The notion of Mr Darcy, of “one day my knight will come”, seems to be hard-wired into women. Strong and powerful, he gallops out of the forest in shining armour and offers us true love. We know that it’s absurd but still we hope and wait – sometimes, alas, too long for our own good. The tall, dark, handsome stranger is the fortune teller’s stock in trade: we read our horoscope hoping to find the demon lover there, no matter that we’re long married with kids. Mills & Boon knows it and profits by it and gives us what we want. Romance.

And Then He Kissed Me, its centenary exhibition, starting this week at the Manchester Library before going on a national tour, serves as a guide to the changing patterns of courtship through the decades. The covers alone are illuminating. Fashions in drawing change, from 1922’s Fugitive Millionaire by way of 1980’s Lucifer’s Angel, to Ruthless Boss, Hired Wife in 2008. Quality may vary – none perhaps rivals what Joyce Dennys did for The Fortune Hunters in 1922.

It was so strong and striking that she was able to leave out the man altogether – those pouting cupid lips and provocative glance being quite enough without, thank you. Joyce Dennys started her career designing propaganda in the First World War (some of her drawings are in the Royal Naval Museum in Portsmouth), and her work stands on the borderline of what counts as art, and what is simply advertising, or at any rate “persuasive illustration”.

But, apart from The Fortune Hunters, everywhere is the clean-cut, prosperous man, the radiant girl, the glance between them that declares true love. Respectable love at that – even in today’s Ruthless Boss, Hired Wife at least we know from the title that there’s been a wedding ceremony. Without the title it looks like Becks and Vicky on a beach.

The pull of romance is the pull of evolution: the common understanding that romance leads to weddings, and weddings lead to socially sanctioned babies. Even though in real life we deny it, fly to the contraceptive Pill or even the abortionist, the need to procreate is ingrained in the species, which is why, I reckon, the jackets have such appeal and the exhibition will be so popular. Why Mr Darcy is the archetype, why we so love Pride and Prejudice – rich, powerful man marries humble Elizabeth, with whom we identify. Not so pretty as her sister Jane, but lively and quirky. Well, we can all be that.

Mills & Boon celebrates that other “right to choose” – the young girl’s ability to choose a mate, at the one and only time in her life when young men queue up and she does the picking and choosing. But what is going on in Utility Wedding (1946)? Is this the body language of the tormented man, back from the war? It looks like it. Behind, in the houses, lies the possibility of domestic bliss, but will she say yes? In her eye we read: “Do I mean to put up with him and his neuroses, or shall I say no?” For once, doubt and emotional distress enter in. But mostly what comes next, true and lasting love, is never in doubt.

So see on these jackets – Sweet Destiny, Maiden Flight, Choose the One You’ll Marry – a promise of evolutionary bliss, the union of the fittest and most beautiful, and the understanding of bouncing babies to come. It is through romance that the species improves itself. What we do when we “fall in love” is serve the Darwinian impulse to breed for brains and beauty. The love hormone oxytocin floods our system, our eyes shine, our skin glows, judgment fails and simple adoration takes its place, as we move on automatic pilot towards a species upgrade. The girls towards the highest status, the boys towards the slimmest waist. (Alas, research proves it.) Our body melts into his . . . even in this year’s Safe in His Arms. A likely tale.

Notice that the older the books are – certainly those from the pre1960s, before the Pill arrived – the more “romantic” they seem to the viewer. The demurer the skirt, the tweedier the jacket, the greater the thrill of the forbidden. The longer the time between the first glance of love and actual sexual congress, the more powerful the erotic tension suggested. The lovers in On-Call Sister, in 1958, will move slowly, after initial ructions, from engagement ring to wedding to home to babies. In 2008’s The Reckoning it looks as if it will be a mere ten minutes before they’re at it. On-Call Sister wins the erotic stakes any time, with his anger, her defiance, their sizzle – though no one’s even touching.

It’s a delicate balance: today’s girl still has to judge, as Elizabeth Bennett did once, whether it’s better to say yes to dull Mr Collins or wait for glamorous Mr Darcy to come along. To hold out too long may mean missing the boat and joining the ranks of the singletons. On the other hand to settle for a baker’s boy today might be to lose a banker tomorrow: compare The Fugitive Millionaire, 1922, and Utility Wedding, 1944, with 2008’s The Billionaire Boss’s Secretary Bride. What changed?

With the years the millionaire may turn in to a billionaire, the girl’s arms get more anorexic, sex may happen before marriage, and the feisty heroine replaces the modest girl, but the story and the advice remain the same. Holding out is best. Elizabeth, such is the Mills & Boon promise, will meet her Mr Darcy, if only just in time.

But that’s the world of Mills & Boon. Many a woman today opts out of the whole messy business, resolves never to commit, never to have children, and takes pleasure in the company of friends and family. It’s a perfectly decent option. And then, of course, he comes along.

And then he kissed her… 100 years of Mills & Boon, Manchester Central Library, from Friday, until July 31 2008, then touring. A book, The Art of Romance: Harlequin Mills & Boon Cover Designs, will be published by Prestel in August at £14.99. Fay Weldon’s novel The Spa Decameron is published by Quercus at £7.99.

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