When a conditioned response to a stimulus has been acquired, other similar stimuli will evoke me response. A dog that learns to salivate to the sound of a tuning fork producing a tone of middle will also salivate to higher or lower tones without farther conditioning.
The more nearly alike the new stimuli are to the original, the more completely they will substitute for it. This principle, called generalization, accounts for our ability to react to novel situations in-so-far as they are similar to familiar ones. Careful study shows that the amount of generalization falls off in a systematic man as the second stimulus becomes more and more dissimilar to the original conditioned stimulus.