LIVING AND NON-LIVING OBJECTS. 41 



may be aroused into action by a stimulus, which is not 

 sufficient to injure it, in such a way as to put forth more 

 energy than is involved in the stimulus itself. For ex- 

 ample, light, gravity, or a mild chemical action or a 

 touch may bring about a response on the part of the 

 living object, or on the part of the cells of which it is made, 

 much more powerful than could be accounted for directly 

 by the stimulating force. 



There is no non-living substance in nature which 

 possesses this quality in anything like the degree that 

 protoplasm shows it. 



54. The Powers and Activities of Protoplasm (and of 

 Living Objects). The chief difference between a living 

 animal and one that is just dead is apparently not so 

 much one of organization as of powers. The animal 

 still has its shape, its organs, its cells; and its cells even 

 seem, on examination, to have their characteristic form 

 and contents, but the peculiar irritability and the activity 

 and powers of protoplasm dependent on it are gone. 

 Chief among these common powers of living protoplasm 

 are the following : 



a. The power to change certain foreign materials into 

 its own substance, and thus to produce growth of an 

 internal and intimate sort. This is in contrast with the 

 growth of a crystal or a bed of sandstone which takes 

 place merely by adding, externally, substance already 

 like its own. 



b. Power of adjustment to external conditions, through 

 its irritability to external stimuli and its power to respond 

 to them. 



c. Power of undergoing an orderly cycle of life, 

 including birth, youth (growth) , maturity (reproduction) , 



