Charles Darwin, author of On The Origin Of Species, was a Nazi. Well, not quite – but his ideas, according to Dennis Sewell, provided the intellectual seedbed in which National Socialist eugenics could thrive. “The evidence that Darwin’s evolutionary theories had a significant influence in shaping the ideology of National Socialism,” he argues, “and was even at the root of its genocidal practices, is so abundant and well attested that to ignore it would be perverse”. Oh dear, Edinburgh University had better tear down all those Darwin anniversary posters and stop holding exhibitions about the old man if they don’t want to be accused of giving intellectual comfort to the BNP.

Of course, Sewell – a former BBC journalist and producer – doesn’t come right out and declare Darwin as Hitler’s granddad. “I have not set out to indict Darwin for any crime,” he says in his introduction. He then presents a highly selective case for the prosecution. He accuses Darwin of fathering the doctrine that races can be purified and improved by selective breeding, just like farm animals. He quotes a passage of from Darwin’s Descent Of Man where the writer appears to say that allowing “the weak members of civilised societies to propagate their kind” could be to “the detriment of the domestic race”.

What he doesn’t say is that Darwin explicitly rejected the application of crude natural selection to humans because we are sentient beings and can use education and medical science to counter any harm done through the propagation of inferior genes.

Where Sewell has a point, however, is in the way Darwin allowed his friends Francis Galton and Herbert Spencer – two influential Victorain scientific writers – to misuse his evolutionary science. They reduced Darwinism to the phrase “survival of the fittest” and applied it directly to humankind in the form of eugenics – a term coined by Galton, who set up the Eugenics Society in 1885. Social Darwinism was influential in intellectual circles in the late 19th century. Eugenics was popularised by a range of prominent Edwardian writers. The novelist HG Wells said: “It is in the sterilisation of failures and not in the selection of success for breeding that the possibility of an improvement of the human stock lies.” But it took the iconoclastic playwright George Bernard Shaw to propose the use of “the lethal chamber” – though I suspect the old joker was just seeking attention as usual.

Sewell is a Catholic polemicist, and sees eugenics as a vice of atheists. This is not an even-handed and balanced assessment of the influence of Darwin’s ideas, but a broadside against those who refuse to see God’s work in the face of man. However, that doesn’t invalidate his critique of eugenics. It was a repellent pseudo-science which was used by Herbert Spencer to oppose poor relief and even proper medical treatment for the underclass of his day, on the grounds that this would lead to degenerate genes polluting the true seed of England.

All the more astonishing then that eugenics was popular on the political left. The Fabian socialist Sydney Webb, the founder of the London School of Economics, was a eugenicist, according to Sewell, who wanted to prevent “thriftless and irresponsible” immigrants from breeding. Even David Beveridge, the liberal reformer who pioneered the welfare state, wrote in 1909 that the undeserving unemployed should be denied citizenship rights “including not only the franchise but civil freedom and fatherhood”.

Really, this book should have been called The Eugenics Conspiracy – Sewell finds them everywhere. John Maynard Keynes, Winston Churchill, Neville Chamberlain. Even the British Medical Association argued against National Insurance on the grounds that it would encourage degenerates to “multiply their breed at the expense of the healthy and intellectual members of the community”. The Mental Deficiency Act of 1913 established colonies for the segregation of “idiots, imbeciles, the feeble minded and the moral defective”. Unmarried mothers, petty criminals and homosexuals were included in the net. The system remained in place until 1959 and some 60,000 people were locked up in institutions.

But it took the Nazis to carry eugenics to its logical conclusion and start ­building the “lethal chambers” that Shaw had talked about. In 1934 they set up a nationwide system of eugenic courts to decide who should be sterilised, using IQ tests as a means of assessment.

Sewell says there was never an explicit order to exterminate the genetically undesirable, only a permissive edict “Aktion T4” which licensed doctors to do so. Sewell claims they adopted the idea enthusiastically because eugenics was so influential in the profession. Pretty soon gypsies, Jews and gays were being gassed along with Downs Syndrome children and people with malformed limbs.

The charge sheet is pretty damning, However, when Sewell arrives at the 1967 Abortion Act you suddenly realise that this paranoia is really getting the better of him. He claims that this was the work of eugenicists, which is simply untrue. I know the politician who moved the bill, David (now Lord) Steel, and he is no eugenicist. This conspiracy theory goes a long way to discrediting Sewell’s entire case. It is only at the end of this book that Sewell reveals himself as a supporter of a form of Intelligent Design, the Christian counter-theory to Darwinism. He hints that contemporary atheists like Richard Dawkins are also infected with the Darwin-eugenic virus.

The truth is that eugenics was a dangerous fad that captivated the chattering classes of the Edwardian era, but had largely burnt itself out by the 1920s – only for it to be revived by Hitler as a means of social control.

Darwin, like Karl Marx, cannot be held to blame for what people did with their ideas. They could perhaps have been more careful, but how were they to know that Hitler and Stalin would come along to justify mass slaughter on pseudo-scientific grounds? Sewell commits the same error as those who say that Islam is to blame for 9/11 or that Christ is to blame for the Spanish Inquisition. This is unfortunate because this is actually a very valuable book. It exposes one of the liberal left’s dirty little secrets – lest we forget.