In the Ether Day Address the phylogenetic key supplied by Darwin was utilized to formulate the principle that the organism reacts as a unit to the stimuli of physical injury, of emotion, of infection, etc.Pain, like other phenomena, was probably evolved for a particular purpose-- surely for the good of the individual; like fear and worry, it frequently is injurious. What then may be its purpose?ContentsPhylogenetic association in relation to certain medical problems -- Phylogenetic association in relation to the emotions -- Pain, laughter, and crying -- The relation between the physical state of the brain-cells and brain functions: experimental and clinical -- A mechanistic view of psychology -- A mechanistic theory of disease -- The kinetic system -- Alkalescence, acidity, anesthesia: a theory of anesthesia.