By interpreting primary energy forms as commodities, competing worldwide, Marchetti (1977) has revealed a remarkably stable substitution process. Figure 3.1 identifies the historical shifts from wood and farm wastes via coal to crude oil and natural gas. It is important to note that the regularity of these structural changes was not really influenced by such exceptional events as the First World War, the Second World War or the economic crisis of the 1920s and early 1930s. The 35 years following 1914 limited the growth of total energy consumption (Figure 3.2) but they were not able to interrupt the striving for ever cheaper and easier forms of energy.