tion occurs most quickly when the response brings actual
_pain_; the animal makes the avoiding reaction to the pain and
quickly comes to make this response to the place where the pain
occurred; and thus the positive reaction to this place is
eliminated.

(b) Elimination occurs more gradually when the response, without
resulting in actual pain, brings _failure_ or delay in reaching a
goal towards which the animal is tending. The positive response of
entering and exploring a blind alley grows weaker and weaker, till
the blind alley is neglected altogether.

(c) Elimination of a response also occurs, slowly, through _negative
adaptation_ to a stimulus that is harmless and also useless.

{311}

(2) New _attachments_ or _linkages_ of stimulus and response occur in
two forms, which are called "substitute stimulus" and "substitute
response".

[Footnote: The writer hopes that no confusion will be caused by
his use of several words to express this same meaning.
"Attachment of stimulus and response", "linkage of stimulus and
response", "connection between stimulus and response", and "bond
between stimulus and response", all mean exactly the same; but
sometimes one and sometimes another seems to bring the meaning
more vividly to mind.]

(a) _Substitute stimulus_ refers to the case where the natural
response is not itself modified, but becomes attached to another
stimulus than the one that originally aroused it. This new linkage
can sometimes be established by simply giving the original stimulus
and the substitute stimulus at the same time, and doing so
repeatedly, as in the conditioned reflex experiment.

(b) _Substitute response_ refers to the case where the stimulus
remaining as it originally was, a new reaction is attached to it in
place of the original response. The conditions under which this
takes place are more complex than those that give the substitute
stimulus. A tendency towards some goal must first be aroused, and
then blocked by the failure of the original response to lead to the
goal. The dammed-up tendency then facilitates other responses, and
gives trial and error behavior, till some one of the trial responses
leads to the goal; and this successful response is gradually
substituted for the original response, and becomes firmly attached
to the situation and tendency.

(3) New _combinations of responses_ occur, givin