 What is the governing principle that classifies the characters in the story?
"There are at least a hundred kinds of humans, each supposedly originating in strict seclusion on as many widely scattered planets. "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. When we project back into time those star systems on which humans existed prior to space travel, we get a certain pattern.
 ancestral bloodline
 ancestral bloodline  physical biology  galactic prevalence  intellectual status