A belief maintained by some psychologists is in direct opposition to this general law that disuse causes deterioration. It is usually stated something like this, that periods of incubation are necessary in acquiring skill, or that letting a function lie fallow results in greater skill at the end of that period, or briefly one learns to skate in summer and swim in winter. To some extent this is true, but as stated it is misleading. The general law of 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.