What transfigures the individual as the years go by is no simple wear and tear of the tissues, nor the replacement of old cells by new.  It is the rearrangement of relationships among the ductless glands, the shifting of influences from the predominant to the subordinate, and vice versa, in the constellation of the internal secretions, that determines the unfolding of the personality.  The transformations raise doubt sometimes as to the reality of personal identity.  What actually happens in the changes from childhood to adolescence, from adolescence to maturity, and so on, is the sloughing of one internal glandular dominance for another.

Growth, as a general name for the mutations, the ensemble of somatic and psychic differentiation, from year to year, passes through five epochs that are standard for the normal.  The normal is the being who harmonizes with his environment, and yet reacts with it because of recurring needs within him.  His endocrine equation settles what is unique and different in him.  But the gland which flourishes during the epoch as its time of triumph, when it has its day, determines what makes him like his fellows.

From this point of view it becomes permissible to speak of the five Endocrine Epochs.  Similarities and resemblances of mind and body between people at a given period of life, childhood, youth, maturity must be put down to their common government by the salient endocrine of the epoch.  So one may list: 

Infancy as the epoch of the thymus Childhood as the epoch of the pineal Adolescence as the epoch of the gonads Maturity as the epoch of whatever gland is left in control as the result of the life struggle.  Senility as the epoch of general endocrine deficiency.

Infancy as the epoch of the thymus explains why, in any given geographic locality, the babies look alike and act alike.  Specialists in the observation and treatment of infants have noted that not until after the second year is any tendency to differentiation discernible to any extent among them.  It is only after the second year, or somewhere around that time, that the child begins to individuate, and distinct individual traits and a personality manifest their outlines.  The thymus is the great inhibitor of all the glands of internal secretion.  By its checking activity upon the other members of the endocrine system, the thyroid and pituitary in particular, it gives the baby time to grow in bulk, which is its chief business during the first two years of its existence.  It quadruples its birth weight.  The brain and nervous system complete their growth in mass by the end of the fourth year.  Recall the experiments of Gudernatsch working with tadpoles, who showed that feeding with thymus produced giant tadpoles whose metamorphosis into frogs was inhibited, while feeding thyroid produced frogs the size of flies.  Differentiation occurred without the preliminary increase in mass usual.  As differentiation and bulk thus appear antagonistic, at least at the beginning of growth, the function of the thymus, at a maximum during infancy, seems then to be to restrain the differentiating endocrines, until sufficient material has been accumulated by the organism upon which the differentiating process may work.