EVOLUTION AND ENTROPY 317 



meaningless unless it refers to equilibrium states." '^ Heat 

 tends of itself to flow from a hotter body to a cooler one — never 

 the reverse — and the flow ends, when the temperatures of the 

 two bodies are equal. Entropy is this tendency to equality, 

 to equilibrium, to uniformity. 



It will be appropriate later on to spell out more carefully the 

 fine-print meaning of equilibrium or uniformity in order to 

 show how general this tendency must actually be and to 

 indicate that this entropic drive is a dedifferentiation by con- 

 trast to the differential structure in an evolutionary process. 

 But let us tighten our hold on the meaning of entropy so far 

 attained. Our second law of thermodynamics reads that the 

 entropy in any closed system always tends to increase to the 

 maximum. The augmentation of entropy is a measure of the 

 use of energy. In a steam engine, for example, some of the heat 

 is dissipated through the machinery and cannot be recovered 

 for use; more generally, in any closed system, energy exchange 

 always involves in the end a dissipation of heat throughout 

 the parts of the system, so that no machine is one hundred 

 per cent efficient. 



The second law of thermodynamics thus records the degrada- 

 tion of energy and, if our cosmos is finite, the downhill drive 

 of the universe itself. The law of the conservation of energy 

 expresses the constancy in quantity of the energy involved in 

 any closed process; but the law of entropy records the quali- 

 tative change in such energy. Some of it passes into a state in 

 which it is no longer available for work. In any machine there 

 is a loss of available energy because of the dissipation of heat 

 through the machine itself. 



Now the law of entropy, like all the laws of thermodynamics, 

 is not of merely local significance like an equation that applies 

 only to electricity or magnetism. All energy can be converted 

 to mechanical energy, and hence all energy transformations — 

 thus all the motions in our universe — involve a change of some 

 of the energy into unusable heat. In other words, since all 



^^ R. Lindsay and H. Margenau, Foundations of Physics (New York, 1936) p. 215. 



