he distinction between the two main cases
of learning is of some importance, since sometimes the changed
stimulus, and sometimes the changed response, is the interesting
fact.]

{401}

I. SUBSTITUTE STIMULUS EXPLAINED BY THE LAW OF COMBINATION

Here the response, without being itself essentially changed, becomes
attached to a new stimulus. We distinguish two cases under the general
head of substitute stimulus. In the one case, the substitute stimulus
was originally extraneous, and unnecessary for arousing the response,
while in the other case it was originally necessary as part of a team
of stimuli that aroused the response.

A. Substitute Stimulus Originally Unnecessary for Arousing the
Response

1. Conditioned reflex.

This is the very simplest case belonging under the law of combination.
The dog that responded to the bell by a flow of saliva, after the bell
plus a tasting substance had acted together on him time after time, is
the typical instance; and another good instance is that of the little
child who was "taught" to shrink from a rabbit by the sounding of a
harsh noise along with the showing of the rabbit. [Footnote: See p.
303.] The explanation of all instances of conditioned reflex is the
same. We have an effective stimulus acting, i.e., a stimulus strongly
linked with the response; and we also have acting an ineffective
stimulus, which gets drawn into the same reaction. The effective
stimulus determines what response shall be made, and the other
stimulus finds an outlet {402} into that response, being, as it seems,
attracted towards the activated response, sucked into it. The weak
linkage from the ineffective stimulus to the response, being thus used
and strengthened, later enables this stimulus to arouse the response
single-handed.

This sort of thing is best presented in a diagram. A full line in the
diagram denotes a linkage strong enough to work alone, while a dotted
line denotes a weak linkage. Letters stand for stimuli and responses.
In the diagram for conditioned reflex, A is the original effective
stimulus (the rasping noise in the instance of the child and the
rabbit), and B is the ineffective stimulus (the sight of the rabbit).
R is the shrinking response, linked strongly to the stimulus A and
only weakly to the stimulus B, which has several other linkages fully
as good as the linkage B-R. But A arouses the response R; and R, being
thus activated, draws on B and brings the linkage