404 FURTHER EVOLUTION 



This includes all animals, both higher and lower, among 

 them most of the protozoa, the vast majority of bacteria and 

 all fungi. This fact by itself is very significant. It is, in fact, 

 hardly possible to imagine the evolution of all these multi- 

 farious living things, entirely in accordance with the simpli- 

 fied scheme suggested by Bateson, as the complete loss of that 

 ability to nourish themselves autotrophically which they once 

 possessed.* This is also contradicted by intensive biochemical 

 studies of the whole metabolic system of these organisms. In 

 the heterotrophs we do not find the specific enzymic com- 

 plexes and concatenations of reactions which are character- 

 istic of autotrophs. On the other hand, the metabolism of 

 autotrophs is based on the same internal chemical mechan- 

 isms as that of all other organisms which can only exist by 

 consuming organic substances. This is what allows autotrophs 

 under some conditions to revert so easily to heterotrophy. 

 This can be confirmed, not only by rather intricate bio- 

 chemical analyses of the metabolism of different organisms, 

 but even by comparatively simple physiological observations 

 on their nutrition. 



The colossal amount of factual material at the disposal of 

 contemporary students of vitamins and essential amino acids 

 shows clearly how widely the requirements for specific, ready- 

 made, organic compounds are distributed among all the 

 inhabitants of the world. Of course, these requirements may 

 arise secondarily in a number of individual cases, as a result 

 of a certain regression, the dropping out of particular syn- 

 thetic mechanisms which had previously been built up in the 

 organism. For example, it is possible by certain procedures 

 to cause some particular bacteria, which were previously 

 able to synthesise all the essential amino acids which they 

 required, to lose this ability.^^ The relative ease with which 

 ' mutants ' of this sort can be obtained indicates that the 

 synthetic mechanism in question is not fundamental to the 

 metabolism which enables the organisms to remain alive. 

 The mechanism can be destroyed or removed but the organ- 

 ism continues to exist so long as the surrounding medium 

 contains the amino acids or vitamins which it needs. 



* An extended critique of hypotheses of this sort is given in V. Polyanskii's 

 interesting article. ^ — Author. 



