lar mobility due to removal of sluggishness by
repeated stimulation--would appear to be applicable in this case also.
It would appear, then, that in all the phenomena which we have studied
under the heads of 'staircase' effect, increase of response after
continuous stimulation, and fatigue, there is a similarity between the
observations made upon the response of muscle and nerve on the one hand,
and that of metals on the other. Even in their abnormalities we have
seen an agreement.

But amongst these phenomena themselves, though at first sight so
diverse, there is some kind of continuity. Calling _all_ normal response
_positive_, for the sake of convenience, we observe its gradual
modification, corresponding to changes in the molecular condition of the
substance.

Beginning with that case in which molecular modification is extreme, we
find a maximum variation of response from the normal, that is to say, to
_negative_.

Continued stimulation, however, brings back the molecular condition to
normal, as evidenced by the progressive lessening of the negative
response, culminating in reversion to the normal _positive_. This is
equally true of nerve and metal.

In the next class of phenomena, the modification of molecular condition
is not so great. It now exhibits itself merely as a relative inertness,
and the responses, though positive, are feeble. Under continued
stimulation, they increase in the same direction as in the last case,
that is to say, from less positive to _more positive_, being the reverse
of fatigue. This is evidenced alike by the staircase effect and by the
increase of response after tetanisation, seen not only in nerve but also
in platinum and tin.

The substance may next be in what we call the normal condition.
Successive uniform stimuli now evoke uniform and equal positive
responses, that is to say, there is no fatigue. But after intense or
long-continued stimulation, the substance is overstrained. The responses
now undergo a change from positive to _less positive_; fatigue, that is
to say, appears.

Again, under very much prolonged stimulation the response may decline to
zero, or even undergo a reversal to _negative_, a phenomenon which we
shall find instanced in the reversed response of retina under the
long-continued stimulus of light.

We must then recognise that a substance may exist in various molecular
conditions, whether due to internal changes or to the action of
stimulus. The responses