There are numerous other examples. When the sailors first landed in Mauritius and saw the gentle dodos, it never occurred to them to do anything other than club them to death. They didn't even want to eat them (they were described as unpalatable). Presumably, hitting defenceless, tame, flightless birds over the head with a club was just something to do. Nowadays such behaviour would be unthinkable, and the extinction of a modern equivalent of the dodo, even by accident, let alone by deliberate human killing, is regarded as a tragedy.

Just such a tragedy, by the standards of today's cultural climate, was the more recent extinction of Thylacinus, the Tasmanian wolf. These now iconically lamented creatures had a bounty on their heads until as recently as 1909. In Victorian novels of Africa, 'elephant', 'lion' and 'antelope' (note the revealing singular) are 'game' and what you do to game, without a second thought, is shoot it. Not for food. Not for self-defence. For 'sport'. But now the Zeitgeist has changed. Admittedly, rich, sedentary 'sportsmen' may shoot wild African animals from the safety of a Land-Rover and take the stuffed heads back home. But they have to pay through the nose to do so, and are widely despised for it. Wildlife conservation and the conservation of the environment have become accepted values with the same moral status as was once accorded to keeping the sabbath and shunning graven images.

The swinging sixties are legendary for their liberal modernity. But at the beginning of that decade a prosecuting barrister, in the trial for obscenity of Lady Chatterley's Lover, could still ask the jury: 'Would you approve of your young sons, young daughters — because girls can read as well as boys [can you believe he said that?] — reading this book? Is it a book you would have lying around in your own house? Is it a book you would even wish your wife or your servants to read?' This last rhetorical question is a particularly stunning illustration of the speed with which the Zeitgeist changes.

The American invasion of Iraq is widely condemned for its civilian casualties, yet these casualty figures are orders of magnitude lower than comparable numbers for the Second World War. There seems to be a steadily shifting standard of what is morally acceptable. Donald Rumsfeld, who sounds so callous and odious today, would have sounded like a bleeding-heart liberal if he had said the same things during the Second World War. Something has shifted in the intervening decades. It has shifted in all of us, and the shift has no connection with religion. If anything, it happens in spite of religion, not because of it.

The shift is in a recognizably consistent direction, which most of us would judge as improvement. Even Adolf Hitler, widely regarded as pushing the envelope of evil into uncharted territory, would not have stood out in the time of Caligula or of Genghis Khan. Hitler no doubt killed more people than Genghis, but he had twentieth-century technology at his disposal. And did even Hitler gain his greatest pleasure, as Genghis avowedly did, from seeing his victims' 'near and dear bathed in tears'? We judge Hitler's degree of evil by the standards of today, and the moral Zeitgeist has moved on since Caligula's time, just as the technology has. Hitler seems especially evil only by the more benign standards of our time.

Within my lifetime, large numbers of people thoughtlessly bandied derogatory nicknames and national stereotypes: Frog, Wop, Dago, Hun, Yid, Coon, Nip, Wog. I won't claim that such words have disappeared, but they are now widely deplored in polite circles. The word 'negro', even though not intended to be insulting, can be used to date a piece of English prose. Prejudices are indeed revealing giveaways of the date of a piece of writing. In his own time, a respected Cambridge theologian, A. C. Bouquet, was able to begin the chapter on Islam of his Comparative Religion with these words: 'The Semite is not a natural monotheist, as was supposed about the middle of the nineteenth century. He is an animist.' The obsession with race (as opposed to culture) and the revealing use of the singular ('The Semite. He is an animist') to reduce an entire plurality of people to one 'type' are not heinous by any standards. But they are another tiny indicator of the changing Zeitgeist. No Cambridge professor of theology or any other subject would today use those words. Such subtle hints of changing mores tell us that Bouquet was writing no later than the middle of the twentieth century. It was in fact 1941.

Go back another four decades, and the changing standards become unmistakeable. In a previous book I quoted H. G. Wells's Utopian New Republic, and I shall do so again because it is such a shocking illustration of the point I am making.

That was written in 1902, and Wells was regarded as a progressive in his own time. In 1902 such sentiments, while not widely agreed, would have made for an acceptable dinner-party argument. Modern readers, by contrast, literally gasp with horror when they see the words. We are forced to realize that Hitler, appalling though he was, was not quite as far outside the Zeitgeist of his time as he seems from our vantage-point today. How swiftly the Zeitgeist changes — and it moves in parallel, on a broad front, throughout the educated world.

Where, then, have these concerted and steady changes in social consciousness come from? The onus is not on me to answer. For my purposes it is sufficient that they certainly have not come from religion. If forced to advance a theory, I would approach it along the following lines. We need to explain why the changing moral Zeitgeist is so widely synchronized across large numbers of people; and we need to explain its relatively consistent direction.

First, how is it synchronized across so many people? It spreads itself from mind to mind through conversations in bars and at dinner parties, through books and book reviews, through newspapers and broadcasting, and nowadays through the Internet. Changes in the moral climate are signalled in editorials, on radio talk shows, in political speeches, in the patter of stand-up comedians and the scripts of soap operas, in the votes of parliaments making laws and the decisions of judges interpreting them. One way to put it would be in terms of changing meme frequencies in the meme pool, but I shall not pursue that.

Some of us lag behind the advancing wave of the changing moral Zeitgeist and some of us are slightly ahead. But most of us in the twenty-first century are bunched together and way ahead of our counterparts in the Middle Ages, or in the time of Abraham, or even as recently as the 1920s. The whole wave keeps moving, and even the vanguard of an earlier century (T. H. Huxley is the obvious example) would find itself way behind the laggers of a later century. Of course, the advance is not a smooth incline but a meandering sawtooth. There are local and temporary setbacks such as the United States is suffering from its government in the early 2000s. But over the longer timescale, the progressive trend is unmistakeable and it will continue.