4 PROTOPLASMIC ACTION AND NERVOUS ACTION 



(i.e., definitely characterized and unique) in its structural 

 and active characters. This building-up, by means of 

 metabolic construction, of a complex system, specific in 

 chemical composition, structure, and activity, out of 

 relatively simple non-specific materials taken from the 

 surroundings (food, water, salts) is the fundamental 

 general peculiarity which distinguishes living organisms 

 from non-living systems. Constantly associated with 

 the constructive group of reactions is the destructive or 

 catabolic group by which substances contained in the 

 protoplasm are broken down, usually oxidized, to yield 

 the energy freed in vital activity. A great diversity of 

 compounds are thus utilized by protoplasm as sources of 

 energy; the catabolic process is non-specific; i.e., sugars, 

 fats, and proteins are metabolized to yield the same end- 

 products (CO2, water, urea, etc.) and energy in organisms 

 of all kinds. 



2. GROWTH, MAINTENANCE, REPRODUCTION, 



AND HEREDITY 



It is essential at the beginning of any study of 

 fundamental vital properties to recognize the dependence 

 of the various phenomena designated by the four terms 

 above upon the fundamental process of specific construc- 

 tive metabolism. In every organic individual normal 

 self-maintenance, by which the material lost as a result 

 of metabolic destruction is replaced by new construction, 

 involves the same specific synthetic reactions as those 

 concerned in growth. And growth is obviously a 

 highly specific process; this becomes evident whenever 

 a seed or an egg ''grows into" the specifically organized 

 adult. Organic growth thus involves or implies ''hered- 



