Even as Europe conquered or exploited much of the world in the 1800s, the “primitive” art that came streaming on to its markets exacted an aesthetic revenge. Artists preferred it to their own “civilised” conventions. When the Belgian artist James Ensor depicted a variety of non-European masks in Astonishment of the Mask Wouse in 1889, Belgium was running the most brutal of colonial businesses in the Congo, working thousands of people to death. André Derain owned a Fang mask from central Africa that directly influenced him and his friends including Matisse and Picasso. Derain’s 1906 painting The Dance portrays fantasy dancers, one with a painted body, another with a shadowed, mask-like face, another in an ancient Greek dress, in a golden paradise where they gyrate with unrepressed abandon.

If the first moderns sought to be “savage”, they also loved sex. This was the age of Sigmund Freud. Matisse’s Dance – no, not that one: who knows when any of us will see his masterpiece in the Hermitage again? – but a carved wooden relief from 1907, releases raw, ecstatic sexuality in its wild naked romp. And this is just one of the exhibition’s nude explosions. In Berlin they were fixated on flesh – tottering, quivering masses of it. Lovis Corinth’s Nana, Female Nude looks as if she’s been left over from the recent Lucian Freud show in this same space. Corinth’s Perseus and Andromeda, in which a knight in armour unveils a female nude, may not seem very modern at all: but nor does Wagner until you hear him.

Why does Degas make it into this show while other impressionists get left behind? Sex. Degas pervs with the best, or the worst. Facing Gauguin’s pensive isolated tropical nude Nevermore are his painting of a woman lost in red ecstasy as her long hair is combed, and pastel of a naked model curled up reading.

It’s a shame Pissarro doesn’t make the cut when he worked so closely with Cézanne and later helped pioneer the divisionist or pointillist style. It is equally baffling that Seurat’s compelling essays in this perceptual art get less space than Paul Signac’s painting-by-numbers attempts. And Munch is shoved into the Berlin section, oddly, which doesn’t stop The Death Bed harrowing your heart.

I could go on, and get cross, but it would miss the point. For some of the unevenness is in the period itself. What this show reveals is that modernism was an end, as much as a beginning. Five hundred years of European pictorial art – the very tradition the National Gallery displays – were breaking and decaying, and what was born in their stead was difficult, elusive, as daunting and inescapable as Picasso.