It's estimated that the number of people of Scottish descent living outside Scotland - the 'ancestral diaspora' - is between 25 and 40 millions.

Seven million Americans self-identify as having solely Scottish ancestry, another 21 million as having Scottish ancestry in combination with another nationality. Nearer home, there are 800,000 Scots-born people living in England. London, they say, is the third biggest Scottish city in the world.

The skills, hard work and ingenuity of these Scots have been a tremendous boon to the USA, England and all the other places they have gone to. But at what cost to Scotland? So much talent lost. So many ties broken.

The figures themselves are scary.

In the 19th century, Scotland lost to emigration between 10% and 50% of its natural population increase every decade. The drain of humanity diminished Scotland's standing in the Union. In 1801, the Scottish population made up one in six of the UK population. Today, it's one in twelve.

The scale of the loss at that time was only greater in two other European countries: Ireland and Norway. However, even emigration from these countries was dwarfed by Scotland's in the years before the First World War and again in the 1920s. In that decade, the 550,000 leaving actually exceeded the entire natural increase and constituted one-fifth of the total working population.

During the age of mass emigration, 1841-1931, over two million Scots emigrated abroad and another 750,000 moved to other parts of the UK. Even as late as the 1960s, 300 Scots emigrated every day.

Things seem to improve in the final decades of the last century. Much has been made of the fact that Scotland's population reached a historic peak of 5,295,000 in 2011. But let's not get too carried away.

Between 1960 and 2011, Scotland's population increased by just 2.6%. Compare that with those other traditional exporters of people. In this period, Ireland's population went up by 62%, Norway's by 40%. Other countries put Scotland's figure to shame: Iceland's population increased 84%, Sweden's 27%, Switzerland's 50% and England and Wales's 24%.

Tellingly, Scotland's slightly improved demographic position this century has only been achieved through immigration.

In 2010-11 alone, about 42,300 people came to Scotland from overseas. Around 16,900 Scots left to go overseas, giving a net migration gain from overseas of around 25,400. Over the decade 2001-11, Scotland's foreign-born population almost doubled, from 191,600 to 369,300. There are now more native speakers of Polish in Scotland than Gaelic.

These immigrants, mainly young and energetically ambitious, are very welcome. We benefit from them enormously. But we need something better than a plug-the-gaps approach which depends upon other economies in the EU being weaker than ours. The Polish government is currently aiming to establish an economy that by the 2020s will not only stem their flow of emigrants but attract back many of the departed. Then what will we do?

While these young immigrants arrive in Scotland, it's estimated that 30,000 young Scots leave every year for England or abroad. Of course, it's good that our young people go to live and work in foreign countries. It's the time of life to see the world, broaden the mind and develop new skills. It's just a pity that, in the main, Scotland won't benefit.

Most of the 30,000 probably intend to return to Scotland. Few of them will.

So many will find that it's much, much easier to leave Scotland than to return. Once a job is found elsewhere, a career path beckons. Training opportunities are offered, promotions gained. Alternatives in Scotland are limited or non-existent. Eventually, a partner comes along, children, in-laws, new friends. The ties that bind. Location is no longer an individual whim. The prospects of returning home slowly recede.

Scotland's population statistics over the past three centuries are a searing indictment of how the Union has failed our country. If September's No vote means business as usual and more hellish austerity economics, we can be sure of one thing.

In the years ahead, tens of thousands of Scots will continue to leave their homeland for ever.