Shock of the old

If Modernism is the driving force in the visual arts this century, it's appropriate that Berlin should play host to its first great survey. The city is a powerhouse of renewal, a building site of collaged styles, seeking to make a new identity out of past division. "The Age of Modernism: Art in the 20th Century" includes over 400 works of art by 130 artists and is staged in Berlin's monumental Martin-Gropius-Bau. Here is the history of Modernism from 1907 to the present day, as interpreted by the exhibition's organisers Norman Rosenthal and Christos Joachimides. This month, Rosenthal celebrates 20 years as Exhibitions Secretary at the Royal Academy in London, and he has known and worked with Joachimides for even longer. This epic exhibition was supposed to come to London, but was prevented by last-minute uncertainties. It's our loss.

The show is divided into four sections, vast themes with fluctuating boundaries, which serve to group the works only roughly but allow for fascinating and fruitful juxtapositions. Each in itself could be a separate exhibition: the scale of this visual and intellectual feast is awe-inspiring. The opening theme is Reality-Distortion, in which Picasso and Matisse set the pace. Brancusi edges in beside the Cubists and the Futurists. The Expressionist maenads of Emil Nolde greet Kirchner's marvellous high viewpoint depiction of the Berlin Victory monument, watched sardonically, sorrowfully, by Wilhelm Lehmbruck's Head of a Thinker. Then around the corner comes the "recall to order" of the 1920s and 1930s, and one of the surprises of the exhibition: Alexander Deineka. Seen from a distance, his painting of athletes on a running track is reminiscent of Muybridge and Bacon, and it's no surprise to discover that Deineka (1899-1969) worked for a while as a photographer. He also kept alight the modernist flame under Stalin: we should see more of his work.

This kind of exhibition has perhaps a duty to the public to show masterpieces, but the side paths of the avant-garde also yield treasures. For instance, Jean Fautrier, whose paintings are rarely shown in Britain. Four are here, all sensitive colour, texture, massy presence. Also, within this first section, is another self-contained show. This "Portrait Gallery" includes that fantastic and horrifying last Picasso self-portrait. Matisse's powerful depiction of Sarah Stein, and a strong British contingent including Freud, Hockney and Kitaj.

The second section, Language-Material, is dominated by Duchamp, with a wall of exquisite Schwitters collages, and a group of similarly high quality Ernst paintings. Dada takes us through to Pop, to Rauschenberg, Johns and Warhol. The Duchampian tendency overwhelms the exhibition, continuing today through the work of Polke, Richter and Bruce Nauman (irritatingly pretending to be a bluebottle on video). One of the most satisfyingly poetic works in this section is Gary Hill's tape-loop of a breaking wave.

Upstairs is Abstraction-Spirituality and the most startlingly beautiful Kandinsky paintings; particularly Composition VII from Moscow. Moving from that to the three minimal black-and-white abstractions by Malevich (a square, a circle, a cross), is an emotional as well as an aesthetic rite of passage. The credit for developing abstraction is placed firmly with the Russians, and when you look at Olga Rozanova's Green Stripe and then check its date (1917-18), you see why; it anticipates Barnett Newman by 30 years. The fourth and final section is called Dream-Myth and includes de Chirico and Magritte, Moore and Balthus, Hopper and Kieter. Of the many excellent things, a wall of Morandi still-lifes stands out in the memory.

It's a stupendous presentation, but is it coherent? The exhibition is not signed as clearly as it might be, and it's not always easy to follow its argument. But on first viewing that doesn't particularly matter, for the sheer density of key works of art justifies a gradual approach. The visitor should be prepared to go back, to ponder, to question, to join the debate. "People think of exhibitions as bibles," says Rosenthal, "they're not, they're essays." The show is not supposed to be a definitive statement, but it is certainly a provocative one. The intention is to present only those artists who have "changed the agenda". Did Robert Delaunay, for instance, alter the course of Modernism? His Eiffel Tower pictures are an impressive application of Cubism, but did they really exert more influence than Juan Gris's entire oeuvre, none of which is featured in the show? Was Oskar Schlemmer of such importance? However much one admires Chillida and Stanley Spencer, is their contribution of this order? There are four paintings by Wols, who always looks minor next to Twombly. There's only one by the great Arshile Gorky. And so on.

Part of the exhibition's effect comes through confrontation. The innovations of Modernism relied upon their shock value to propel them to a central place in our consciousness. But by now many of these works of art have become familiar to us, icons even, like Duchamp's urinal. Their shock value can only be recaptured through a change of context. In some cases, new and unexpected affinities emerge. Bacon's Study for Portrait of van Gogh VI is daringly hung with three large Baselitz paintings; de Kooning is juxtaposed with Mondrian; Yves Klein sponge pictures are shown with Rothko stain paintings; Tatlin's weird flying machine hangs above a Joseph Beuys installation. These meetings have to be seen to understand just how enlivening they are, The art speaks together and then speaks to us in a new way.

The exhibition is to all intents and purposes a temporary Museum of Modernism. It's an unparalleled collection, to some extent defined by the loans which were available, but in the end it's invidious to complain of omissions or wrong inclusions. The achievement of bringing these works of art together is not only unique but unprecedented, and the only disappointment is that London missed out. Book your seats now for Berlinn

'The Age of Modernism: Art in the 20th Century' is at the Martin-Gropius-Bau, Stresemannstrasse 110, D-10963 Berlin (030 32 33 45 2), 10am-8pm daily. To 27 July

King of the shock tactic: Marcel Duchamp's 'Roue de Bicyclette' Hessisches Landesmuseum, Darmstadt