ng effective
after repetition, in the relation between stimulus and response, and in
the effects produced by temperature; also in the phenomenon of
after-oscillation. These similarities went even further, the very
abnormalities of retinal response finding their reflection in the
inorganic.

[Illustration: FIG. 117.--ABOLITION OF RESPONSE IN NERVE, PLANT, AND
METAL BY THE ACTION OF THE SAME 'POISON'
The first half in each set shows the normal response, the second half
the abolition of response after the application of the reagent.]

Thus living response in all its diverse manifestations is found to be
only a repetition of responses seen in the inorganic. There is in it no
element of mystery or caprice, such as we must admit to be applied in
the 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 convenient 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 may possibly carry us
one step further, proving to us that these things are determined, not by
the play of an unknowable and arbitrary vital force, but by the working