Y'know, rather strangely, it doesn't seem like the world actually works anymore.
I lost my very first fortress thanks pretty much entirely to a water-moving accident that drowned basically everyone except for two children and one adult, all male. That kinda of ended any chance of the dwarven race living on via this fort, so I abandoned and tried again.
As early as Summer in the first year though, this new fort got immigrants from the previously non-existant Whips of Omens. The civilization just spontaneously reappeared in the world along with them. I thought maybe it had to do with my having abandoned a fortress and the three survivors being left behind still counting as members of the Whips of Omens, thus meaning the civilization now truly existed again.
I tried regenerating the world from scratch, but I was still getting immigrants. Even using the saved version world I uploaded earlier got the same results. Very odd. Had my first immigrant-less fortress been some sort of fluke? Where the heck had all these new dwarves come from?
The first fort had existed for more than 12 years, long enough for the first two children to become adults, had a total value that was nearing half a million, and yet never did a single immigrant appear. Meanwhile, different forts in 'clean' versions of the exact same world would get them all the time. O_o