 What is NOT true of the crew?
"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. 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.
 They could all mate with each other (it was explicitly discussed as a possibility in the story)
 They are all different species  They are all focused on finding evidence to support an important theory  They are all respected in their field  They could all mate with each other (it was explicitly discussed as a possibility in the story)