Or why is Aerth so empty?
Of all the guides in Dangerous Journeys Mythus is most likely the one with the most, and the most advanced, magick. This has consequences. One is, Mythus is high magick (at least as far as I see), and second, through the use of magick the worlds of Ærth has learned things one may not expect them to.
For the most part the reason why comes down to divination that works. Thanks to that Ærth has learned things. A number of things, but the most important ones for our purposes have to do with agriculture. Such as the superiority of Sung agriculture, which is now used by most everyone, including the Lemurians (while they are racist pigs, they are not stupid). And divinations directed at discovering good times to plant, and to reap, and the best way to transport crops to market. Learning the best crops to grow is a big part of this.
And this has one huge consequence, it only takes a quarter of the land that would other wise be needed to support a population.
In part 2 of this series we’ll have a look at how this affects just exactly how much territory a population of 500 million requires.