METABOLISM. ^3 



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. 



§ 236. 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 ; 



