Estelle Morris, so far as I can discover, is the first ministerial head of the education department to resign sinceSir Charles Trevelyanin 1931.
Joan Lestor resigned from Harold Wilson's government in 1976 over education cuts, but she was an under-secretary in the department. Lord Longford, in 1968, also resigned from a Wilson government, because of a delay in raising the school-leaving age, but he did so as Lord Privy Seal.
So we must go back more than 70 years to find a true precedent for Morris's decision. Trevelyan - a sort of early 20th century Tony Benn, who went from a solid establishment background to passionate socialism - resigned when his Education Bill, which would have raised the school-leaving age to 15, fell in the Lords.
The Labour Prime Minister Ramsay MacDonald, he felt, had given him insufficient support. But his disgruntlement went wider. "In the present disastrous condition of trade," he wrote in his resignation letter, "it seems to me that the crisis requires big Socialist measures". (Treve-lyan was an old hand at principled resignation; he had left Asquith's Liberal government in 1914 when Britain declared war.) Even then, ministerial office was daunting. When Trevelyan first got the job, his father, George Trevelyan, advised him that to be the one man in a great office who has to make the decisions "is a most bewildering business".
George's own father (and Charles's grandfather) "said that in his first fortnight at the Treasury he saw the snakes coming out of the papers in his dreams at night".
But that was the Treasury; I doubt that education ministers in the 1930s, or even the 1960s, suffered the sleepless nights that reportedly afflicted Morris. No government department has undergone so radical a change in its profile and responsibilities. Until the late 1970s education was mainly for political has-beens, time-servers and lightweights. Save for Anthony Crosland and one or two others, even the exceptions tended to prove the rule. In the wartime coalition, RA Butler was sent to education because it seemed a safe hideaway for a pre-war appeaser.
In 1970, Edward Heath sent Margaret Thatcher to education because he was advised to have one woman in his cabinet, not because he rated her. Thatcher herself seemed to doubt that it was a job for a serious person.
During a 1983 election campaign press conference, Max Wilkinson, a former education correspondent who was by then a Financial Times political pundit, rose to ask a question. The following exchange took place. Thatcher: "Ah, Mr Wilkinson, you've come a long way since education." Wilkinson: "So have you, Mrs Thatcher."
We all know what has changed. Education has been nationalised. It has not yet become as great a distributor of life chances as inherited wealth, but politicians (who all now profess a belief in meritocracy) aspire to make it so.
It has already become an enormous consumer of public money, having long ago left defence behind. And above all, politicians need things to meddle with.
They have lost significant overseas influence (though you wouldn't believe it to listen to Tony Blair); their powers over the economy are constrained by global markets; they have stepped back from labour disputes and private industry; they are still too frightened of doctors to plunge wholeheartedly into the NHS.
MacDonald, Churchill and Macmillan took only a passing interest in education; but now, Blair virtually has a shadow education department in Downing Street.
There is poetic justice in Morris's departure; the pressures of all those targets and all that scrutiny, which have driven so many teachers out of the profession, eventually proved too much for the minister.
But for a truly red-blooded politician, education, with its weak and divided unions, castrated local authorities and compliant workforce, is the best train set around. No wonder Charles Clarke couldn't wait to get his hands on it.
Peter Wilby is editor of the New Statesman