'em some food. But some gang of young men, I'm not saying it was that lot and I'm not saying it wasn't, set the Brown's farm alight, just for the hell of it ...'

In any cowboy film we find the message that barbarism is opposed to tribalism, that honour and tradition are not good bedfellows. And that, having selected himself or herself for imagination and the ability to endure pain for future pleasure, Homo sapiens is now prepared to die for his or her beliefs, for his or her gang, for honour, for hatred, or for love.

Civilisation, as we know it, seems to combine elements of both ways of human culture, tribal by tradition and barbaric for honour, for pride. Nations are internally tribal, but present a barbarian face to other nations. Our extelligence tells us stories, and we tell our children stories, and the stories guide us about what to be or do in what circumstances. Shakespeare is the ultimate civiliser, in this view. His plays were composed against the barbarian background, in a city where you could see heads on spikes and ritually dismembered bodies; all of them were set on the tribal, traditional base that is most of human life, most of the time. He tells us very persuasively that evil fails in the end, that love conquers, and that laughter -the greatest gift that barbarism brought to tribalism - is one of the sharpest weapons, because it civilises.

Cohens are the hereditary High Priest lineage of the Jews. Jack was once asked, in Jerusalem, whether he was not proud to be a Cohen, in view of the noble Jewish history that the High Priests had promoted. Jack sees this nobility as based in about six inches of blood in the streets, nearly all of it other people's, so he is not proud. Instead, to the extent that any of us is responsible for what their ancestors did, he is ashamed. He loves Small Gods, in much the way he enjoys the Jewish Day of Atonement, Yom Kippur: it engenders a feeling of repentance, and he can always find plenty to repent. He is sure that this emotion -guilt -is a legacy of the Morgan/Campbell selection of his ancestors through tribal rituals.

Tribesmen aren't 'proud'; for them, everything that isn't mandatory is forbidden, so what is there to be proud about? You can praise your children for doing things right, or admonish or punish them for doing things wrong, but you can't take pride in what you -a fully fledged member of the tribe -do. That comes with the territory. However, you can be guilty about not having done the things that you should have done. Having said that, High Priests waging war on dissenters or neighbouring tribes, leading to atrocities like heads on spikes, is straight barbarism.

The distinction between tribalism and barbarism is illuminated by the story of Dinah in chapter

34 of Genesis. Dinah, an Israelite, was the daughter of Leah and Jacob, and 'when Schechem the son of Hamor the Hivite, prince of the country, saw her, he took her, and lay with her, and defiled her'. Then Schechem fell in love with her, and wanted to make her his wife. But the sons of Jacob felt that maybe Schechem had gone about things in the wrong order: "... the men were grieved, and they were very wroth, because he had wrought folly in Israel in lying with Jacob's daughter, which thing ought not to be done'. So when Hamor, the father of Schechem, asked for approval of the marriage, and for an intermingling of his tribe with the Israelites, the sons of Jacob came up with a cunning plan.

They told the Hivites that they would agree to the proposal, but only after the Hivites had circumcised themselves, so that they were just like the Israelites. The Hivites were willing to go along with this, because they told themselves that 'These men are peaceable with us, therefore let them dwell in the land, and trade therein; for the land, behold, it is large enough for them; let us take their daughters to us for wives, and let us give them our daughters'. The decision was made, and 'every male was circumcised, all that went out of the gate of the city'. And they stood around in pain for a couple of days. At that point, Dinah's brothers Simeon and Levi hauled Dinah out of Schechem's house, put all the Hivite men to the sword, destroyed their city and took all their domestic animals, their wealth, their children and their wives. This story of deceit and betrayal has not been given much circulation in recent years; it doesn't appeal to people's sense of humour any more, as it once did.

At any rate, in that story, the Hivite response to Schechem's crime is tribal, but the Israelites behave like barbarians. The Hivites, after their initial mistake, want to make amends and coexist peacefully, and they're prepared to offer dowries and other concessions to try to make up for what Schechem did. But all that matters to the Israelites is a twisted kind of 'honour', in which cruelty, murder and theft are justified to protect Dinah's reputation. Or, more likely, their own sense of manhood.

A favourite Discworld character is Cohen the Barbarian, a satire on sword-and-sorcery heroes like Conan the Barbarian, all muscles and trolls' teeth necklaces and testosterone-propelled heroism. He first appears in the second Discworld novel The Light Fantastic:

'Hang on, hang on,' said Rincewind. 'Cohen's a great chap, neck like a bull, got chest muscles like a sack of footballs. I mean, he's the Disc's greatest warrior, a legend in his own lifetime. I remember my grandad telling me he saw him ... my grandad telling me he ... my grandad ...'

He faltered under the gimlet gaze.

'Oh,' he said. 'Oh. Of course. Sorry.'

'Yesh,' said Cohen, and sighed. 'Thatsh right, boy. I'm a lifetime in my own legend.'

Cohen, by then 87, is the sort of barbarian whose hordes ride into town, set the houses on fire and look wistfully at the women. But he's no softie: as he ages, he goes hard, like oak. In Interesting Times he explains to Rincewind why, in the area known as the Ramtops, there's no future in the Barbarian business any more:

'Fences and farms, fences and farms everywhere. You kill a dragon these days, people complain.

You know what? You know what happened?'

'No. What happened?'

'Man came up to me, said my teeth were offensive to trolls. What about that, eh?'

According to Jewish tradition, Cohens are the true Cohanim, the lineal descendants of Aaron.

Recent research into the genetics of Cohens has turned up some interesting findings about the very prideful (barbaric) issue of Cohen heredity. Professor Vivian Moses (yes, indeed ...) and a group of scientists in Israel decided to check whether the tradition has any factual basis. Just as the mitochondrial DNA sequence traces female heredity, so the Y-chromosome, possessed only by males, can be used to trace male heredity.

There has been an interesting division of the Jewish peoples, and that provides a scientific check on the story of the Cohanim. During the Diaspora, some Jews remained in North Africa, but one large population went into Spain. They are known as Sephardi, and the Rothschilds, Montefiores and other banking families are all Sephardic. Another, more diffuse population went into middle- Europe, especially Poland, and they are known as Ashkenazi. Moses and his colleagues looked at the Y-chromosomes of representative Sephardi and Ashkenazi Cohens and non-Cohens

('Israelites'). They found characteristic DNA sequences, specific to Cohanim, in about half of the Cohens that they tested, but with small and characteristic differences in the three groups. From these differences it is reasonable to suppose that Ashkenazi and Sephardi Jews separated rather less than 2,000 years ago, and that all Cohens were a single group only 2,500 years ago.

This looks like a very nice story, with the DNA evidence supporting the expected history. But science is the best guard against believing things because you want to. There is a factor that Moses and his colleagues didn't explicitly consider, and it needs to be explained away, because it makes those figures much too good.