10 HEREDITY AND ENVIRONMENT 



complex substances known to chemistry. 

 Protoplasm is not a homogeneous substance, 

 but it always exists in the form of cells, which 

 are minute masses of protoplasm composed of 

 many distinct parts, the most important of 

 these being the nucleus and the cytoplasm 

 ( Fig. 1 ) . Protoplasm is therefore organized, 

 that is, composed of many parts all of which 

 are integrated into a single system, the cell. 

 Higher animals and plants are composed of 

 multitudes of cells, differing more or less from 

 one another, which are bound together and 

 integrated into a single organism. Living cells 

 and organisms are not static structures which 

 are fixed and stable in character, but they are 

 systems which are undergoing continual 

 change. They are like the river, or the whirl- 

 pool, or the flame, which are never at two con- 

 secutive moments composed of the same 

 particles but which nevertheless maintain a 

 constant general appearance ; in short they are 

 complex systems in dynamic equilibrium. 



The principal physiological processes by 

 which all living things maintain this equilib- 

 rium are: 



