68 SYMBIOGENES1S 



and concurrently I have shown work in the production of 

 social values to be the central issue of the evolutionary 

 process, and shown how untenable are those views which are 

 based on a neglect of these two fundamental factors. From 

 the detailed study of symbiosis it must now have become more 

 apparent than before that unless the fact of the subordination 

 of most other factors to the bio-economic factor is consistently 

 taken into account, only very inadequate and partial or local 

 views respecting the significance of biological phenomena are 

 likely to prevail. 



Having now, as I hope, seen sufficient reasons for a more 

 discriminating judgment upon symbiosis and its role in the 

 organic world generally, we are now in a position better to 

 appreciate the case of the lichens in particular. 



The symbiotic association between a (generally higher) 

 fungus and a (generally unicellular, sometimes filamentous) 

 alga forming together what is called a lichen, presents a 

 striking illustration of the surpassing significance of 

 co-operation and symbiogenesis generally. It has needed 

 years of painstaking research and of controversy to establish 

 the compound and dual nature of the lichen. The special 

 fungi which take part in the association are, with rare excep- 

 tions, not found growing separately, whilst the algal forms 

 are constantly found free. The reproductive organs of the 

 lichen are of a typically fungal character. The algal cells are 

 never known to form spores whilst forming part of the lichen- 

 thallus, but they may do so when separated from it and grow- 

 ing free. 



"The fungus," says the Encyclopaedia Britannica, "thus 

 clearly takes the upper part in the association." 



The algal cells in this case are like the majority of the 

 (vicarious) plantagens of a tree, which remain content with 

 asexual reproduction in order the better to subserve the pur- 

 poses of domestic and biological symbiosis, and they are 

 satisfied to depute both the high privilege and the onerous 

 duties of sexual reproduction to the more expert fungus. 



