66 THE STORY OF LIFE'S MECHANISM. 



THE VITAL FACTOR. 



With all this explanation of life processes it 

 can not fail to be apparent that we have not 

 really reached the centre of the problem, We 

 have explained many secondary processes, but 

 the primary ones are still unsolved. In studying 

 digestion we reach an understanding of every- 

 thing until we come to the active vital property 

 of the gland-cells in secreting. In studying 

 absorption we understand the process until we 

 come to what we have called the vital powers 

 of the absorptive cells of the alimentary canal. 

 The circulation is intelligible until we come to 

 the beating of the heart and the contraction of 

 the muscles of the blood-vessels. Excretion is 

 also partly explained, but here again we finally 

 must refer certain processes to the vital powers 

 of active cells. And thus wherever we probe the 

 problem we find ourselves able to explain many 

 secondary problems, while the fundamental ones 

 we still attribute to the vital properties of the 

 active tissues. Why a muscle contracts or a 

 gland secretes we have certainly not yet answered. 

 The relation of the actions to the general prob- 

 lems of correlation of force is simple enough. 

 That a muscle is a machine in the sense of our 

 definition is beyond question. But the problem 

 of why a muscle acts is not answered by showing 

 that it derives its energy from broken food 

 material. There are plainly still left for us a 

 number of fundamental problems, although the 

 secondary ones are soluble. 



What can we say in regard to these funda- 



