In the US, rumours such as the claim that Barack Obama had been born in Kenya, not America – the birther conspiracy – can sometimes enter the mainstream swiftly. In modern Europe, insidious ideas tend to take longer to creep in and be taken seriously. But creep in they do: which is why news of a demography conference held by Hungary's Viktor Orban last week is so disturbing.
It is not just what the prime minister himself said, although that was worrying enough.
“If Europe is not going to be populated by Europeans in the future and we take this as given, then we are speaking about an exchange of populations, to replace the population of Europeans with others,” he said. “There are political forces in Europe who want a replacement of population for ideological or other reasons.”
This was a reference to the "great replacement" theory, which posits that white populations with low birth-rates on the continent are being systematically substituted by non-white immigrants, specifically Asian and Arab Muslims, whose tendency to have more children means that they will one day end up as the majority. Naturally, as with many scaremongering far-right plots, this involves the complicity of "evil elites" who are supposedly scheming to undermine their own indigenous cultures.
We are used to hearing this kind of ethno-nationalist message from Mr Orban and his ideological fellow travellers. What made this instance of it even more unfortunate was that Mr Orban was applauded by a guest at the conference – the former Australian prime minister Tony Abbott, who praised his host for having "the political courage to defy political correctness".
In his own speech Mr Abbott said: "The problem with the people who have been swarming across the borders in Europe in very recent times is that you don't get any impression that they come to join. They are not there to be grateful, they are there with a grievance."
The language was ugly. Insects swarm, people do not.
Further, while no one doubts that large-scale immigration can pose problems of integration, there is plenty of evidence that new or more recent arrivals are not only grateful but seek to demonstrate it. A survey by the UK-based Policy Exchange think tank in late 2016 found, for instance, that Britain’s Muslims (only some of whom are immigrants of course) were “amongst the country’s most loyal, patriotic and law-abiding citizens”. They were also more likely to oppose terrorism and take part in elections than the population as a whole.
All this seems to have been lost on Mr Abbott, who warned darkly of "the implications of the shrinking West". This is so troubling because Mr Abbott cannot be dismissed as either an eccentric or an extremist. He is the former leader of a highly developed country, a regional power with the 10th-highest per capita income in the world. Mr Abbott may have run a very tough anti-immigration policy when in office, but the Liberals – the party he once led – are one of Australia's two major parties, and are the local equivalent of mainstream centre-right groups such as Germany's Christian Democrats or Conservative parties in the UK and Canada. So for him to provide support for Mr Orban's replacementism is truly shocking.
Ten years ago, such ideas were little aired in mainstream discussions because their proponents were mainly either crackpots or members of the extreme right. True, a few fundamentalist atheists and militant secularists gave some credence to the Eurabia thesis of Gisele Littman, an Egyptian-born writer who under the pen name of Bat Ye’or promoted the notion that the European Union and Arab countries had a decades-long secret plan to "Islamise" the continent.
Reviewing one of her books in these pages in 2011, however, I still felt confident that her views were so self-evidently preposterous that all I had to do was quote her. When she wrote that "the blind termites in the chancelleries of Europe and America are working assiduously" to "bring a worldwide caliphate to power", it seemed to me that no one could take such propositions seriously. Although the British historians Niall Ferguson and Sir Martin Gilbert had a curiously soft spot for her, they appeared to be outliers, misled in an area outside their expertise.
But today Bat Ye’or’s views have moved from the margins. The Alternative for Germany – a far-right organisation, yes, but also the third-largest group in the country’s parliament – ran a poster campaign in elections this year with the caption: “Europeans, vote for AfD, so that Europe will never become ‘Eurabia’.”
The British author Douglas Murray, who regularly writes for mainstream publications such as the Spectator magazine and the Daily Mail, has praised Bat Ye'or as "a great scholar". His book, The Strange Death of Europe, is described by his publishers as an "account of a continent and culture caught in the act of suicide" due to "declining birth-rates, mass immigration and cultivated self-distrust and self-hatred".
Mr Murray is a highly intelligent man who is considered to be part of the respectable right. His views are not identical to Mr Orban's, but the Hungarian prime minister has endorsed his book, and far-right wingers from whom Mr Murray may wish to disassociate himself will find much they agree with in it.
This is a poison that is entering the mainstream. Never mind that there is no evidence whatsoever for Mr Orban’s claim that “there are political forces in Europe who want a replacement of population for ideological or other reasons". If enough people believe it, that is good enough in our post-truth world.
You could dismiss Mr Abbott as a has-been. Or you could say that it was incumbent on a former leader of a great country who represented an honourable conservative tradition to say: Stop. The facts matter. And there is a boundary beyond which the respectable right will not go. There is a barrier between us and the far right.
For if people like Mr Abbott will not man it, who will?
Sholto Byrnes is a commentator and consultant in Kuala Lumpur and a corresponding fellow of the Erasmus Forum