INTRODUCTION. I/ 



tion of evolution. Certain it is that, had it been 

 found that no correlation could be traced between 

 vital and non-vital forces, the doctrine of evolu- 

 tion could not have stood, and even now the 

 special significance which we shall in the end give 

 to evolution will depend upon how we succeed in 

 answering the questions above outlined. The fact 

 is that this problem of the mechanical explanation 

 of vital phenomena forms the capstone of the 

 arch, the sides of which are built of the doc- 

 trines of the conservation of energy and the 

 theory of evolution. To the presentation of these 

 problems the following pages will be devoted. 

 The fact that both the doctrine of the conserva- 

 tion of energy and that of evolution are practi- 

 cally everywhere accepted indicates that the me- 

 chanical nature of vital forces is regarded as 

 proved. But there are still many questions which 

 are not so easily answered. It will be our pur- 

 pose in the following discussion to ascertain just 

 what are these problems in dynamical biology and 

 how far they have been answered. Our object will 

 be then in brief to discover to what extent the 

 conception of the living organism as a machine is 

 borne out by the facts which have been collected 

 in the last quarter century, and to learn where, if 

 anywhere, limits have been found to our possibility 

 of applying the forces of chemistry and physics 

 to an explanation of life. In other words, we shall 

 try to see how far we have been able to under- 

 stand living phenomena in terms of natural force. 

 Outline of the Subject. The subject, as thus 

 presented, resolves itself at once into two parts. 

 That the living organism is a machine is every- 

 where recognized, although some may still doubt 

 as to the completeness of the comparison. In the 



