Riffing off Steve Sailer's prime example of an instance where a picture is worth a thousand words, in 1950 sub-Saharan Africa's population density was on par with that of contemporary Idaho (8 people per square kilometer). By the turn of the next century, UN population projections predict that Africans will be packed in more tightly (170 p/sqkm) than people in New York are today. That shakes out to more than a twenty-fold increase in 150 years, a mere six or seven generations.
Europe's population density, in contrast, is projected to remain nearly static over that same period of time, from 24 p/sqkm in 1950 to 28 p/sqkm in 2100, or from today's Mississippi density to that of West Virginia.
Take a moment to dwell on this. In 1950, there were three times as many Europeans in any given place in Europe as there were black Africans in sub-Saharan Africa. In less than a century, there will over six times as many black Africans in any given place in sub-Saharan Africa as there will be Europeans in Europe.
Put in another way, at the close of the 21st century it is estimated that for every one extant descendant of a European alive in 1950, there will be more than 18 living descendants of a sub-Saharan African living at the same time. This is a veritable Darwinian rout.
Parenthetically, the contrast is even starker than it appears at first blush, since over the intervening 150 years net migration has been and will continue to be from sub-Saharan Africa to Europe. In other words, in the year 2100 virtually all of those 170 p/sqkm in sub-Saharan Africa will be black Africans. A lot of those 28 p/sqkm in Europe, in contrast, will be of non-European ancestry.
For some reason I'm not confident that a twenty-fold increase in sub-Saharan Africa's vibrancy over a century and a half is going to be enough to incentivize black Africans to stay put. Excepting Europe's abrupt (and politically unthinkable) adoption of Israeli-style perimeter security on a continental scale, how does camp of the saints not become the story of the 21st century?