In fact, it is this internal condition in the living creature that is the most important single link in instinct. In the non-mating season the sight of the female has no effect on the male. But periodically his internal organs become tense with procreative cells; these change his coenaesthesia; that starts desire, and desire sets going the mechanisms of search, courtship, the sexual act and the care of the female while she is gravid. All instinctive acts have back of them either a tension or a deficit of some kind or other, brought about by the awakening of function of some glandular structure, so that the organism becomes ready to respond to some appropriate outside stimulus and inaccessible to others. During the mating season, with certain animals, the stimulus of food has no effect until there is effected the purposes of the sexual hunger. Changes in the body due to the activity of sex glands or gastric juices or any other organic product have two effects. They increase the stimulation that comes from the thing sought and decrease the stimulation that comes from other things. In physiological language, the threshold for the first is lowered and for the other it is raised. But this does not explain HOW the changes in glands MAKE the animal seek this or that, except by saying that the animal has hereditary structures all primed to explode in the right way. We may fall back on Bergson's mystical idea that all life is a unity, and that instinct, which makes one living thing know what to do with another--to kill it in a scientific way for the good of the posterity of the killer--is merely the knowledge, unconscious, that life has of life. That pleasant explanation projects us back to a darker problem than ever: how life knows life and why one part of life so obviously seeks to circumvent the purpose of another part of life. For us it is best to say that instinct arises out of the racial and individual needs; that physically there occur changes in the glands and tissues; that these set up desires which arouse into action simple or elaborate mechanisms which finally satisfy the need of the organs and tissues.[1] [1] Kempf in his book on the vegetative nervous system goes into great detail the way the visceral needs force the animal or human to satisfy them. Life is a sort of war between the vegetative and the central nervous system. There is just enough truth in this point of view to make it very entertaining.