Reversing the order of our reasoning, let us now glance at the patient who is unconscious and who, therefore, has lost much of the power of association. His mouth is usually dry, the digestive processes are at a low ebb, the aroma of food causes no secretion of saliva, tickling the nose causes no sneezing; he catches no cold. The laryngeal reflex is lost and food may be quietly inhaled; the entire process of metabolism is low. The contrast between a man whose associational centers are keen and a man in whom these centers are dulled or lost is the contrast between life and death. In accordance with the law of adaptation through natural selection, phylogeny, and association, one would expect no pain in abscess of the brain, in abscess of the liver, in pylephlebitis, in infection of the hepatic vessels, in endocarditis. This law explains why there are no nociceptors for cancer, while there are active nociceptors for the acute infections. It is because nature has no helpful response to offer against cancer, while in certain of the acute pyogenic infections the nociceptors force the beneficent physiologic rest. Could we dispossess ourselves of the shackles of psychology, forget its confusing nomenclature, and view the human brain, as Sherrington has said, "as the organ of, and for the adaptation of nervous reaction," many clinical phenomena would appear in a clearer light. Natural Selection and Chemical Noci-association in the Infections Thus far we have considered the behavior of the individual as a whole in his response to a certain type of noci-influences. We have been voicing our argument in terms of physical escape from GROSS physical dangers, or of grappling with GROSS NERVE-MUSCULAR enemies of the same or of other species. To explain these phenomena we have invoked the aid of the laws of natural selection and phylogenetic association. If our conclusions be correct, then it should follow that in the same laws we may find the explanation of immunity, which, of course, means a defensive response to our MICROSCOPIC enemies. There should be no more difficulty in evolving an efficient army of phagocytes by natural selection, or in developing specific chemical reactions against _*microscopic enemies_, than there was in evolving the various nociceptors for our nerve-muscular defense against our _*gross enemies_. That immunity is a chemical reaction is no argument against the application of the law of natural selection or of association. What essential difference is there between the chemical defense of the skunk against its NERVE-MUSCULAR enemies and its chemical defense (immunity) against its MICROSCOPIC ENEMIES? The administration of vaccines becomes the adequate stimulus which awakens phylogenetic association of a chemical nature as a result of which immune bodies are produced.