406 FURTHER EVOLUTION 



constant presence of algae within the bodies of some infusoria 

 and other kinds of animals. The scientific literature also 

 contains references to large numbers of cases of parasitism, 

 not only among green algae, but also among blue-green, 

 diatomaceous and brown and purple forms. ^* 



In this, as in other forms of parasitism, regression undoubt- 

 edly takes place, the loss of the internal chemical abilities 

 which the original organism possessed and which enabled it 

 to build up the necessary organic substances autotrophically. 

 However, this return to the past could not occur so readily 

 unless there were already present some phylogenetically 

 earlier mechanism for heterotrophic nutrition. 



The higher green plants which have a very highly devel- 

 oped apparatus for the synthesis of different substances have, 

 to a large extent, freed themselves from dependence on pre- 

 formed vitamins, if we consider the organism as a whole. 

 Nevertheless, separate parts of such plants grown in isolation 

 in tissue cultures are absolutely dependent on an exogenous 

 supply of vitamins and other organic substances." 



Not only do many green photoautotrophs need specific 

 organic substances but, in general, they can all very easily 

 be induced to nourish themselves on ready-made exogenous 

 organic compounds, notwithstanding the fact that during the 

 process of evolution, they long ago acquired the ability to 

 synthesise these substances for themselves from mineral salts 

 at the expense of energy derived from sunlight. Such an easy 

 transition to ordinary heterotrophism demonstrates once 

 more that the metabolism of photoautotrophs is based on 

 chemical mechanisms which can derive energy from ready- 

 made organic substances. 



It is understandable that the less highly organised photo- 

 autotrophs are specially liable to manifest their tendency to 

 heterotrophism and revert to it with particular readiness 

 under both laboratory and natural conditions. 



As an example of complete transition from autotrophic to 

 heterotrophic nutrition we may mention the experiments of 

 C. Ternetz-" and a number of later authors on Euglena. 

 Starting from the green forms of this organism it is possible 

 to obtain completely colourless forms which can only nourish 

 themselves heterotrophically. This is done by cultivating 



