COMPOUND ORGANISMS 203 



encountered in nature are just those which are 

 fitted for actual environmental conditions. 

 Plants not so adapted are unable permanently 

 to occupy any position at all. The positively 

 unfit are speedily exterminated, and only 

 those combinations which give good results 

 can persist. But the " good results " are 

 primarily the result of particular operation 

 of internal factors. They arise as the in- 

 evitable consequence of the particular algal 

 and fungal combination, and they are quite 

 independent and irrespective of ultimate 

 adaptation to light or other external conditions. 

 These external conditions are the tests which 

 largely determine not the origin, but the 

 persistence (or extinction) of each and every 

 individual sort of lichen. 



It may not be possible to push very far 

 our analysis of the factors involved in the 

 genesis of form and structure on the one hand, 

 and those correlations of growth wherein so 

 many " adaptive modifications " consists on 

 the other. It may well be, however, that 

 an experimental study of lichens is destined 

 to throw light on much that is now obscure. 

 For in witnessing the synthesis of a lichen, 

 and the modification in structure and habit 

 which results from the association of the two 

 symbionts, we seem to have caught a glimpse 

 of the secret methods and processes which 

 direct the evolution of organic form. 



