SYMBIOSIS 43 



remain indispensable to the animal. Truly the summary 

 exploitation of the coloured cells by the Convoluta amounts to 

 killing the goose which laid the golden eggs ! It can be done 

 with a certain amount of regularity (though even so, 

 physiologically, not with impunity) so long as a sufficient 

 supply of free cells is always offering itself. Thus these primi- 

 tive animals, instead of relying wholly and solely on biological 

 symbiosis which requires give and take have managed 

 somewhat retrogressively to obtain essential supplies of photo- 

 synthesised food-materials by a kind of return to an ancestral 

 mode of domestic symbiosis, which allows them both to supple- 

 ment other supplies and to replace them at times by a less sur- 

 feiting and physiologically more suitable nutrition. It is a 

 compromise which suffers from its one-sided nature and conse- 

 quent lack of stability, a consortism (rather than a symbiosis) 

 which is opposed to the main lines of evolutionary progress. 



It is worth noticing, again, how in the case of both 

 C. roscoffensis and C. paradoxa even a limited reliance upon 

 the seasonal gifts, however ill-requited, of the green plant 

 implies : 



(a) Extraordinary powers of resisting starvation ; 



(b) An astonishing degree of independence an income 

 which, though but for a time, renders predaceous and blood- 

 thirsty departures (an intense "struggle for existence") 

 unnecessary ; 



(c) Proper bio-chemical material and stimulation for 

 growth and (by providing the proper food substance for the 

 eggs) for fertilisation, i.e., for rejuvenescence and sexual 

 reproduction. 



But notwithstanding all these benefits, forbearance and 

 return of service on the part of the animal are inadequate to 

 set up such a genuine union as exists between the parts of a 

 green plant. The association of cells in Convoluta fails to pro- 

 vide opportunities for the establishment of such mutual 

 reproductive expenditure as would be required for the propaga- 

 tion of the union as a whole. 



