SYMBIOSIS 51 



We may reasonably assume that it was due to the increas- 

 ing photosynthetic efficiency and general productiveness of the 

 plant that changes of form and of character with subsequent 

 progressive development generally became possible in the 

 organic world. 



As regards the primitive Euglena viridis, we can see from 

 Prof. Keeble's remarks how the character of an organism 

 fluctuates with fluctuations in its modes of nutrition. 



In certain circumstances chlorophyll disappears from the body, and 

 Euglena viridis passes into a colourless phase. When in this state the 

 animal, if it is to feed at all, must do so by ingesting ready-made food. 

 That is, from being a holophytic organism one with a typically plant- 

 like mode of nutrition it becomes heterotrophic, that is, it feeds on 

 ready-made organic materials, obtained from its environment. After 

 a time it may reconstruct its chlorophyll and become free once more to 

 manufacture by photosynthesis its organic food-substances from the raw, 

 inorganic materials of its environment. 



Here we have only an alternation of holophytic and hetero- 

 trophic habits and of the respective characters. The power of 

 photosynthesis is not, however, permanently lost. The 

 organism is even capable of a double state of what I have 

 termed cross-feeding. In course of progressive plant-animal 

 development the differentiating animal parts had to justify 

 their existence by becoming useful adjuncts to, rather than 

 devourers of, the strenuous green cells ; nor could they indulge 

 in reckless and indiscriminate " heterotrophism." 



As regards the alternations of feeding habits and of form, 

 Prof. Keeble puts the question : ; 



If one species of animal can do this, why should not another even 

 more highly developed species possess like powers? Why should there 

 not appear, here and there, animals which resume the habit possessed 

 by their ancestors, construct chlorophyll and become independent, photo- 

 synthesising organisms ? 



The answer is that this would constitute a retrogression 

 from the bio-economic point of view, and that the bio-economic 

 factors now established in the world are too powerful to permit 

 of such retrogression. Moreover, animals are debarred from 



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