"Young people are always asking what it's like to be on a spaceship... Well, what does it feel like? Because that's what you're on. The Earth is a very small spaceship hurtling through space. ...This spaceship is so superbly designed that we've had men on board here for about two million years reproducing themselves, thanks to the ecological balance ...all the vegetation is breathing up [out] all the gases needed by the mammals and all the mammals are giving off all the gases needed by the vegetation ..."