436 FURTHER EVOLUTION 



Consideration of the evolutionary aspect of all this material 

 leads one to concur with the opinion of E. S. Guzman 

 Barron^^^ that "... the complexity of the regulating mechan- 

 isms that link fermentation to respiration diminishes as the 

 cells go down the phylogenetic scale." Analysis of the onto- 

 genetic data leads to the same conclusion. ^^* In particular, 

 a study of the ontogenesis of carbohydrate metabolism in the 

 brain of birds and mammals shows that the metabolism of 

 the brain has evolved from being anaerobic to being aero- 

 bic. ^^^ This may be confirmed by the resistance of embryos 

 and new-born animals to anoxia, a resistance which dimin- 

 ishes considerably as the animal becomes mature. It has also 

 been shown that as the animal becomes older the oxidative 

 processes in the brain become more intense while anaerobic 

 glycolysis becomes less intense. N. Verkhbinskaya^^* made a 

 direct study of the intensity of the respiratory and glycolytic 

 processes in the isolated brains of cyclostomes, selachians, 

 sturgeons and bony fishes, amphibians, reptiles, birds and 

 mammals. In this way she was able to show that in the brains 

 of the lower, cold-blooded animals the intensity of anaerobic 

 glycolysis is great, while oxidative respiration only occurs to 

 a relatively slight extent. In warm-blooded animals the 

 relationship between the intensities of respiration and glyco- 

 lysis in the brain seems to be reversed. Respiration increases 

 significantly while glycolysis decreases. This led the author 

 to suggest that during the phylogenetic development of 

 animals there had been a change from the predominantly 

 anaerobic type of energy metabolism in the brain to the 

 oxidative type. 



Thus, intensive comparative study of the metabolism of 

 contemporary organisms shows that, though the conditions 

 of existence on the Earth are different now from what they 

 were when life first arose, nevertheless we find, in any con- 

 temporary representative of the living world, the relics of a 

 primitive organisation which has been inherited from the 

 first organisms and which is, therefore, now common to all 

 the inhabitants of the Earth. These are: in the first place, 

 heterotrophy, the ability to use organic substances as sources 

 of the energy and of primary structural materials needed 

 for the synthesis of the components of protoplasm ; in the 



