The explanation is obvious: having colonised the world from its birthplace in an English Midlands public school via the British Empire, the two places where the sport most thoroughly permeated the local culture - South Africa and New Zealand - lie in the deep south of the planet.

With Australia and Argentina also nearly always among the top 10 rugby union-playing nations over recent years, the "north versus south" preoccupation makes perfect sense.

Throughout my life, the south has usually held sway, the most noteworthy exceptions that spring to mind being the glorious 1971 British and Irish Lions tour to New Zealand, when the Home Nations teamed up to beat the All Blacks in their backyard, and England’s pragmatic victory in the 2003 Rugby World Cup.

The other eight World Cups to date have been split between the southern hemisphere titans.

If you award marks for World Cup performance all the way back to the inaugural tournament in 1987 – four marks for the champions, three for the runners-up, two for beaten semi-finalists and one for reaching the last eight – New Zealand come out top, ahead of Australia.

England and France are tied with South Africa in third place, but the Springboks did not feature in either the 1987 or 1991 competitions.

So, the fact that all four pools at the ongoing 2023 Rugby World Cup in France have been topped by northern-hemisphere sides - a first - feels like a bit of a moment.

Indeed, the sense of a possible turning of the tide was heightened in the very last pool game in Toulouse on Sunday night (October 8), when combative and sprightly northern-hemisphere Portugal beat error-prone southern-hemisphere Fiji 24-23 to record their very first Rugby World Cup win.

Granted, the Pacific islanders - who have won two consecutive men’s rugby sevens Olympic gold medals - still progressed to the quarter-finals; but the scoreline was nevertheless one of the most eye-catching of the competition so far.

For all that, it might be wise to wait for at least one more week before proclaiming that the sport’s natural order of things - with the south on top and the north for ever scrambling to catch up - really has been subverted.

Yes, the two biggest clashes so far - France’s opening day bust-up at the Stade de France against New Zealand and Ireland’s impressive 13-8 subduing of the Springboks two weeks later - have both gone the north’s way.

That essentially is why Six Nations teams came out on top of all four pools.

But, on the evidence of the rugby played to date, the four teams involved in those two mega-clashes look head and shoulders above the rest; each looks capable of beating the others on days when the bounce of the ball goes in their favour; and two of them - 50 per cent - are from the southern hemisphere.

Because of a draw upon which my colleague Mike Rowbottom pronounced at length in this space yesterday (October 9), the classy quartet are also quarter-final opponents: on Saturday (October 14) Ireland take on New Zealand before hosts France clash with South Africa the following day.

It is possible that the four northern hemisphere pool-winners will go on to become the four semi-finalists; but it is possible too, though I think less likely, that the last four survivors will be composed entirely of southern-hemisphere teams.

At all events, that will be the most sensible juncture at which to assess whether the north has at last overthrown the long-held supremacy of the deep south.

France versus South Africa should be a match for the ages, with Bleus’ fans hoping that their team can emulate the 1958 French tourists who crossed the equator and battled to an epic Test series victory, as chronicled so memorably by Denis Lalanne in a classic book, The Great Fight of the French Fifteen.

But Ireland against the All Blacks the previous day is almost more intriguing.

The Europeans suffered 111 years of hurt before travelling to Chicago and at last beating the New Zealanders for the first time in 2016.

The men in green - who, remember, represent the whole island of Ireland - have won four of the seven games between the two sides since then.

And yet, when it mattered most, four years ago in another World Cup quarter-final in Japan, the All Blacks simply blew their opponents away, notching seven tries in a 46-14 triumph.

If Ireland and their New Zealand-born star, centre Bundee Aki, can reverse that defeat on Saturday in the Stade de France and qualify for a first World Cup semi-final, it would amount to a stunning turnaround - and a strong signal that, yes, perhaps the north really is finally wresting rugby union supremacy away from the south, its traditional home.