190 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 
