 What is the governing principle that classifies the characters in the story?
His four flat legs looped out and in, the ends tucked under his wide, thin body, which constituted the knot at the middle. Of all his features, only his head had appreciable thickness and it was crowned with a dozen long though narrower ribbons. "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. There's a corresponding span of fertility. 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. "Something had to distribute one species so widely and it's not the result of parallel evolution—not when a hundred human races are involved, and only the human race." 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. "We've got new data and are able to interpret it better. The theory is that humans who can mate with each other were once physically close. 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. They were spaced at fairly equal intervals along a regular curve, a horseshoe loop that didn't close, though if the ends were extended, the lines would have crossed. "The adjacency mating principle. On some worlds on which there are no other mammals, we're able to place the first human fossils chronologically.
 ancestral bloodline
 ancestral bloodline  physical biology  galactic prevalence  intellectual status