 What is both a gift and curse of humanity?
"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. 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." The theory is that humans who can mate with each other were once physically close. "And whereas there are humans who are relatively near and not fertile, they can always mate with those they were adjacent to two hundred thousand years ago !" It was true that Kelburn was the most advanced human type present, but while there were differences, biological and in the scale of intelligence, it wasn't as great as once was thought.
 They have a tendency to unify against a potential threat, but are unaware of how they could overtake other species through strength in unification
 They are intellectually superior, but unable to mate with other species (and therefore grow their population) due to galactic mating laws  They are biologically superior, but their restrictive mating abilities prevent them from growing their population as fast as other species  They have a tendency to unify against a potential threat, but are unaware of how they could overtake other species through strength in unification  They have a tendency to fall in love easily, which is difficult to manage when they become attracted to 'higher-level'  species, who are off limits