard University. In his autobiography, Naturalist, Wilson originally described Watson, fresh from his Nobel success, arriving at Harvard's biology department and 'radiating contempt' for the rest of the staff. He was 'the most unpleasant human being I had ever met,' Wilson recalled. 'Having risen to fame at an early age, [he] became the Caligula of biology. He was given licence to say anything that came into his mind and expected to be taken seriously. And unfortunately he did so, with casual and brutal offhandedness.'

That is a fairly grim description, to say the least. However, there is a twist. There has been a rapprochement. 'We have become firm friends,' Wilson told The Observer last week. 'Today we are the two grand old men of biology in America and get on really well. I certainly don't see him as a Caligula figure any more. I have come to see him as a very intelligent, straight, honest individual. Of course, he would never get a job as a diplomat in the State Department. He is just too outspoken. But one thing I am absolutely sure of is that he is not a racist. I am shocked at what has happened to him.'

As to Watson's own prospects today, those can only be described as unpromising. Cold Spring Harbor Laboratory's campus is set in rolling wooded hills on the north shore of Long Island, dotted with buildings set amid the trees turning bright orange and red as autumn sets in.

Executives there have already revealed just how seriously they are taking the Watson case and on Friday triggered a compete PR lockdown at the campus: students and staff were warned not to talk to journalists, security guards were on the lookout for unannounced reporters and interlopers were picked up and taken to the campus PR department.

However, some students and staff did speak and expressed surprise at Watson's remarks. 'It is all a bit of a shock. I suppose it is possible that he was misquoted, but it is still shocking. The whole thing is just amazing that anyone would say something like that,' said Louis Nguyen, a graduate student taking courses on the campus and whose college is affiliated with Cold Spring. Some members of staff were also upset at the furore surrounding Watson.

'I agree with what the college has done so far in suspending Watson. They acted well. It may be that he has to lose his job. That is a really serious thing to say,' said Jean Wang, a science technician at the college.

Responsibility for dealing with the PR crisis fell to Jim Bono. On Friday, the director of the college's public affairs unit acknowledged there had been a massive interest in the incident. 'We are getting phone calls from all over the planet,' he said. Given the nature of Watson's comments, that was not surprising.

However, Bono would not be drawn on a timetable for further action or the outcome of the board's deliberations on Watson's future. 'The leadership [of Cold Spring] continues [its] debates. The leadership has been working around the clock and has been deeply involved in dealing with this situation,' he said.

It sounds ominous. Nevertheless, the laboratory's behaviour has been supported in the local Long Island press. America is facing a spate of nooses being left in public places - including in Long Island: the noose is a symbol of anti-black feeling and intended to rouse memories of lynching. The prospects of a return to such extremism alarms authorities and commentators alike.

Hence a thundering editorial in Newsday, Long Island's main newspaper: 'The lab condemned its chancellor and key fundraiser, but that statement and Watson's apology yesterday will probably not be enough. He has to go.'

Thus Watson - who today should have been accepting a warm welcome from his public at Newcastle's Centre for Life - now finds himself at home, alone and in trouble. His reputation as a brilliant, radical thinker has been transformed in a few days into that of an unstable maverick. The stain on his career is unlikely ever to be washed clean.

James Dewey Watson - The CV

Born in Chicago, Illinois, on 6 April 1928, Watson was a precocious child who gained a place at Chicago University aged just 15. He graduated with a zoology degree in 1947.

In October 1951 he arrived at the Cavendish Laboratory, Cambridge, to carry out genetics research and met Francis Crick. They struck up a friendship that became one of the most famous partnerships in science and led them, in 1953, to the discovery that DNA, from which human genes are made, has a double helix structure.

In 1962, the pair - with Maurice Wilkins, of King's College London - were awarded the Nobel Prize for Medicine for this work. Six years later Watson published his racy account of the DNA story in The Double Helix. Dropped by its original publisher amid objections by Crick, Wilkins and others, its opening sentence was: 'I have never seen Francis Crick in a modest mood.' It became a bestseller and the basis of the 1987 TV film Life Story, in which Jeff Goldblum played Watson and Tim Pigott-Smith Crick.

In 1988 Watson was made head of the US Human Genome Project, America's arm of the international programme for sequencing human DNA. Four years later he resigned over US plans to try to take out patents on gene sequences discovered by US scientists, saying 'the human genome belongs to the world's people'.

He was suspended last week as Chancellor of Cold Spring Harbor Laboratory, Long Island.

The week that ruined Watson's reputation

14 October The Sunday Times Magazine publishes an interview with Dr James Watson. He says he is 'inherently gloomy about the prospect of Africa' because 'all our social policies are based on the fact that their intelligence is the same as ours - whereas all the testing says not really'.

His hope is that everyone is equal but says that 'people who have to deal with black employees find this is not true'.

17 October The front page of the Independent runs a story on Watson's theory with the headline 'Africans are less intelligent than Westerners says DNA pioneer'. It creates a furore.

18 October The Science Museum cancels Watson's talk planned for 19 October. In a statement, the museum says 'James Watson's recent comments have gone beyond the point of acceptable debate'.

At a launch for his book at the Royal Society in London, Watson withdraws the words attributed to him: 'To all those who have drawn the inference from my words that Africa, as a continent, is somehow genetically inferior, I can only apologise unreservedly.'

19 October Watson seeks to justify his theory that there is a genetic basis behind differences in IQ in an interview with the Independent.

20 October Watson is forced to return to New York after his employers at Cold Spring Harbor Laboratory, Long Island, suspend him from his duties because of his apparent views.