"Some take the Venice Biennale very seriously indeed; others regard it as the Eurovision Song Contest of Art."

Every two years more than a thousand artists, museum directors, art dealers, press officers, political activists, millionaires, alcoholics, art critics and gatecrashers - in other words that uncertain group known corporately as the international art community - gather to celebrate the world's oldest festival of contemporary art in the world's most conservative city.

Some people take the Venice Biennale very seriously indeed, while others regard it as the Eurovision Song Contest of art exhibitions. But, given that it takes place in Venice, in June, more or less everyone has a good time - and that is probably the chief reason for the survival of this peculiar and intriguing cultural anachronism.

Last week saw the opening of the 46th Biennale, a hundred years after the first. Although generalisations about the work of scores of different artists from scores of different countries are, generally speaking, unwise, it may be said that a mood of morbidity and melancholia hangs over much of the art on display.

The largest single exhibition of this year's Biennale, which has been organised by a French curator of great energy and ambition called Jean Clair, is a show somewhat ponderously titled Identity and Otherness. According to the official Biennale press release, written in officially opaque Biennale English, its theme is "the possibility of finding the human identity and the conditions that make it reachable". In fact it is an exhibition without a theme, unless something as hubristically broad as life in the 20th century can be described as one.

The scope of Clair's show is so large that it is less like a temporary exhibition than a temporarily convened museum of modern art. Running from the end of the19th century to the present day, including painting and sculpture and many other of the potential media of modern art, it fills the Palazzo Grassi on the Grand Canal and the Museo Correr in St Mark's Square before spilling over into the Italian Pavilion in the Giardini di Costello. Its chief faults, which are a mania for inclusiveness and a habit of inscrutably placing extremely unlike objects next to one another, are partially compensated for by the remarkably high quality of many of the works of art which it contains. This is a curious and sometimes inspired collage of works which is constantly lapsing into incoherence.

Reflecting on the century of atrocities, Clair has included many artists' responses to the First and Second World Wars, among them Jacob Epstein's famous mutant robotic soldier and Otto Dix's scabrous, seething heads of the dead, dying and wounded, as well as some surprisingly vivid studies of the disfigured faces of infantrymen painted by Sir Henry Tonks. Then, turning introspective on the subject of introspection, Clair has included memorable suites of self-portraits by Giorgio de Chirico, Max Beckman and Pierre Bonnard. And so the haphazard pattern of the show goes. Bonnard for his part, painting himself like a frail owl in his bathroom mirror amid the yellow and purple mists of his South of France, seems to be wondering what on earth he is doing in this company.

The question is rather difficult to answer, because the overall, declared aim of the show - to prove that the 20th century has seen much remarkable art focussed on the human face and form - seems almost too banal to be true. Identity and Otherness is accompanied by a violent, blustering rhetoric which aims to disguise commonplace thoughts as revelations. This exhibition, we are given to believe, is meant to shake us out of the complacent received opinion that all modern painting stems from Cezanne and tends to abstraction. But it is preaching to the converted; that old, evolutionary myth of modernism which Clair is so keen first to parody and then refute was discredited in most quarters long ago.

Identity and Otherness is nevertheless the show that steals the show at this year's Biennale. The trouble is that it amounts to less than the sum of its parts, and although it bleeds into the present, the exhibition is fundamentally a historical survey. Its dominance gives the whole Biennale itself a slightly benighted air.

The national pavilions in the Giardini di Costello, where the works of living artists are gathered - and the work of living artists is supposed to be the raison d'etre of the Biennale - feel somehow like afterthoughts to Monsieur Clair's impresario musings.The decision to do away with the Aperto section of the Biennale, where relatively inexperienced artists used to be able to show their work, enhances the sense that this is a Biennale feeling its old age as much as celebrating it.

Several of the exhibitions in the national pavilions feel uneasily, themselves, like afterthoughts. The Russian pavilion looks like a footnote to the Russian pavilion of the last Biennale, when Ilya Kabakov mockingly turned the place into a building site. This year, three Russian artists have collaborated on another exercise in knowing national self-humiliation. Their exhibition is made up of sad bits and pieces, scraps of Russian pornographic magazines tacked to pinboards and melancholy snippets of archive film projected on to a bare wall. The show culminates in a room containing nothing but a collections box sardonically labelled with the information that all donations will be gratefully received.

The Japanese pavilion, for by no means the first time, seems rather too much like a demonstration of the potential applications of Japanese technology to the visual arts. Swathed in multi-coloured plastic, its basement contains vast quantities of television sets on which are broadcast, variously, images of human skin and hair shot in extreme closeup. A film upstairs exhibits interesting new possibilities in the reordering of computer- generated imagery. The images themselves, of course, are of no interest at all.

There is a lot of "technologically assisted" work, as the phrase has it, in this year's Biennale. Bill Viola's, in the American pavilion, rises most effectively above the unpromising technology and makes of it simply a tool for art. His Greeting is a homage to Florentine cinquecento painting shot on video, a slow motion film on the theme of the Virgin greeting Saint Anne which might sound perverse but is genuinely entrancing. Three ordinary people, shot on a stageset that recalls the ideal cities painted by Raphael, have been made to seem numinous through slow-motion. A smile and a touching of hands, extended over five minutes, become holy gestures.

Elsewhere, the work exhibited in the Giardini amounts to the usual extremely uneven conspectus of art today, ranging from the good, bad and indifferent to the hopelessly dated and the plain hopeless. In the last category, which is as usual and inevitably the strongest, Cesar's crushingly banal contribution - it includes several tons of cars cubed for scrap and piled up like haybales - just about wins the palm for conspicuous waste of resources.

Cesar manages to make Leon Kossoff, whose clotted North London cityscapes and portraits hang in the British pavilion next door, look positively economical with materials. Kossoff's work is uneasily suspended between the turgid and the energetic, and although his are plainly the paintings of a serious, intent man, he often sees so keen to prove that they were painted with feeling that he leaves the viewer doubting it. Many of his recent pictures are so covered with marks that conventionally indicate stress or excitement that the technique can look awkwardly like a form of mannerism. Occasionally, something like a sense of epiphany struggles through the heaviness of the paint.

A widespread sense of morbidity among the exhibiting artists is reflected in the Biennale's body count. Dead bodies, rotting flesh, open wounds and spilled guts are more or less everywhere - although much more often in film or photography than in painting, of which this Biennale contains relatively little. Several artists exhibit photographs taken in morgues or hospital operating rooms. Jake and Dinos Chapman make a good stab at stealing the young British artists' show, General Release with a spectacularly repugnant life-size reconstruction of a Goya Disasters of War print made from shop window mannequins. Mona Hatoum, invited to show in the Italian pavilion, contributes an intriguing self-portrait in the form of an extremely intimate and impersonal film of various parts of her body, made with the assistance of a microscopic surgical camera.

There are a few gentler deaths and gentler ponderings of the theme of mortality besides. The Danish pavilion is given over to the unpredictably touching works of John Olsen, who makes drawings and assemblages out of the found skeletons of dead animals and creates sculptures that look like huge beached bones or shells, without in the least recalling the work of Henry Moore. But much more often the bodies seem victimised, the emblems of a paranoid or miserable sensibility. The Venezuelan pavilion is exemplary, containing a series of portrait photographs manipulated so as to wall up the orifices of the face with solid flesh: horrible dreams of a living death, dreamed up by Sammy Cucher and Anthony Aziz.

There are those who attribute the obsession with twisted, damaged bodies to the AIDS epidemic, and those who see it as a natural metaphor for concern about the earth and what we have done to it. There are others again who find in the trend a deeper and more eschatological morbidity - a natural preoccupation for those living at the bloody end of a bloody century. The real reasons may be altogether less high-sounding.

Perhaps the gloomy preoccupations of one fin de siecle have been mysteriously inherited by another. But the angst of late 20th-century artists seems, in all but a few cases, more sentimental, more self-pitying, more callous, more facilely sensationalist and more lazily expressed than that of their late 19th-century counterparts. But that might just be my own fin de siecle gloominess talking.