It was the coming of the carnivores, therefore, that accelerated the development of brain matter, and started the process which created man. But in the millions and millions of years of conflicts, instincts grew into being that sank deep into bone and marrow. The most fundamental reflexes, those immediate responses to irritation or danger, were laid down, and among them the drive and check system. When the animal had decided to fight its enemy or was forced to fight, or determined to prey, then was the time for the drive system to do its utmost to speed up everything that would help in the fight, while the check system came into play to hinder whatever would interfere or burden in the fray. First the drive mechanism must have been hit upon, and then the value of the check devices must have been found in fear and flight, and especially in hiding and simulation of death, when even breathing had to be inhibited. Until finally there developed, for everyday use, a complete check and drive nerve machinery for every organ, to be used according to the exigencies of the moment, with the thyroid as the primary stimulant and controller of the drive system and the adrenal as the primary dictator over the check system. THE HARMONY OF THE HORMONES All the glands, in fact, work in unison, with a distribution of the balance of power that diplomatists might envy. In the co-ordinating synchronism, the vegetative nervous system plays the part of an agent that acts as well as is acted upon. The chemical interaction of the internal secretions is not the only way in which they influence each other. For, as the case of the thyroid and the adrenal so well shows, secretions which, when directly interacting, are mutually reinforcing, when affecting nerves, may become clashing opponents. The Kinetic Chain is about as good a case as there is of the glands of internal secretion co-operating. The Check and Drive systems, with the adrenals and thyroid opposed, are one of the best instances of their antagonisms. Besides, there are a number of other relationships between them that might be cited. They all bear with more or less pressure, positive or negative, upon the sex glands which will be considered in its place. If one wished to consider all the glands in their pro and anti relations, a separate volume would be required. THE VEGETATIVE APPARATUS The combination of the internal secretions and the vegetative system has been spoken of as the vegetative or autonomic apparatus. The vegetative apparatus is the oldest part of the nervous system. And some acquaintance with its constitution is necessary to any understanding of the possibilities of control of human nature.