LIVING MATTER 21 



distinguished by physiological qualities dependent upon 

 the peculiar characteristics of the protoplasm, or living 

 substance of which it is composed. 



This substance is distinguished from non-living sub- 

 stances by three qualities: irritability, or the power to 

 move ; assimilation, or the power to use food substances ; 

 and reproduction, or the power to form new protoplasm. 

 These qualities of living matter are responsible for the 

 behavior of organisms as independent beings. 



Living matter responds to a stimulus with some form 

 of motion. This motion may take the form of a circula- 

 tion within the cell, or of a contraction. If the contrac- 

 tions are organized they will result mechanically in a 

 movement of the whole, or of some part, of the independ- 

 ent organism from place to place. Food substances are 

 used by independent organisms in such a way that new 

 protoplasm is formed, endowed with the power of 

 becoming differentiated into the specific tissues of a spe- 

 cific organism. Food that a sheep eats is never trans- 

 formed into the muscles of a fish. Reproduction of 

 independent organisms involves the formation of a cell 

 endowed with the power to become a new individual like 

 the parent. 



These functions are controlled by physical and chem- 

 ical laws. The most important of these are (1) All 

 matter is composed of particles that move and have 

 spaces between them. (2) This motion is the equiva- 

 lent of heat which may be transferred without loss from 

 particle to particle and from substance to substance. 



As the particles move faster they move farther away 

 from each other, and the substance occupies a larger 



