Ronald Searle: Graphic Master, Cartoon Museum, London

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Scratch, scribble, scrawny, scruffy, scrape, scrawl – Searle's lines often begin with scr-, conveying an abrasive feeling. He is also a comedian, and can fill up a body like a bag of sugar, a helpless sagging blob that swells and slops and spreads. This mixture became his signature mode, the savage and the genial together, and it hasn't failed him in almost seven decades. His retrospective at the Cartoon Museum, "Ronald Searle: Graphic Master", marks his 90th birthday.

His beginnings were first as a juvenile cartoonist, then an art student, then a Japanese POW. He took pen and paper to the verge of death. His image of an emaciated man – Prisoner dying of Cholera, Thailand 1943 – is one of the greatest drawings of the 20th century. And it might have promised a career in serious art. But Searle refused to stick to "seriousness". His most famous works of the 1950s are his illustrations of St Trinian's and St Custard's, where pain is sublimated into a schoolchild's wicked naughtiness. How could the same hand do both?

Another artist might have decided limitation was compulsory; Searle pursued ever more, and ever more confident, variety. The range is amazing: invention, observation, lightness, darkness. Look at the grim and sober reportage of Eichmann's trial in Jerusalem, early 1960s. And then, pretty well simultaneously, there's a Punch cover; a picture of beautifully, innocently absurd birds, with stick-on fluff-ball popping eyes, and a man with a "lobster-claw" head, a long pointed nose one pincer, a long pointed chin the other.

People sometimes ask: can he or she draw? The implication is that there's a single secret. But Searle's performance shows how multiple the matter is. There are so many different ways of being accurate – and then, even more strangely, these different registers can interbreed. The Cartoon Museum is a small space. Its highlights are well chosen, and examples from the history of caricature are added, to show his ancestors. But ideally Searle's work would have a more complete survey, and with wider comparisons. Then we might wonder if there's any clear distinction between cartoon and art.

Meanwhile, we can talk about his diverse originalities. There are his wildly grotesque transformations of the human head and body, which have influenced Scarfe and Steadman. Or there are his smart conceptions, often in the form of art-about-art jokes. A Bigger Slash: Hommage à David Hockey has a row of scrawny naked gents, peeing up arcs of multi-coloured piss into a West Coast pool – an exact bit of visual-verbal wit. Or there's a genre that is almost unique to Searle's practice, caricatural documentary: the draughtsman-witness-journalist visits Las Vegas or East Berlin, takes scenes with his pen, acid but humorous. The sour face of the border guard can't quite conceal the smirk of Nigel Molesworth.

Searle can do so many things, and blend them. But if there is one thing that he does essentially, and all through, it is to animate. He fills his subjects with a life. He finds it and then exaggerates it. Keats, in a letter, writes: "I go among the Fields and catch a glimpse of a stoat or a field-mouse peeping out of the withered grass – the creature hath a purpose and its eyes are bright with it – I go amongst the buildings of a city and I see a Man hurrying along – to what? The creature hath a purpose and his eyes are bright with it."

This is Searle's vision – a counterweight, perhaps, to the early death he saw closely and narrowly escaped. He brings out the vitality, and not only in creatures, whether human or animal, but in furniture, cars, architecture. Whether it's a jalopy, a shark-finned gas-guzzler, a rickety shantytown or a cathedral, he finds the action in the shape. His world is gesture. His eyes are still bright with it.

To 4 July (020 7580 8155)