PHOTOSYNTHESIS 463 



engine could only appear on the basis of pre-existing 

 machines. Before the invention of the cylinder and the 

 dynamo, even the most ingenious constructor could not have 

 built such an engine. 



The appearance of photosynthesis constituted an extremely 

 important stage in the process of evolution of life on our 

 planet. It radically changed all the relationships which had 

 previously existed. Alongside the formation of oxygen in 

 the atmosphere there began a rapid increase in the quantity 

 of organic substances in the biosphere which could once 

 more be put into metabolic circulation by the old hetero- 

 trophic methods. It allowed the main current of evolution 

 to revert to the old channel, with the further development 

 of organisms adapted to nourishing themselves on organic 

 materials. The period of acute scarcity of these substances 

 passed, and there only remained, as a souvenir of it, a small 

 group of autotrophic organisms capable of chemosynthesis, 

 which constituted a side branch of the main evolutionary 

 stream. 



The main channels for this stream now became the green 

 plants (photoautotrophs) and colourless organisms, animals 

 in particular, w'hich were adapted to the earlier, more primi- 

 tive heterotrophic habit. However, after the emergence of 

 photosynthesis, the evolution of even those organisms which 

 used ready-made organic substances for their vital processes 

 took place under entirely different biochemical conditions 

 from those which prevailed before this emergence. 



The decisive condition in this regard was the presence of 

 oxygen in the atmosphere. This allowed a considerable 

 rationalisation and intensification of the process of mobilisa- 

 tion of the energy of organic substances. Naturally, this 

 rationalisation took place on the basis of the same anaerobic 

 mechanisms on which the energy metabolism of the earlier 

 heterotrophs had been founded. ^^^ However, in the process 

 of evolution under the new aerobic conditions, natural 

 selection guaranteed the survival and further development 

 of those organisms in which there arose accessory enzymic 

 complexes and systems of reactions which allowed them to 

 obtain from the exogenous organic substances far greater 

 amounts of high-energy compounds than had earlier been 



