32 THE FITNESS OF THE ENVIRONMENT 



temperature, pressure, and chemical consti- 

 tution of environment and organism are 

 really essential to life, and that there is great 

 advantage in many other regulations and in 

 finer regulations. Finally, a living being must 

 be active, hence its metabolism must be 

 fed with matter and energy, and accordingly 

 there must always be exchange of matter 

 and energy with the environment. 



Returning to the concept of the organism 

 as a durable form through which flow matter 

 and energy, it is now possible to make these 

 ideas more vivid. The complex structure of 

 the living being is relatively stable, alike in 

 the chemical composition of its individual 

 constituent molecules, in their proportions 

 and amounts, in their aggregation into the 

 invisible structural elements of protoplasm, 

 in the visible parts of the cell, in the organs 

 and tissues, and finally in toto, as a man or a 

 tree. Similarly stable are the physical con- 

 ditions within this structure: temperature, 

 pressure, alkalinity, and osmotic pressure. 

 Finally, that which surrounds it, the imme- 

 diate environment, possesses also a high de- 

 gree of stability, or if the organism be very 

 complex, it may be that it has an efficient pro- 

 tection against change of environment; a skin 

 which insulates, for instance. But in this case 



