question:  What is the governing principle that classifies the characters in the story?
context: "There are at least a hundred kinds of humans, each supposedly originating in strict seclusion on as many widely scattered planets. Obviously there was no contact throughout the ages before space travel— and yet each planetary race can interbreed with a minimum of ten others ! "Outside of your own world, there's nothing like your species, except superficially, and that's true of all other creatures, intelligent or not, with the sole exception of mankind. Actually, the four of us here, though it's accidental, very nearly represent the biological spectrum of human development. It's reasonable to assume that, if men were set down on a hundred such worlds, they'd seem to fit in with native life-forms on a few of them. That's what happened on Earth; when Man arrived, there was actually a manlike creature there. "But there are other worlds in which humans who were there before the Stone Age aren't related to anything else there. Man was the most numerous though not always the most advanced—Ribboneers had a civilization as high as anything in the known section of the Milky Way, and there were others—and humans were more than a little feared. When we project back into time those star systems on which humans existed prior to space travel, we get a certain pattern. This is one arm of it as it is today and here are the human systems." And while stars in a given region tend to remain together, there's also a random motion. "The adjacency mating principle. Anyway, non-humans weren't trained in the fine distinctions that men made among themselves. We have men who could do the job in a makeshift fashion, but the region we're heading for, while mapped, is largely unknown. It was a paradox that some biologically higher humans hadn't developed as much as lower races and actually weren't prepared for the multitude of life-forms they'd meet in space.
output:  ancestral bloodline
options:  ancestral bloodline  physical biology  galactic prevalence  intellectual status