340 ELEMENTS OF COMPARATIVE ZOOLOGY. 



favorable conditions we can see its particles changing their 

 relative position, or we may see the mass move as a whole. 

 It moves also in response to external influences, or, as the 

 physiologist expresses it, it reacts to stimuli. Thus some 

 protoplasm will turn to the light, other kinds will try to 

 avoid it. Heat, up to a certain degree, will increase its 

 action, while electricity will cause it to contract. 



Protoplasm has the power of reproduction, by which we 

 mean that portions can separate from the parent mass and 

 can then carry on all the processes which could be per- 

 formed before the separation took place. 



These, and a number of other features not so easily de- 

 scribed, are characteristic of protoplasm, and they occur in 

 no non-living substance. These are, too, the phenomena 

 of life, and hence protoplasm has been aptly termed the 

 physical basis of life. 



