LIFE AND LIVING THINGS 35 



active because the functioning of such an organ depends 

 upon its surroundings. 



4. Metabolism is the name for the complex of chemical 

 changes continually taking place in living things. Noth- 

 ing which is alive can stand still, it must change. Some 

 changes are constructive (anabolism), some are destructive 

 (catabolism). In general there is an orderly sequence in 

 the transformation of substance. Material is appro- 

 priated, digested (changed chemically and dissolved), and 

 assimilated (or converted into living substance). If any 

 of the material is not fit for food, it is rejected as fceces. 

 In order to furnish energy to transform materials and 

 build substance, oxidation takes place, and this makes 



FIG. 21. Scheme showing the life cycle through which every living thing must 



pass. 



respiration necessary to supply the oxygen. After living 

 substance has been broken down a residue is left which 

 passes out as an excretion (Fig. 20). 



In addition to this continual daily rhythm there is 

 another series of periodic changes which is characteristic 

 of living things. This is called the Life Cycle, during which 

 an organism passes through the stages of youth, maturity, 

 and old age. A young animal has great vitality it grows 

 rapidly and has an abundance of energy. A mature animal 

 is at its best the machine is complete and perfect. Usu- 

 ally at this stage reproduction takes place and a new 

 cycle is started by setting free a young animal. In old 

 age the machine wears out and death finally puts an end 



