1 9 o RESPONSE IN THE LIVING AND NON-LIVING 



assumption of a hypermechanical vital force, acting in 

 contradiction or defiance of those physical laws that 

 govern the world of matter. Nowhere in the entire 

 range of these response-phenomena inclusive as that 

 is of metals, plants, and animals do we detect any 

 breach of continuity. In the study of processes 

 apparently so complex as those of irritability, we must, 

 of course, expect to be confronted with many difficulties. 

 But if these are to be overcome, they, like others, must 

 be faced, and their investigation patiently pursued, 

 without the postulation of special forces whose con- 

 venient property it is to meet all emergencies in virtue 

 of their vagueness. If, at least, we are ever to understand 

 the intricate mechanism of the animal machine, it will 

 be granted that we must cease to evade the problems 

 it presents by the use of mere phrases which really 

 explain nothing. 



We have seen that amongst the phenomena of 

 response, there is no necessity for the assumption 

 of vital force. They are, on the contrary, physico- 

 chemical phenomena, susceptible of a physical inquiry 

 as definite as any other in inorganic regions. 



Physiologists have taught us to read in the response- 

 curves a history of the influence of various external 

 agencies and conditions on the phenomenon of life. By 

 these means we are able to trace the gradual diminution 

 of responsiveness by fatigue, by extremes of heat and 

 cold, its exaltation by stimulants, the arrest of the life- 

 process by poison. 



The investigations which have just been described 



