The Origin of Life and Photosynthesis 105 



water for any length of time without becoming oxidized. We could 

 only get over this difficulty if we suppose that the composition of the 

 atmosphere has also changed with time and that there is much more 

 oxygen now than in previous epochs. Where could oxygen have come 

 from? There is much oxygen and nitrogen in the sun and it is not im- 

 possible that atomic particles of these substances which are shot out 

 from the sun are captured by the earth in sufficient quantities to 

 change the atmosphere over long periods of time. Hydrogen is too 

 light to be permanently held in the earth's atmosphere. 



LIFE AND ENTROPY 



There is no doubt that under the influence of life there has been a 

 great elaboration of complex materials on the earth. Can this be ex- 

 plained as a purely natural process, operating under the known laws 

 of physics? At first sight it seems to be contrary to the tendencies 

 which we observe in the non-living universe. Here the direction in 

 in which spontaneous processes occur is always down-hill; energy 

 always becomes more dissipated; materials always pass from more 

 organized states to more mixed-up states. 



This tendency was recognized by Lord Kelvin and formulated as 

 the law of dissipation of energy. Now better known as the second law 

 of thermodynamics, it can be expressed in a number of different 

 ways. One of them, as formulated by Willard Gibbs, makes use of the 

 concept of entropy. Entropy is a measure of mixed-up-ness or dis- 

 order, or of the degree of dissipation of energy, and the mathemati- 

 cians' statement of this law is that entropy is always increased in 

 natural processes which occur of their own accord. 



The growth and elaboration of organisms seem at first to be a 

 direct contradiction of this law. We see in living processes an increase 

 in elaboration and order. It is only when the organism dies that dis- 

 integration occurse and disorder increases. It would thus seem that 

 the whole progress of life on this planet, which has led to the evolu- 

 tion of more and more complex structures, is contrary to the laws of 

 the inanimate world. 



It therefore seems that life has found a way of evading the other- 

 wise universal tendency to dissipation and decay. Kelvin, when he 

 formulated the law of dissipation of energy, made a possible excep- 

 tion in favour of the operations of life. J. N. Lewis thought that living 

 things are 'cheats in the game of entropy'. However, scientific opinion 

 at the present day takes the view that such an exception is unneces- 

 sary. The entropy law only requires that on the whole energy is dis- 

 sipated and entropy is increased. There can be a local decrease of 



