30 THE STORY OF LIFE'S MECHANISM. 



physiology of man and his allies than any 

 other animals. The reason for this is plain 

 enough. In the first place, there is a value in 

 the knowledge of the life activities of man 

 entirely apart from any theoretical aspects, and 

 hence human physiology has demanded atten- 

 tion for its own sake. The practical utility of 

 human physiology has stimulated its study 

 for centuries ; and in the last fifty years of scien- 

 tific progress it has been human physiology 

 and that of allied animals that has attracted 

 the chief attention of physiologists. The result 

 is that while the physiology of man is tolerably 

 well known, that of other animals is less under- 

 stood the further we get away from man 

 and his allies. For this reason most of our 

 knowledge of the living body as a machine must 

 be derived from the study of man. This is, how- 

 ever, fortunate rather than otherwise. In the 

 first place, it enables us to proceed from the 

 known to the unknown ; and in the second 

 place, more interest attaches to the problem as 

 connected with human physiology than along 

 any other line. In our discussion, therefore, we 

 shall refer chiefly to the physiology of man. If 

 we find that the functions of human life are 

 amenable to a mechanical explanation we cannot 

 hesitate to believe that this will be equally true 

 of the lower orders of nature. For similar 

 reasons little reference will be made to the 

 mechanism of plant life. The structure of the 

 plant is simpler and its activities are much more 

 easily referable to mechanical principles than 

 are those of animals. For these reasons it will 



