On this basis we may see that the activities of life depend upon the ability of the parts of the body mechanism to respond adequately to adequate stimulation. This postulate applies not only to stimuli from visible forces, but to those received by the invasion of the micro-bodies which cause pyogenic or non-pyogenic infections. In the case of dangerous assaults by visible or invisible enemies, the brain, through the nerves and all parts of the motor mechanism, meets the attack by attempts at adaptation. Recovery, invalidism, and death depend upon the degree of success with which the attacking or invading enemies are met. Questions regarding disease become, therefore, questions in adaptation, and it is possible that, when studied in the light of this conception, the key to many hitherto unsolved physical problems may be found. Perhaps no more convincing proof of our thesis may be secured than by a study of that ever-present phenomenon--pain. In whatever part of the body and by whatever apparent cause pain is produced, we find that it is invariably a stimulation to motor activity-- whose ultimate object is protection. Thus by the muscular action resulting from pain we are protected against heat and cold; against too powerful light; against local anemia caused by prolonged pressure upon any portion of the body. So, too, pain of greater or less intensity compels the required emptying of the pregnant uterus and the evacuation of the intestine and the urinary bladder. It should be noted that in every instance the muscular activity resulting from pain is specific in its type, its distribution, and its intensity, this specificity being true not only of pain which is the result of external stimulation, but also of the pain associated with certain types of infection. Pain, however, is not the only symptom of the invasion of the body by pyogenic or parasitic organisms. Fever, invariably, and chills, often, accompany the course of the infections. Can these phenomena also be explained as adaptations of the motor mechanism for the good of the individual? As the phenomena of chills and fever are most strikingly exhibited in malaria, let us study the course of events in that disease. It is known that the malarial parasite develops in the red blood-corpuscles, and that the chills and fever appear when the cycle of parasitic development is complete and the adults are ready to escape from the corpuscles of the blood plasma. Bass, of New Orleans, has proved that the favorable temperature for the growth of the malarial organism is 98'0, and that at 102'0 the adult organisms will be killed, though the latter temperature is not fatal to the spores. The adult life of the malarial parasite begins after its escape into the blood plasma, and it is there that the organism is most susceptible to high temperature. We must infer, therefore, that the fever is an adaptation on the part of the host for despatching the enemy.