LIVING ORGANISMS 29 



processes, by the interaction of the elements of 

 food and of the oxygen of the air. There is 

 thus action and reaction between the living 

 being and the conditions in which it lives. 

 These conditions are termed the environment. 

 The living being has, within limits, the faculty 

 of suiting itself to the surrounding conditions. 

 Such a power, which is a necessary condition 

 of the existence of every living being, is known 

 as its variability. It is the power of adaptation 

 to external conditions. 



