It seems to me that not enough consideration has been given to the fundamental importance of the conditioning to which our physical as well as our mental being have been submitted due to the fact that they develop within a certain gravitational field. All the long evolution of the species seems to proceed from our desire for the impossible abolition of our own gravity, or at least for its partial alleviation. Many centuries before the appearance of man compensatory means were tried and adopted, ranging from the acquatic bladder to the wing and including the simple hopping of the marsupials.