98 MAN --AN ADAPTIVE MECHANISM 



thus begun in the lowest forms of life, continues 

 throughout all species of animals, and is waged per- 

 petually with varying success for each participant. 

 There occurs now the death of the "host," now the 

 defeat of the "parasite"; and now, again, a mutual 

 compromise symbiosis which is of benefit to both. 

 There are many interesting examples of this constant 

 warfare in the bodies of men and animals. In fact, 

 infection is but an illustration in miniature of that 

 great principle which is being demonstrated on a 

 larger scale throughout all nature, namely, that it is 

 not man alone, but all the world besides, which is under- 

 going "adaptation to environment." 



Symbiosis and Parasitism 



Out of these reciprocal adaptations of animals to 

 plants, plants to other plants and animals to other 

 animals result some of the most characteristic phe- 

 nomena of life. Thus, the color, the shape and the 

 nectar of flowers are undoubtedly determined by the 

 physical forms and habits of the fertilizing insects 

 which visit them. On the other hand, the proboscis 

 of the insect, its wings and perhaps its sense of smell 

 have, in turn, been evolved by the existence of the 

 flower from which it gets its chief nourishment. The 

 nectar of the flower supplies the insect with the re- 

 quired carbohydrate fuel to make its flight that is, 

 the flower furnishes the motor power, the "gasoline." 

 These interrelations and interdependencies of one 

 organism upon another become so firmly established, 

 after a time, that to alter their status quo in any respect is 

 often to alter the life equation for numerous organisms. 



