52 SYMBIOGENES1S 



such a reversion by the very fact of their protracted specialisa- 

 tion as independent plant complements. Specialisation must 

 entail limitations of some kind. Evolution has proceeded by 

 biological symbiosis superposed on domestic or primitive 

 symbiosis. Primitive symbiosis proved useful wherever 

 biological symbiosis for one reason or another was impractic- 

 able, as in the early ages, and possibly for purposes of pioneer 

 work after glacial periods, as it does now in the case of the 

 arctic lichens. 



Prof. Keeble himself suggests that the circumstances 

 making for the adoption of a communal mode of life in the 

 remote past were much the same as in the case of many lichens 

 in the present day, and that whilst the colourless tissues 

 specialised increasingly as purveyors of supplies to the chloro- 

 plasts, the latter specialised increasingly as experts in photo- 

 synthesis. This I believe to be true of all normal instances 

 of primitive symbiosis. If this explanation does not meet the 

 case of the Convohita, as Prof. Keeble evidently thinks, we 

 must remember that we have here essentially a retrogression, 

 a degeneration, and that, too, in the most important, the bio- 

 economic, sense. The case of the lichen stands quite 

 differently in so far as it represents a proper and permanent 

 not a one-sided working arrangement. The partners remain 

 active and co-operative to a great extent, and the result is that 

 the total domestic and bio-economic output of work is 

 increased. 



In the case of the Convoluta, the plant cells by relaxing 

 their normal activity and depending partly on ready-made 

 and surfeiting nitrogenous (waste) material coming from the 

 animal tend to lose their vital photosynthetic power, a loss too 

 great to be made up by their gains in other directions. The 

 animal partner, on the other hand, remains only just 

 sufficiently active to obtain the barest amount of " love-food " 

 indispensable to its sexual reproduction, i.e., the "infection" 

 by the algal cell means scarcely more than a safeguard against 

 complete loss of sexual reproduction, a preventive against 



