It is convenient to divide the field of Psychology into four main parts: (1) Adult Human Psychology, (2) Child Psychology, (3) Animal Psychology, (4) Social Psychology. The first and second parts can each be divided again into, (a) Normal Psychology, and (b) Pathological or Abnormal Psychology. Although this scheme is not the only one which may be used, it is the most convenient one for practical purposes.
It is true that this scheme as given is not an accurate, logical one, as "animal", in the wide sense, includes both the child and the adult human animal, as well as the so-called "lower animal". We actually use the terms to mean animal other than human. Social Psychology, again, is usually understood as Human Social Psychology; but it makes extensive use of the results of experiments and observations on animals. We might extend the scheme still further by dividing Animal Psychology into Normal and Pathological, but at the present time there is no practical advantage in such division; whereas there is an advantage in separating Abnormal Human Psychology from Normal Human Psychology. There is an advantage also in separating the psychological treatment of the pathological child--the child which either has insane tendencies, or is mentally deficient--from the treatment of the normal child: but in the case of the lower animal it is not at present useful to study the psychology of the young animal apart from that of the adult. It is also true that there is in Human Psychology an intermediate field between the Psychology of the Child and the Psychology of the Adult, which is frequently given separate