iv] MODES OF NUTRITION 93 



body are being formed and the reserve food substances 

 are being oxidised, or are being decomposed, to 

 supply heat or energy to the organism. Now the 

 living cell must be supplied with food to compensate 

 for its waste ; to form new structures or tissues, or 

 new individuals during reproduction ; or to form the 

 skeletal structures ; or to be oxidised for heat and 

 energy production. The substances which are brought 

 to the cells for these purposes are the foods, and 

 their incorporation in the cell substance is the process 

 of assimilation. 



We recognise in the tissues of an animal substances 

 called proteids, fats and carbohydrates. The fats are 

 relatively few in number, and when not incorporated 

 with the proteids are fairly simple in composition. 

 So are the carbohydrates, and we know the chemical 

 structure of most of these substances. But the 

 proteids are exceedingly complex, more so than any 

 other chemicals known to us, and in spite of long- 

 continued research we do not know what is their 

 constitution. But we do know that the proteids of 

 every species of organism must be different from 

 those of any other species, and it is possible that this 

 holds true even for individuals. The facts of heredity 

 can hardly be explained in any other way, and indeed 

 there is a certain amount of experimental proof in 

 support of this assertion. We also know that the 

 proteids can be split up, or decomposed in various 



