EVOLUTION AND ENTROPY 315 



expanding. They hold to the formation of heavier elements 

 out of hydrogen as one of the general principles for matter's 

 development. They re-interpret the evolutionary movement so 

 that it is endless in both directions and so that any deficit 

 caused by " evolution " is continually being overcome. But 

 they do recognize some kind of cosmic advance, and hence our 

 later assessment of evolution will include the evolutionary 

 aspect of the steady-state theory itself. 



Our only point so far is that the two leading contemporary 

 cosmologies are theories of evolution. 



II 



The concept of evolution, an up-hill tendency, forms one part 

 of the paradox in modern cosmology; the other is the law of 

 entropy, the second law of thermodynamics. This law was 

 actually stated by Carnot in 1824 and hence it is also called 

 the Carnot principle. Although still invoking the caloric theory, 

 Carnot likened a heat engine to a hydraulic system, say a mill 

 wheel. The gist of Carnot's argument did not become explicit 

 until subsequently, and so his views will be here summarized 

 in the later and more polished form taken from other pens.*^ 

 In the case of the hydraulic engine, to restore a quantity of 

 water to an earlier position at the top of the wheel, energy 

 must be supplied to the wheel from some source besides the 

 quantity of the water in question. Simplifying the analogy still 

 further, let us imagine a source of water and a sink below it. 

 To drive the water from the sink back to the source, the water 

 and the sink are not enough; we have to supply energy from 

 the outside, for instance with a pump or a heater. Were it 

 possible, from within any closed mechanical system, to restore 

 the system to its initial state after a disturbance of this original 

 set of conditions, a perpetual motion machine could be con- 

 structed, and one of the ways of phrasing the Carnot principle 



*'' Carnot's principle is discussed in P. Bridgman, The Nature of Thermodynamics 

 (Cambridge, Mass., 1950) chap. 2. 



