Stimulus & Mechanism as Factors in Organisation. 199 
inference that in some sort it has a real existence when we reflect 
upon the diverse results produced on different organisms or on 
different parts of the same organism, by relatively simple and 
identical stimuli, e.g. gravitation, or by the change in the character 
of response that may under appropriate conditions be induced in the 
organisation of many plants. 
The term stimulus is an awkward one, for it is (at least im¬ 
plicitly) often used in two senses that do not necessarily correspond. 
The application of an external agent, such as light, may act indi¬ 
rectly as a stimulus, and its application be followed by a movement. 
But I take it this is often only a partial expression of the whole 
truth, and is perhaps a rather misleading part. The physically 
applied external stimulus serves to produce a change in the recep¬ 
tive sensitive structure, and here it is translated into an alteration 
of molecular condition and it is this that is propagated and finally 
excites the executive structure to perform the appropriate response. 
Whether the response be visible as a molar movement or not, it is 
pretty certainly invariably associated with some change in chemical 
structure. 
Apart from a stimulus there can be no rearrangement of parts;; 
no growth, no waste, no movement. It is conceivable that this 
inactivity might be induced in various ways. Either the proximate 
stimuli might be lacking, or the real ultimate stimuli, of which I 
have just spoken, might be unable to exist owing to lack of lability 
in the living substance itself, such as does happen, for example,, 
when the watery contents fall below the minimum. We have such, 
an instance in dried seeds that are capable of withstanding temper¬ 
atures that under moister conditions would inevitably lead to the 
disruption of the vital complex of which the living substance is 
composed. 
But given a stimulable condition, the particular result that will 
follow on any impulse will certainly depend on the nature of the 
mechanism affected, and it may also be modified by the kind of 
stimulus applied. In the latter case it perhaps depends upon the 
capacity of the transmitting zone to propagate different kinds of 
disturbances or decompositions when differently affected, in the 
alternative case any irritating agent can only affect this interme¬ 
diate zone by setting up one kind of change. The optic nerve, how¬ 
ever stimulated, only gives rise to the impression of light, and a 
Mimosa leaf makes the same response whether the proximate 
stimulating agency be a lighted match ora pair of forceps. 
