Ever since Darwin, there has been a troubled but changing relationship between two great achievements of nineteenth century science, thermodynam­ics and Darwinian evolutionary theory. The second law of thermodynamics, particularly in its Boltzmannian statistical formulation, predicts that as en­tropy increases to a maximum, so will disorder. Eventually, energy differ­ences in the universe will disappear, so that no more work will be possible. Darwin's theory, by contrast, implies the possibility that biological systems can, though not necessarily, increase in ordered complexity over time through the action of natural selection. The increase in heterogeneity that is a hall­mark of life does not seem to follow the arrow of time that points toward the heat death of the universe.