According to an authoritative survey this week, the number of British Jews has dipped below the 300,000 mark for the first time this century, down to just 285,000, as a result of assimilation ( or "marrying out"), emigration to the US and Israel, and just plain drift from the synagogues. It is not entirely clear how the numbers are estimated - how much is based on membership of a synagogue, how much is based on census numbers and so forth, but there is no doubt that the Jewish community in Britain thinks itself to be in numerical decline.
This, by the way, is not necessarily a mark of falling morale; one spokeswoman, with admirable chutzpah, observed that we should focus on quality rather than quantity, a fair observation given the astonishing success and self- confidence of this community. It is regarded by no less an authority than Bernie Grant MP as a valuable role model for other immigrant groups.
I don't intend here to get into the issue over whether or not a non-practising Jew ceases to be Jewish. The arguments seem to defeat the rabbis; there is no reason why an outsider should be able to shed new light. But what is certain is that there comes a point in any community's life when its numbers are falling so rapidly that it can no longer be a self-sustaining, distinct entity.
Two questions arise. First, should we care, and if so, what should we do about it? You don't have to be Einstein to work out that my own answer to the first question is yes.
The struggle to maintain the tribe is one of the most ancient in humankind. But there are moments in history when that struggle becomes critical. Entire peoples disappear without trace; who knows, for example, where the Etruscans went?
Today, the descendants of the Parsees, the Zoroastrian fire-worshippers driven out of Persia by the rise of Islam, are said to be fewer than 75,000 world-wide. It is an unusually talented and well resourced disappearing tribe; many of India's great industrial and commercial families are Parsees and their most famous living son, the conductor of the Three Tenors, Zubin Mehta, has become, in his own way, as much of a global figure as the late Firdous Bulsara, better known as Freddie Mercury. But within the next century it is almost certain that this will all be history.
Even larger, more distinctive groups, such as my own, the black Britons, are not as they were. Our numbers are more or less stable; but our character is changing, principally as a result of intermarriage. By the middle of the next century, the so-called "black" person will probably be a rare specimen in this country; most of our grandchildren will be of mixed race.
This has happened before. By the late 19th century, the entire black population of London - some 25 000 strong in Georgian times - had effectively disappeared. Today, the idea of calling yourself a Huguenot would seem ridiculous, though thousands fled Catholic persecution to settle here in Tudor times.
You could dismiss all this as simply a consequence of the march of time. Indeed, casting an eye to France now, it is arguable that the less historical baggage we carry, the less room there is for conflict. And as I have argued many times here, none of us wants to be trapped by our ancestral traditions. However, in our hearts of hearts, none of us wants our communities to disappear either. We may not want to be part of the ethnic or religious action, but we want to be sure that there is some action going on.
Even if we don't care about our own specific traditions, there is much to be said in general for human cultural diversity. Just as in the rainforest, there is an ecology in human affairs which we neglect at our peril. Our culture is built of many historical fragments, in many cases represented by the persistence of small communities; lose any one of those communities, and something disappears from all our lives.
Anybody can appreciate Jewish-mother jokes, or love Jamaican reggae, or thrill to Irish dance; but without the core of a community to sustain those traditions and constantly renew them, these elements of culture become stale, old-hat and ersatz. And without this rich array of authentic cultures, what is to stand in the way of the remorseless march of Disneyfication and McDonaldisation?
That is why, of course, it is in all our interests to sustain cultural diversity. But how?
No number of official receptions, Arts Council grants and essays on multiculturalism can match the influence of two key institutions: the temple (or church or synagogue) and the family. And these are the principal sites of the problem.
Family continuity is, for example, at the mercy of intermarriage. But the old ties are loosening; children insist on their rights and it is becoming almost impossible to legislate for the purity of your grandchildren.
So perhaps the temple needs to be the focus of our interest. But the temple has its rules which are themselves shrinking the communities. For those who marry into communities where the mark of belonging is passed on either through the father or the mother, the chances are that their children's interest in that community will be arrested pretty early; it is impossible, for example, to become a Parsee unless you are born to a Parsee father. Even today there are still many hours spent arguing about whether someone of mixed race can be regarded as "black" or not.
For many centuries, such defensive rules were vital to maintain the survival of the tribe. Today we need to look at these rules again, and reflect on the possibility that if we have an ethnic identity it may have to be shared with and kept alive by people who were not born into it, in order for it to survive. The disappearing tribes have to find a way to embrace those who come wanting to share their history and their traditions. The choice is straightforward: open the temples, or die in them.