g more is will be clear if we ask ourselves why a
substitute response should ever be made. Evidently because there is
something wrong with the original response; if that were entirely
satisfactory, it would continue to be made, and there would be no room
for a substitute. The original response being unsatisfactory to the
individual, how is he to find a substitute? Only by finding some
stimulus that will arouse it. This is where trial and error come in,
consisting in a search for some extra stimulus that shall give a
satisfactory response.

Suppose now that the extra stimulus has been found which arouses a
satisfactory substitute response. The original stimulus, or the
reaction-tendency aroused by it, still continuing, participates in
arousing the substitute response, playing the part of the originally
ineffective stimulus in the conditioned reflex. Thus the original
stimulus becomes strongly linked with the substitute response.

The process of reaching a substitute response thus includes three
stages: (a) original response found unsatisfactory, (b) new stimulus
found which gives a satisfactory substitute response, (c) attachment
of the substitute response to the original stimulus.

{409}

There are two main cases under the general head of substitute
response. In one case, the substitute response is essentially an old
response, not acquired during the process of substitution, but simply
substituted, as indicated just above, for the original response to the
situation. This represents the common trial and error learning of
animals. The second case is that where the substitute response has to
be built up by combination of old responses into a higher unit.

C. Substitute Response, but not in Itself a New Response

I. Trial and error.

Our much-discussed instance of the _cat in the cage_ need not be
described again, but may simply be illustrated by a diagram.

[Illustration: Fig. 60.--How the cat learns the trick of escaping from
the cage by unlatching the door. S is the situation of being shut up
in a cage, and T is the tendency to get out. R1 is the primary
response aroused by this tendency, which response meets with failure,
not leading to the end-result of the tendency. Responses are then made
to various particular stimuli about the cage, and one of these
stimuli, the door-latch, X, gives the response R2 which leads to the
end-result. Now the response R2 was in part aroused by T, and its
pre-existing weak li