Middlemarch, an invitation: Tomorrow evening the BBC begins its six-part serialisation of George Eliot's novel. Here, the literary critic Claire Tomalin celebrates a book which grows greater with each reading

It is one of those books that expands and grow greater in its readers' minds with each reading. The tale of a country town, yes, but what makes it so remarkable? Its breadth of knowledge, to begin with; the range and fullness of its sympathies; its grasp of the way in which private and public things interact; its astonishing insight into human passions and behaviour, and its capacity to show them in detail and in action. Young readers are sometimes impatient with it. Perhaps you need to be as old as George Eliot was when she wrote it - she was in her mid-fifties and had not lived a sheltered life - to fully appreciate the way she animated her mixed society of young and old, aristocrats, clergymen, businessmen and labourers, wayward, diligent and corrupt men and women; or to appreciate her vision of the slow workings of seemingly insignificant choices over months, years and decades. When you have experienced these things for yourself, you see how good she is.

The picture of provincial England in Middlemarch is still in many ways valid. Life is built round committees on sanitary matters, housing and reorganised hospitals, in which hatred and distrust of foreigners and strangers is predominant and abroad is the right destination for anyone who earns disapproval; and in which self-righteous gossip is used to bring down the mighty. But it is also a place that tolerates a good deal of eccentricity, throws up good and disinterested men and women, and leaves them room to do their work.

Eliot was the great moralist of daily life; fortunately for her television adapter, there is nothing abstract here, everything is shown through a kaleidoscope of action in which the main characters meet, talk, exchange stories and work upon one another. This is intrinsically dramatic, as the narrative flows from one crucial and passionate exchange to another. The drama will be easier to handle than the changes of tone. For instance, in the first chapters we seem to be squarely in the world of Jane Austen - two sisters with contrasted characters: Dorothea, the elder, all passion and idealism; Celia, the pragmatist, content with her lot. The conversation in which they discuss their late mother's jewels could be straight from Sense and Sensibility. So could Dorothea's dismissal of her first suitor, a baronet whose land lies with hers; swiftly followed by his discovery, engineered by his mother, that Celia will suit him better. So too could Celia's wifely statement that 'of course men know best about everything, except what women know better'.

In fact this first part of the book began as a separate short story; later the book comes closer to Ibsen. Eliot's impatience with the contemporary view of female capacities runs through the whole book like a leitmotif. Dorothea's discontent with 'that toybox history of the world adapted to young ladies which had made the chief part of her education' is the main reason she sees the dried-up scholar Casaubon as an ideal husband, because he can give her the education she has been denied. From him she will learn languages, history, art. The doors that have been shut on her will be opened. Or so she believes.

Eliot does not need to tell us that Dorothea's ignorance extends to sex, in which Casaubon is also to be her tutor. Most Victorian novelists were enraged by the restrictions on discussing such things, but Eliot simply leapt over them, informing us of Casaubon's failure through a brilliant deployment of language and imagery. We are told of his private feeling that 'the poets had exaggerated the force of masculine passion'; and shown Dorothea on her honeymoon in Rome, looking like a nun and with tears on her face, as she stands alone in a great museum next to the 'pale and glowing' statue of a reclining Cleopatra. Her husband has not led her into the 'spacious places' she had imagined but only into 'curious little winding passages leading nowhere'. Nothing more needs to be said: can television do as well, or as subtly? We shall see.

What it can do is follow Eliot in her shifting viewpoints. Where a lesser novelist would have let sympathy for Dorothea exclude sympathy for Casaubon, she suddenly lets us look through his eyes. Withered in spirit as he is, absurd in his hopeless labour to produce a 'Key to all Mythologies', he suffers too, fearing humiliation and doubting his own capacities. He even grows frightened of letting the enthusiastic Dorothea help him with his work, and begins to view her as a potential critic, a spy even, who will uncover the deficiencies in his scholarship. You can hardly help sympathising.

Dorothea commands the early pages of the book, but her story is very soon woven into others, the most important being that of Lydgate, the young doctor who comes to Middlemarch determined to pursue scientific research and make the place a centre of excellence in medical practice. Lydgate and Dorothea have a shared idealism, and give one another support. Are they, as is often said, the twin poles of the book? I am not so sure. Rosamond, the mayor of Middlemarch's daughter, who traps Lydgate into marriage, threatens to take over its centre with her display of the Victorian womanly virtues.

Rosamond is the nastiest female figure in 19th-century fiction - far worse than Becky Sharp and the rest - because she is passive, sweet-natured, correct, beautiful, and never raises her voice. When Lydgate calls her his 'little pot of basil' towards the end, he means it: she is flourishing on a dead man's brains, like the basil in Keats's poem, for she might as well have murdered her husband as do what she has done to subdue him to her will. Middlemarch does not see her like this; it is Dorothea who is labelled 'not a nice woman'. Eliot restores the balance by withholding from Rosamond any trace of the sympathy she allows every other character, giving the narrative a curious tilt, as though acknowledging that her type must always win, but that this author at least would not forgive her.

She applies a wonderful phrase to Dorothea as she looks beyond her personal distress to find comfort in awareness of 'the largeness of the world and the manifold wakings of man to labour and endurance'. Eliot valued and thoroughly understood the working world of men, even its least exalted aspects - the horse sales, committee meetings, elections and auctions which provide many of the best scenes in the book. Among its working people, Bulstrode, the most powerful businessman, who first stands above his fellow citizens and then is brought crashing down, is another tremendous character: his fall is terrifying and, through its effect on his simple wife, intensely moving.

Middlemarch moves steadily away from the brightness of the early part. Certain groups of characters - the good Garth family especially - are preserved in a glowing sentimentality, but the general tone darkens. The book is not usually spoken of as a tragedy, and yet those who emerge unscathed from its pages are those who have not aimed too high or risked too much, and the final presentation of Rosamond in triumph is as cynical as something out of Thomas Hardy. And Dorothea? Although she is not destroyed, as Lydgate is, and is left with a kindly word of approval from her creator, she remains unfulfilled, feeling 'there was something better she might have done, if she had only been better and known better'. But she must settle for romantic and domestic bliss. Is this a happy ending for her? No - it is only the standard Victorian one - and Eliot produces some awkward, apologetic words to cover her diminished state.

This is one of the problems of the book: how could she create a Dorothea and think she would accept this brushing aside of her ambitions? You want her, like Frankenstein's creature, to get up and shout her objections; then go off to found a woman's college, business or medical school. I shall be waiting curiously to see how Andrew Davies sees and settles Dorothea's fate.