the effect of disuse on a memory is true, but under some circumstances its effect is mitigated by the presence of other factors whose presence has been unnoted.  Sometimes this improvement without practice is explained by the fact that at the last practice period the actual improvement was masked by fatigue or boredom, so that disuse involving rest and the disappearance of fatigue and boredom produces apparent gain, when in reality it but allows the real improvement to become evident.  Sometimes a particular practice period was accompanied by certain undesirable elements such as worry, excitement, misunderstandings, and so on, and therefore the improvement hindered or masked, whereas at the next period under different conditions there would be less interference and therefore added gain.  All experimental evidence is against the opinion that mere disuse in and of itself produces gain.  In fact, all results point to the fact that disuse brings deterioration.

In the case of memory, as has already been described in habit formation, reviews which are organized with the period between repetitions only gradually lengthened may do much to insure permanence.  It is entirely feasible to have children at the end of any school year able to repeat the poems or prose selections which they have memorized, provided that they have been recalled with sufficient frequency during the course of the year.  In a subject like geography or history, or in the study of mathematics or science, in which logical memory is demanded, systematic reviews, rather than cramming for examinations, will result in permanence of command of the facts or principles involved, especially when these reviews have involved the right type of organization and as many associations as is possible.

It is important in those subjects which involve a logical organization of ideas to have ideas associated around some particular problem or situation in which the individual is vitally interested.  Children may readily forget a large number of facts which they have learned about cats in the first grade, while the same children might remember, very many of them, had these facts been organized round the problem of taking care of cats, and of how cats take care of themselves.  A group of children in an upper grade may forget with great rapidity the facts of climate, soil, surface drainage, industries, and the like, while they may remember with little difficulty facts which belong under each of these categories on account of the interest which they have taken in the problem, “Why is the western part of the United States much more sparsely populated than the Mississippi Valley?” Boys and girls who study physics in the high school may find it difficult to remember the principles involved in their study of heat if they are given only in their logical order and are applied only in laboratory exercises which have little or no meaning for them, while the same group of high school pupils may remember without difficulty these same laws or principles if associated round the issue of the most economical way of heating their houses, or of the best way to build an icehouse.