 What is both a gift and curse of humanity?
In repose, Taphetta the Ribboneer resembled a fancy giant bow on a package. 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. Emmer just misses being able to breed with my kind, but there's a fair chance that I'd be fertile with Meredith and a similar though lesser chance that her fertility may extend to Kelburn." "Humans require a certain kind of planet. 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. "And so, to account for the unique race that can interbreed across thousands of light-years, you've brought in the big ancestor," commented Taphetta dryly. "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.
 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