54 THE STORY OF THE LIVING MACHINE. 



plicated machinery, which confuses the problem, 

 and see if we can find the fundamental units which 

 show these properties, unencumbered by the sec- 

 ondary machinery which has hitherto attracted our 

 attention. We must turn now to the problem 

 connected with protoplasm and the living cell, 

 since here, if anywhere, can we find the life sub- 

 stance reduced to its lowest terms. 



CHAPTER II. 



THE CELL AND PROTOPLASM. 



Vital Properties. We have seen that the gen- 

 eral activities of the body are intelligible accord- 

 ing to chemical and mechanical laws, provided we 

 can assume as their foundation the simple vital 

 properties of living phenomena. We must now 

 approach closer to the centre of the problem, and 

 ask whether we can trace these fundamental prop- 

 erties to their source and find an explanation of 

 them. 



In the first place, what are these properties ? 

 The vital powers are varied, and lie at the basis 

 of every form of living activity. When we free 

 them from complications, however, they may all 

 be reduced to four. These are : (i) Irritability, or 

 the property possessed by living matter of react- 

 ing when stimulated. (2) Movement, or the power 

 of contracting when stimulated. (3) Metabolism, 

 or the power of absorbing extraneous food and pro- 

 ducing in it certain chemical changes, which either 

 convert it into more living tissue or break it to 

 pieces to liberate the inclosed energy. (4) Repro- 



