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 wltimate 
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. 
