As regards these latter, however, it must be borne in mind that here (as in most other cases of the treatment of the abnormal in Psychology) the distinction between normal and abnormal is one which is drawn for the sake of practical convenience only, and which indicates merely a difference of degree not a difference in quality, between the phenomena which it distinguishes. Even those manifestations which mark the most extreme departures from the normal are present as possibilities in all of us: it is only a question of the extent of our tendency towards them and of the intensity of the predisposing causes in our environment. A slight alteration in the balance of our mental forces or in the circumstances of our life and upbringing, and we too might fall victims to the aberrations which now seem to us so repulsive, foolish or ridiculous, when displayed by others. The abnormal in Psychology is most frequently only an aspect of the normal magnified beyond its usual dimensions and thus brought out of proportion to the other aspects of the mind. For this reason the study of the abnormal is often the best means of investigating the minute structure of the normal: and in the present case we shall find that when we have reviewed the principal abnormalities and variations in the psychic development of the individual in relation to his family, we shall be in a much more favourable position for arriving at a decision as to our own attitude--theoretical and practical--towards this development than if we had simply considered the process of growth in its strictly normal aspects. [Sidenote: Abnormalities of development at different levels] Byways in human development, both emotional and intellectual, may diverge from the main track at various points in its course--some near its origin in the infantile strata of the mind, some at a later stage of progress. Those which leave the main track at a relatively early point preserve, as a rule, throughout their course some more or less definite indication of their early origin, some trace of infantile or childish character; while those which take their departure at a subsequent stage bear the marks of a later, but still immature, condition of development. As each variation or aberration thus, to some extent, corresponds in nature to the point of development at which it took its rise, it is possible to classify such variations and aberrations according to their point of origin; and to regard each one as a fixation or arrest of development at a certain point in the main track of progress. What is true of human development in general is true more particularly of the development of the individual's relation to his family. The more primitive variations will be found to bear the characteristics of the early stages of the individual's mental growth while the later variations will indicate a more advanced condition of this growth.