In the stomach, for instance, extracts of the adrenal glands have been proved to intensify the function of the sympathetic or check system in different degrees, so that there is a lessening of the amount and acidity of the gastric fluid.  On the other hand, thyroid extracts will intensify the action of the autonomic or drive system, so that the amount and acidity of the digestive juice is increased.

The stomach cell may, therefore, be regarded as a test-reagent for the different internal secretions, as they affect the check and drive systems.

These constitute an automatic device for regulating the activities of every organ.  Three factors enter into the mechanism.  One is the amount of the circulating internal secretions.  Another is the organic and functional integrity of the nerve filaments comprising the check and drive systems.  The third consists of the number and vitality and limitations of the terminal receiving cells acted upon by the nerve filaments, which in their turn have been acted upon by the internal secretions.  Upon every organ, including the mind, through the brain, a stimulus from without or within will act according to its ability to influence one or others of these factors.

Normally, the check and drive systems are properly balanced.  But under stress and strain the balance is upset.  Indeed, the Kinetic Drive may be defined as a mechanism contrived in the course of evolution as the normal, healthy mode for meeting stress and strain.  The Kinetic chain of organs, brain, adrenals, liver, thyroid and muscles, began working together in desperate situations for their possessor ages ago.  Successful in helping him to survive, they have survived as a functional unit.

It was probably evolved in the Post-Tertiary Era, about twenty million years ago, when the coming of the carnivores introduced direct body-to-body conflicts, and their concomitants, a quick and versatile nervous system.  During the Tertiary epoch the earth basked in the heat of a tropical sun nearly everywhere on its surface.  The luxuriant vegetation of the torrid zone flourished and swarmed, for the temperature all over was what it is today at the equator.  Gigantic vegetarians were the animals, creatures like the dinosaurs, enormous, gargoylean monsters, of an incredible size and strength, but clumsy and grotesque, with small brains and little intelligence.  For what need was there for brain and intelligence when food lay about so abundantly at hand for them to gorge themselves.  As there was no competition for food, there were no enemies.

Then as the earth evolved and grew cooler, vegetation failed, the ancestors of the present carnivora appeared, the fathers of the wolf and tiger, light, lithe and pugnacious, with senses acute and ferocious weapons of attack, who set out to destroy everybody.  They destroyed pretty nearly all of the huge leaf-eating species, and only the more plastic and smaller ones, who were more keen-sensed and swift-footed (of whom the deer and antelope, horse and ox are the descendants), escaped.  The smallest either took to the air to become the bat, or, like the forerunners of the squirrel and ape, took to the trees.