 Within the setting of the story, what makes one character more attractive than the other?
"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. The theory is that humans who can mate with each other were once physically close. 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.
 Evolutionary status
 Having a pure genetic line  Reasoning capacities  Evolutionary status  Lineage to the big ancestor