THE PROBLEM 15 



pended particles larger than molecules in the fluid 

 medium, which in the case of protoplasm is water. 

 Living protoplasm may range in its physical condition 

 from a semi-fluid to a stiff jelly-like substance according 

 to the aggregate condition of its particles. This mix- 

 ture of substances, protoplasm, is the visible substratum 

 of the living form, and in it the changes which constitute 

 life occur. Changes in its aggregate condition and in 

 the chemical constitution of one or more of its parts 

 form the basis of specialization and differentiation and 

 so of structure and form, and the energy of the organism 

 originates from certain of the chemical reactions which 

 occur in it. 



Metabolism consists in a complex series of inter- 

 related chemical reactions in protoplasm. On the one 

 hand, nutritive substances are transformed and built 

 up into protoplasm or into other substances characteristic 

 of living organisms, and, on the other hand, portions of 

 the protoplasm and of these other substances are broken 

 down and oxidized, setting free energy, which appears in 

 the various activities of life. What we know of metab- 

 olism indicates that the oxidations are in general of 

 fundamental importance in the whole reaction system. 

 Apparently life cannot continue without them, and the 

 other reactions are to a greater or less extent associated 

 with and dependent upon them. In a given organism, 

 under given external conditions the rate of oxidation is 

 in some degree a measure of metabolic activity and of 

 life. Objection is sometimes made to the term ''metab- 

 olism" because of its vagueness. It is of course true 

 that we do not know all the various reactions and their 

 relations to each other and to other conditions, but we 



