Another characteristic of the learning curve is the presence of the so-called “plateaus.”  Plateaus show in the curve as flat, level stretches during which there has apparently been no progress.  The meaning of these level stretches, and whether or not they can be entirely done away with in any curve, is a matter of dispute.  These pauses may be necessary for some of the habits to reach a certain degree of perfection before further progress can be made.  However this may be, there are several minor causes which tend to increase the number of plateaus and to lengthen the time spent in any one.  In the first place an insecure or an inaccurate foundation must result in an increase of plateaus.  If at the beginning, during the initial spurt, for instance, the learner is allowed to go so fast that what he learns is not thoroughly learned, or if he is pushed at a pace that for him makes thoroughness impossible, plateaus must soon occur in his learning curve.  In the second place a fruitful cause of plateaus is loss of interest,—­monotony.  If the learner is not interested, he will not put forth the energy necessary for continued improvement, and a time of no progress is the result.  The attitude of the learner toward the work is extremely important, not only in the matter of interest, but in the further attitude of self-confidence.  Discouragement usually results in hindering progress, whereas confidence tends to increase it.  The psychological explanation of this is very evident.  Both lack of interest in the learning and the presence of discouragement are likely to result in divided attention and that, as has already been shown, results in unsatisfactory work.  A third cause for plateaus is physiological.  Not only must the learner be in the right attitude towards the work, but he must feel physically “fit.”  There seem to be certain physiological rhythms that may disturb the learning process whose cause cannot be directly determined, but generally the feeling of unfitness can be traced to a simple cause,—­such as physical illness, loss of sleep, exercise, or food, or undue emotional strain.

The older psychology has left an impression that improvement in any function is limited both as to amount and as to the period during which it must be attained.  The physiological limit of improvement has been thought of as one which was rather easily reached.  The loss of plasticity of the nervous system has been supposed to be rather rapid, so that marked improvement in a habit after one has passed well into the twenties was considered improbable.  Recent experiments, however, seem to show that no such condition of affairs exists.  There is very great probability that any function whatsoever is improvable with practice, and in most cases to a very marked degree.  To find a function which has reached the physiological limit has been very rare, even in experimental research, and even with extended practice series it has been unusual to reach a stage of zero improvement even