London,

Trail-blazer: Tom Thomson’s Jack Pine, 1916-17. Thomson was regarded by the Group of Seven as a free spirit, the embodiment of “Canadianism”

Were the new exhibition of paintings in the Dulwich Picture Gallery to be stripped of all sources of information, even the most culturally well-informed of visitors might find themselves confused. They would have no difficulty in dating these pictures to the later Tens and Twenties of the last century; they might suppose them to be Scottish, for some echo the intensity of Arthur Melville and Hornel's decorative reorganisations of reality; equally, some of these landscapes could be English and from the circle of Augustus John, Derwent Lees and James Innes. Against these comfortable conclusions, however, are paintings that seem to echo Van Gogh, Hodler, Signac and other looser Pointillists, paintings that could perhaps be German, Scandinavian or Russian, that might even be American - and in the chilling spectral landscapes of Lawren Harris one could be forgiven for seeing kinship with Walt Disney's much later Fantasia.

Without exception these are landscapes of a country in which vastness is a constant, even if all that can be seen are tree-trunks so close-to and dense that everything beyond them is reduced to colour, to a blue that could as well be water as a mountain range, to a yellow that is the sky of sunset. Trees laden with their autumn glory screen the distance but do not conceal it, and we sense what lies beyond; trees lean with the wind; trees lend architecture to the landscape and remind us of Monet. Here brave painters contend with wind and rain and with laden brushes trowel paint onto unprimed panels, leaving between the brushstrokes glimpses of the brown timber to lend a unifying tone to brilliant and contrasting colours that lie heavy and hard-pressed by every pig's whisker of the brush, or flick into being with fat but lightning touch the rush of the river's tumbling rapids. Everywhere paint is not only paint the medium, but is paint metamorphosed into leaf and log, rock and waterfall, and magicked into light and air, only occasionally and deliberately constrained by an imposed and mannered style.

Who then are these painters? They are Tom Thomson and the Group of Seven who followed in his wake, Canadians all - though not all as Canadian as Thomson by birth or education. Born in Ontario in 1877, trained as an engraver, at 30 he joined a group of commercial artists working for Grip Ltd, a photo-engraving printing firm in Toronto, where the head designer, John MacDonald, encouraged him to paint. He became a summer painter in the wilderness of Algonquin Provincial Park, earning a pittance as a fire-ranger and a guide, returning to work in Toronto in the winter, developing his immediate and often turbulent sketches into gallery landscapes of imposing scale and majesty, exuberant colour and calculated mannerism.

He died mysteriously in July 1917, days short of his fortieth birthday, possibly from a blow on the head, possibly from drowning, possibly both - there were no known witnesses. Standing up in his canoe to empty his bladder when drunk is one supposed cause of the mishap, murder another. His fellow artists at Grip seem to have held him in the same regard as mountaineers here held George Mallory - as a free and heroic spirit, masculine yet mystical, a muscular pantheist, the romantic ideal of Thoreau, Emerson and Whitman, an embodiment of "Canadianism". If not in his honour, it was certainly in his spirit, as it were, that in May 1920 they formed the Group of Seven.

Thomson's mentor, John MacDonald, two years his senior, was the leading founder member. He left Grip in 1911 in order to paint full time, having overlapped with Thomson for only two years); he had encouraged other employees there to paint with him at weekends, teaching themselves and each other. These, half of them immigrants from England, had attended art schools there and on the loose in Italy, Antwerp (then a notable international school), Paris and Berlin at a time when German art was breaking Expressionist boundaries (could Lawren Harris have sat at Max Liebermann's knee?). It is not surprising that with this experience and some knowledge of lively contemporary painting in Britain in the first decade of the century, their work seems something of an international melting-pot.

Meeting constantly, it was their failure to find a better title for their first joint show in May 1920 than An Exhibition by a Group of Seven Painters that brought the Group into existence. Stuck with the name, in the catalogue of their second exhibition, the following year, they denied that they were tied to a "group formula" and maintained that they had widely divergent aims - as indeed they eventually did, though not obviously so at this early stage. They were not discouraged by critical descriptions of their work as the Hot Mush School of Painting, freakish, garish and the stuff of a drunkard's vomit. With a slowly diminishing unity of 13 years, they outlasted the Cubists and most other important art movements of the earlier 20th century, but their divergent aims led to the dissolution of the group in 1933 and their expansion into the larger, looser and far less memorable Canadian Group of Painters, of which Jackson was first president.
How divergent they had become is demonstrated particularly by Lawren Harris, whose early work was very much in the Thomson "house style" of the Group, but in the later Twenties he turned away from specifically Canadian landscapes of the Algonquin type, to adopt an austere, abstracted, hieratic and almost transcendental cold blue mannerism in which to depict the Arctic North and the Great Lakes in winter. To some these are schematic masterpieces and the exhibition's final room must be a place of awe and wonder in which to shiver. I shivered with distaste, for to me they are bleak, contrived and cinematic, the first step on the painter's path to a period of complex, ambiguous and geometric abstraction - indeed, in Canada, Harris was abstraction's father.

All who care for landscape painting will find pleasure in the work of Tom Thomson and the Group of Seven, though not in the quite brutally unsympathetic, even disgusting, frames. Beyond the subject, I cannot identify a specific Canadianism in any of them - indeed all these painters seem very much eclectics of their day and MacDonald in particular could be so much Thomson or Munch or Hodler, even Schiele, that I wonder if he ever had a personal and private style; with their roots in the commercial business of Grip, I wonder if they had privileged access to reproductions of work by European artists. Above all elements Canadian, these are painters who knew how to handle paint and colour, and how to turn a small sketch executed on the spot into a high-pitched studio masterpiece. There it should have stopped, but in our contemporary painter Peter Doig, a darling of Tate Britain who spent his boyhood and later twenties in Canada, Thomson has a slavish follower-cum-imitator who, with (I suspect) the aid of computer-based technology, pitches his near century-old paintings into the flaccid gigantism characteristic of art now, ten times the size of Tom's but not even one tenth of his quality, perfect postmodernist corruptions.

Painting Canada is at Dulwich Picture Gallery, SE21 (020 8693 5254, dulwichpicturegallery.org.uk) until January 8, 2012. Open Tues-Fri, 10am-5pm; Sat-Sun, 11am-5pm; closed Mon. Admission £9 (concs available)

Details are correct at the time of publication - please check with venue before booking.