ABSTRACT
It is useful to contemplate what Darwin termed the fifth revolution. This will occur when humankind exhausts the store of the source of the first revolution: fire.
The sources of fire (heat) that powered the industrial revolution have been mainly coal, oil, and natural gas-the fossil fuels. Both Darwin and Haldane (1923) before him point out that resources of these fossil fuels are but centuries from exhaustion. In the long run, we shall have to find alternatives to fossil fuels to produce energy, and both Darwin and Haldane went on to suggest the usual possibilities of harnessing energy from the sun directly, or indirectly through wind, tides, or rivers. The possibility of atomic power was discussed by both Darwin and Haldane, while Darwin also discussed geothermal heat and the use of the vertical temperature gradient of the ocean, as well as the use of plants to produce alcohol as a convenient fuel.
