If the thymus retrogresses after the second year, what takes its place as a brake upon the forward driving impulses of the other endocrines?

The pituitary, too, begins to retrogress after the period of maturity.

And, while such higher type begins to dominate over lower types, and to spread at their expense, the lower types survive in habitats or modes of life that are not usurped, or are thrust into inferior habitats or modes of life in which they retrogress.