"Under the pretext of 'profit', 'economic development', 'culture', our civilisation is intent on the destruction of life. It attacks it in all its forms, cuts down forests, extinguishes species, wipes out indigenous peoples . . . and degrades those living creatures which it spares into mere merchandise, into the marked objects of an unlimited greed."

Who is the author of this passage? Naomi Klein or George Monbiot? No. It comes from a 1913 pamphlet by the German ideologue Ludwig Klages and is one of the first statements of what has come to be known as environmentalism. So why is Klages not revered today as a prophet and seer? Because he was also an anti-Semitic crank who believed Jewish-American capitalists were destroying the "cosmological Eros". For modern environmentalists, he is a liability.

Then there is Martin Heidegger, whose 1953 lecture "The Question Concerning Technology" depicts the machine as the expression of a blinkered, domineering attitude to nature. Whereas a bridge arches gently over a river, respecting its distinctive mode of being, a hydroelectric power station bends it rudely to our will. It reduces it to what Heidegger calls "inventory". In one of those phrases that some people find cosmically profound, others portentously vacuous, Heidegger urges us to "let Being be".

Unfortunately for modern environmentalists, Heidegger was politically even less reputable than Klages. An early supporter of the Nazi regime, he never repudiated its "inner truth and greatness". And though he later distanced himself from its "outer" misdeeds, he did so in the crassest manner possible. "Agriculture is now motorised food industry," he said in 1949, "essentially the same as the manufacture of corpses and gas chambers." This statement provoked anger, both for its equation of human beings with livestock and for its presentation of the Holocaust as a mere offshoot of machine technology. That has not stopped many others - including, most recently, Peter Singer - from repeating it.

Is it really a surprise, one might ask mischievously, that Germany now boasts the largest green movement in the world? Most of its supporters have no inkling of its tainted origins. The ideas of Klages, Heidegger and others have long since been made over by thinkers of the left, rendering them safe for public consumption. And does it matter, in any case, that many of the early environmentalists were also Nazis? Surely what counts is the inherent truth of an idea, not its historical associations.

Still, that many early environmentalists belonged to the radical right should give us pause. Does environmentalism really merit its current place on the checklist of political correctness? Marx himself would undoubtedly have viewed it as a new kind of religion, dedicated to the enshrinement of class privilege. His political disciples were enthusiastic industrialisers; they polluted lakes and rivers without qualm. Environmentalism is, in truth, a conservative ideology. The passion that animates it is one of return, not progress. Its current association with the political left is mere window-dressing.