Two thousand years of theories:
At the outset of taking an interest in this subject, I had no idea how that it was true, much less how important it would prove to be that urban populations are inherently unstable.  I assumed that a phenomenon of such magnitude could not have escaped notice.  It was only after seeing the proof that I could go back and trace the great conversation that addressed the issue. 

I once saw on television, and alas I cannot say if it was the History Channel or the Discovery channel, that the topic had been discussed long ago.  The printed page has the advantage that something interesting can be marked with a bookmark or copied a few hours later.  It often takes some time for something to sink in.  Television is harder to recover. 

But it was noticed, was discussed and explanations were offered.  Of course whether a woman conceived and gave birth has been attributed to fate or some supernatural influence in many places and at many times, but now I am talking about practical explanations.

As I remember, the lecturer featured on the program described a conversation that took place more than two thousand years ago in Asia Minor between a barbarian and a Greek who lived in one of the cities of the Greek Diaspora.  The barbarian said that anyone who drank from a certain well in the city became effeminate.  The Greek, from who we got the story, countered that the well was in the middle of the town.  Anyone who came that far into town could see the obvious advantages of a civilized life and naturally was eager to adopt a civilized life style.

I do not know what the barbarian meant by “effeminate” or whether the Greek even understood the same thing.  But it seems reasonable to assume that being manly included at the very least making women pregnant.  The conversation seemed to imply that the barbarian thought the well was somehow tainted.  The Greek thought having babies was not so important to a person who was blessed with being civilized.  It was a matter of choice.  So the debate appears to have been as to why there were so few children born of the people in the town, and the opinions were pollution and free choice.

The ancient world collapsed and cities, at least in the West, just about vanished.  That was to be expected if people stopped moving to them.  And people had plenty of incentive to move into ancient cities.  There was free grain distributed because after Augustus the output of Egypt was the private property of the emperor.  Distributing grain boosted his popularity and assured immigration.  The Romans recruited vigorously by other means.  They defeated people in war and brought them home to be slaves.  That ended with the empire.

With the inevitable collapse of the cities, civilization waned and the Dark Ages reigned.  When cities sprang up again, people began to carry swords.  This had not always been true, and the explanation may be this.  They noticed that there was violence in the cities, as there has been violence of one form or another throughout recorded history.  The fact that cities were always recruiting could be attributed to murders, which presumably then as now drew interest out of proportion to the actual numbers of lives lost.

So people started carrying swords, at least the young men in Europe.  And anyone who was worried could get a young man to escort him or her. 

But it didn’t work.  Cities continued to fail to produce enough babies to survive.

So the next obvious step was to create effective police forces.  That probably really did reduce the amount of violence, but more important it created a group of professionals to keep track of the murders.  In doing their job they were in a position to point out just how few the murders were in comparison with the numbers in the population.  So we continue to have police and to keep track of crime, but no one seriously blames falling population on private violence.  (Of course there have been times when government sponsored murders were not trivial numbers, but those have occurred less regularly.)

So with an efficient police force in effect and numbers still declining so that migration into cities continued to be required, they turned to blaming the swords.  Young men do like to play aggressively, and it is great fun to play with swords.  You get to intimidate your friend without having close physical contact.  You are not likely to have a lot of young men carrying around swords for very long before you hear the clink of metal on metal.  And of course it was dangerous.

You can, without the slightest malice, kill your friend with a sword.  I myself have more than once struck a friend over the heart hard enough to have killed had I not been using a blunt blade.  After the second time, in which I accidentally touched a highly expert swordsman, I gave up playing with them altogether.  He was unhurt, but I was horrified.  I thought I had control of the instrument.  You could throw a small chip of wood to me and I could flick it away with the blade most times.  But even with the best of intentions I could not keep complete control.

Yes there are ways to play safely, but they are far less fun.  You have to wear padding and a mask.  If you must fence, do so, but be sure to follow all the safety rules.  Alas, healthy young men died.  I am sure dueling became more formal, more ill tempered and more deadly over time, and much grief was caused.  Otherwise almost indestructible young men were dying needlessly.  And the population could not sustain itself. 

So they blamed it on the swords.  Dueling was banned.  It was made illegal, and finally it effectively vanished. 

But it still didn’t work.  Cities still had to recruit.  So next they invented something called élan vital.  It was some sort of life force that manifested in nature but was depleted in the crowded streets of a town.  Now when I was growing up, you could tell the children from the country.  Their complexions were not the bright clear smooth ones of the children who lived in town.  You could not have convinced me that élan vital made those country boys superior.  Oh they were tough and energetic, maybe a little taciturn.  But they did not radiate health that the town boys lacked. 

Charles Dickens in Oliver Twist has a character remark that there are, “Beef faced children and mealy faced children.”  I suspect Dickens had noticed the same thing I had noticed.  As for me, I was somewhere in between.  We ate what would now be considered an exemplary diet, right in various kinds of vegetables, with adequate but not excessive meat, poultry and dairy foods and no sweets.  I never lacked any life force that I was aware of.  But I did not have the clear complexion of some of my friends on richer diets. 

So what did the country people look like at the time?  Here is a long lost painting by van Gogh of a Dutch peasant girl.  You may differ, and indeed the painting is invisible.  The image was recovered by meticulously scanning beneath another painting looking for the telltale traces of the elements used in pigments at the time and reconstructing the image by knowledge of what colors those pigments were. 

Figure of a Dutch peasant girl from “The Hidden van Gogh,” Phillip Bell, NATURE, vol 454, 31 July 2008, page 563. 

To my eye this is a city girl.  Her muscle mass and tone are good, her color is high, and beneath the fatigue her eyes are bright.  I suspect he moved the left eye as he worked.  So did Dutch peasants have better nutrition than the farm boys I knew?  Maybe.  But this painting was probably made in 1884-85.  The term élan vital had not been coined, but it looks like she’s got it.  In fact, by the time of the painting, and thus long before the concept élan vital was formalized, Louis Pasteur had already done the basic work that showed that germs do not form spontaneously.  Life only comes from life. 

Once the germ theory caught on, we all did a 180 degree turn and decided that it was not that the country was teeming with life but that the city was sickly with it.  The failure of urban populations was blamed on infectious disease.  And that, retroactively, has been applied to all cities at all times everywhere. 

Only that doesn’t work either.  Infectious disease is still with us, but in terms of the numbers lost to it before reproductive age, it simply does not account for the lack of growth of cities from within.

So we are back where we started.  Maybe it’s free choice.  Maybe it is something in modern food preparation.  But there was no evidence for either of those in Asia Minor and there is no evidence now.  What we do have is that there is a genetic limit to population diversity, to effective gene pool size, and that cities time out of mind have heedlessly exceeded the limit with consequences catastrophic to civilization after civilization.

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