100 



Fig. 23 



Circular phylogenetic classification of the Filicales. Families and 

 subfamilies are arranged so that their radial position corres- 

 ponds to their relative advancement (primitive near the centre; 

 advanced near the outside). Broken lines enclose 'areas of 

 affinity', indicating close relationship. The numbers represent 

 successive grades of relative advancement, expressed as a per- 

 centage ('the advancement index'), ranging from the most 

 primitive (0%) to the most advanced (100%) 



characters listed above are taken into account, it is possible 

 to calculate roughly an average 'advancement index' for 

 each family or subfamily, ranging from o per cent (the most 

 primitive) to lOO per cent (the most advanced). "^^ This has 



151 



