This essay was written by Ludwig Boltzmann and published in 1886. In it, Boltzmann writes: “The general struggle for existence of animal beings is therefore not a struggle for raw materials—these, for organisms, are air, water and soil, all abundantly available—nor for energy, which exists in plenty in any body in the form of heat, but a struggle for (low) entropy, which becomes available through the transition of energy from the hot sun to the cold earth.” Lotka cites Boltzmann as one of the first scientists that suggested natural selection should be viewed from a thermodynamic perspective. Lotka uses this perspective to develop his principle of maximum energy flux, which H. T. Odum further develops and renames the maximum power principle.
Boltzmann L. 1886. The second law of thermodynamics. In Ludwig Boltzmann: Theoretical Physics and Philosophical Problems: Selected Writings (ed. B McGinness), pp. 14–32. Dordrecht, The Netherlands: D. Reidel, 1974.