The economic life of the human race had for some time been based on coal, but latterly oil had been found a far more convenient source of power; and as the oil store of the planet was much smaller than its coal store, and the expenditure of oil had of course been wholly uncontrolled and wasteful, a shortage was already being felt. Thus the national ownership of the remaining oil fields had become a main factor in politics and a fertile source of wars.

Chapter I: Balkan Europe; Section 4, The Russo-German War (pp. 29-30) - Last and First Men (1930)