26 PROTOPLASMIC ACTION AND NERVOUS ACTION 



with the environment and explain why so large a part 

 of biological investigation has reference to the inter- 

 relations between organism and environment. 



We may here recall Spencer's characterization of life 

 as essentially a continual adjustment of internal to 

 external relations.^ Such an abstract definition, how- 

 ever, appHes to many other systems found in nature; e.g., 

 to any system in '' dynamic equilibrium," such as a 

 candle flame, a whirlpool, or other physical system in 

 which there is an automatically regulated balance 

 between the material and energy supplied to the system 

 and that lost to the environment. Nevertheless, it is 

 peculiarly true of organisms that their processes are of 

 such a kind as to maintain constantly a certain special 

 complex of structural and active characters in spite of 

 internal and external changes. The requirements for 

 such maintenance vary in the different cases, but certain 

 conditions are universal. The primary condition is 

 that material must be taken from the outside that will 

 serve (i) as a source of energy (to replace substances 

 consumed in supplying this energy) and (2) as building 

 material for the structural substratum (protoplasm) in 

 which the energy-yielding transformations occur; in 

 this second class are included substances which do not 

 serve directly as sources of energy — e.g., inorganic salts. 

 Considered from the most general point of view, there- 

 fore, the living organism exhibits (i) a continual trans- 

 formation of material taken frona its surroundings into its 

 own specifically organized substance; and (2) a continual 

 chemical decomposition of portions of this substance 

 of such a kind as to furnish free energy which is utilized 



^ Principles of Biology. 



