ENERGY METABOLISM 437 



second place, the anaerobic method of degrading these sub- 

 stances. 



As the conditions of existence changed so, in the course 

 of evolution, metabolism became more highly developed. Its 

 primary mechanism became encrusted with more and more 

 new ' accessories ' which were different in different organ- 

 isms, but the basic organisation common to all living matter 

 remained as before. A study of this may therefore help us 

 to some extent to judge of the external conditions which 

 prevailed at the time when life first appeared, and of the 

 ways by which it arose. 



The main, and perhaps the sole sources of organic nourish- 

 ment for the first living things would seem to have been 

 hydrocarbons and their various derivatives which had been 

 formed on the surface of the Earth. The reserves of these 

 substances, though they may have been supplemented to 

 some extent, were, in any case, very limited. In the mean- 

 while the growth and multiplication of organisms led to a 

 greater and greater consumption of organic materials. In 

 part they entered into the composition of living bodies, but 

 an even greater quantity was broken down, degraded, during 

 destructive metabolism. 



Thus, the reserve of organic substances in the external 

 medium available for the nourishment of the first organisms 

 must, all the time, have been diminishing in quantity and 

 becoming qualitatively simpler. This disappearance intensi- 

 fied the struggle for existence and was a potent factor in the 

 later evolution of the original organisms, inducing further 

 integration and complexity in their internal chemical organ- 

 isation. But if the evolution of living things had always been 

 confined to heterotrophic means of nutrition, then, sooner 

 or later, the process must have attained its final conclusion 

 with the complete annihilation of all organic nutrient ma- 

 terial and the destruction of all living things. 



This stimulated the organisms in their struggle for exist- 

 ence, in the process of selection and adaptation to the new 

 conditions of life with ^vhich they were faced, to elaborate 

 within themselves new forms of metabolism which would 

 enable them, not merely to assimilate exogenous organic 

 materials as rationally as possible, but also to use other means 



