ORGANIC ADAPTATION 



337 



creased intelligence, in particular, makes flexible the stereotyped 

 life as exhibited in the lower forms — the human individual is 

 adaptable to the various community tasks. 



2. Symbiosis 



Associations are not confined to members of the same species, 

 nor are all an expression of cooperative adaptations. All grada- 

 tions occur from those which are mutually beneficial to the parties 

 in the pact, to those in which one member secures all the advantage 

 at the expense of the other. 



The most intimate associations in which the organisms involved 

 are mutually benefited, if not absolutely necessary for each other's 



Fig. 221. — The formation of a Lichen, Physcia paratina, by the combina- 

 tion of an Alga and a Fungus. A, germination of a Fungus spore (sp), whose 

 filaments are surrounding two cells (a) of the unicellular Alga, Cystococcus 

 humicola. B, later stage in which spores have formed a web of filaments 

 (mycelium), enveloping many algal cells. Magnified about 400 times. (From 

 Bonnier.) 



existence, are termed symbiotic. A familiar case is the common 

 green Hydra (Chlorohydra viridissima) that owes its color to the 

 presence of a large number of unicellular green plants which live 

 in its endoderm cells. The products of the photosynthetic activity 

 of the plant cells are at the disposal of the Hydra, and the latter 

 in return affords a favorable abode and the material necessary 

 for the life of the plants. 



