 What is NOT a technological advancement involved in this story?
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. "But there are other worlds in which humans who were there before the Stone Age aren't related to anything else there. 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.
 Advanced weaponry
 Rapid healing abilities  Advanced space travel  Rapid mutations  Advanced weaponry