MENTION has already been made of the difficulty in estimating the stages of evolutionary development leading up to the emergence of consciousness, with the development of complete cortical function. In the normal child, the stages are passed through so quickly, and at a time when so little corroboration can be obtained from the object of our study, that little help can be got in establishing a clear picture of the process. In the retarded personality, however, it is possible to obtain a little more indication. In this case, mental growth takes place to a certain point and then stops, as the development of the cortex is arrested, so that some sort of picture is presented, which may be studied at leisure. Even so, however, the problem is not so simple as it might at first sight appear, since cortical function is so complex that the retardation is not necessarily universal in its incidence; as, for example, in those cases referred to in Chapter XIV, in whom intellectual development and control is good, but the control of the affective reactions is deficient. Further bodily development, so far as it comprises structures outside the central nervous system, may proceed normally, and we have already seen the effect of bodily, and especially endocrine activity on mental process. Hence it is obvious that we must not regard the mental reactions of a cortex which has stopped development at the stage of a child of five, when it is under the influence of adult thyroid and sex gland function, as identical with the normal condition of a child of this age, when the cortical function should be under the influence of thymus and juvenile thyroid function.