§ 34. =The Interference and Decay of Associative Tendencies.=—If a set of associative tendencies, such as we have just described, is left to itself, and neither disturbed nor renewed, it _gradually disappears_; the loss is at first very rapid, then proceeds more slowly, and thereafter goes on only at a snail’s pace. To make the matter concrete, we may think of the meshwork of tendencies as a meshwork of channels, deeper and shallower, in the substance of the brain; then the rule is that the channels tend to fill up,—the shallow ones speedily, the deeper ones at first quickly and then more and more slowly,—until everything is smooth again. This is a mere figure, but it carries the meaning that we desire. The same thing happens with _the tendencies set up by meaningful material_; they too slowly die away; but _it is doubtful if they ever wholly disappear_; in their case the brain, if it has been thoroughly impressed, seems never wholly to ‘forget.’ Ebbinghaus learned some stanzas of Byron’s Don Juan, for experimental purposes, and did not look at them again for 22 years; yet he relearned those stanzas in 93 per cent. of the time required to learn new stanzas; a saving of 7 per cent. Some stanzas that he had learned more thoroughly were not read again for 17 years; these were relearned with a saving of nearly 20 per cent. He had no memory whatever of the verses formerly learned; but his brain ‘remembered’; the associative tendencies had not completely disappeared. As a rule, however, a particular set of tendencies is not allowed to die a natural death; it is interfered with by others. _All associative tendencies need a certain time to establish themselves_, to settle down; and if this time is not granted, but stimulus treads on the heels of stimulus, there is no impression of the meshwork, and no connections are formed; we have seen that a series of excessive length simply throws the learner into confusion. A recently acquired connection may even be abolished, as most of us know to our cost, by interruption of the train of thought; you have just got to your point, to the insight, the phrasing, the argument, that will clinch things; you are distracted by some irrelevant matter; and when you come back to your work, the point has gone. So nicely balanced and so easily disturbed are the associative tendencies, that you may never recover it; no wonder that the constructive worker, in literature, in science, in affairs, ‘hates to be interrupted’!