PREFACE. 



THAT the living body is a machine is a state- 

 ment that is frequently made without any very 

 accurate idea as to what it means. On the one 

 hand it is made with a belief that a strict com- 

 parison can be made between the body and an or- 

 dinary, artificial machine, and that living beings 

 are thus reduced to simple mechanisms ; on the 

 other hand it is made loosely, without any special 

 thought as to its significance, and certainly with 

 no conception that it reduces life to a mechanism. 

 The conclusion that the living body is a machine, 

 involving as it does a mechanical conception of 

 life, is one of most extreme philosophical impor- 

 tance, and no one interested in the philosophical 

 conception of nature can fail to have an interest 

 in this problem of the strict accuracy of the state- 

 ment that the body is a machine. Doubtless the 

 complete story of the living machine can not yet 

 be told ; but the studies of the last fifty years have 

 brought us so far along the road toward its com- 

 pletion that a review of the progress made and a 



