Let us take account of stock at this point, before passing to human learning, and attempt to generalize what we have observed in animals of the process of learning.

(1) Elimination of a response, which means detachment of a response from the stimulus that originally aroused it, occurs in three main cases:

(a) Elimination 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, giving higher motor units.