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. After the second year, the thymus begins to shrink. That is to say, officially its involution begins. Careful dissection will demonstrate some thymus tissue even in a normal subject up to the fourteenth year. This refers to the average normal, for the large thymus may continue large and grow larger after the second year in the type of individual designated in a preceding chapter as the thymocentric.