METABOLISM. 63 



these two ultimate results are brought about. The materials 

 forming the tissues of plants as well as the materials con- 

 tained in them, are progressively elaborated from the inor- 

 ganic substances; and the resulting compounds, eaten and 

 some of them assimilated by animals, pass through successive 

 changes which are, on the average, of an opposite character: 

 the two sets being constructive and destructive. To express 

 changes of both these natures the term " metabolism " is 

 used ; and such of the metabolic changes as result in building 

 up from simple to compound are distinguished as " anabolic," 

 while those which result in the falling down from compound 

 to simple are distinguished as " katabolic." These antithetical 

 names do not indeed cover all the molecular transformations 

 going on. Many of them, known as isomeric, imply neither 

 building up nor falling down: they imply re-arrangement 

 only. But those which here chiefly concern us are the two 

 opposed kinds described. 



A qualification is needful. These antithetic changes must 

 be understood as characterizing plant-life and animal-life in 

 general ways rather than in special ways as expressing 

 the transformations in their totalities but not in their details. 

 For there are katabolic processes in plants, though they bear 

 but a small ratio to the anabolic ones ; and there are anabolic 

 processes in animals, though they bear but a small ratio to 

 the katabolic ones. 



From the chemico-physical aspect of these changes we 

 pass to those distinguished as vital; for metabolic changes 

 can be dealt with only as changes effected by that living 

 substance called protoplasm. 



23&. On the evolution-hypothesis we are obliged to as- 

 sume that the earliest living things probably minute units' 

 of protoplasm smaller than any the microscope reveals to 

 us had the ability to appropriate directly from the inor- 

 ganic world both the nitrogen and the materials for carbo- 

 hydrates without both of which protoplasm cannot be formed ; 



