It has been pointed out by Boltzmann that the fundamental object of contention in the life-struggle, in the evolution of the organic world, is available energy. In accord with this observation is the principle that, in the struggle for existence, the advantage must go to those organisms whose energy-capturing devices are most efficient in directing available energy into channels favorable to the preservation of the species.
Lotka, A.J., 1922. Contribution to the Energetics of Evolution. Proceedings of the National Academy of Sciences, 8, pp.147–151.