In bringing forward some evidence which would point to the biological course followed by some forms of nervous disease to be considered, I would first of all accept as a working hypothesis two generalisations which apply to all forms of disease. The first of these generalisations is that there is essentially no difference in kind between a physiological and a pathological process. The distinction is an arbitrary one; the course of disease is distinguished from that of health only in so far as it tends to compromise the continuation of a more or less perfect adaptation between the organism and its surroundings. There is no tendency in Nature either to kill or to cure; she is absolutely impartial as to the result of a conflict between organisms and a host; and it is a matter of complete indifference to her as to whether toxins are eliminated or not. In the same way diseases of the mind are the manifestation of a perfectly natural relation of the organism, such as it is, to the environment. If the mental processes are abnormal, it goes without saying that the brain must be acting abnormally whether the stimuli to abnormal action originate in the brain itself or in some other part of the body. For example, if a child with pneumonia be suffering from delirium and hallucinations, as is not infrequently the case, this must be considered a perfectly natural outcome of the relation of the brain to its environmental stimuli outside and inside the organism. The actual stimuli may originate in the intestine from masses of undigested food and the stimuli may play on the brain rendered hypersensitive by the toxins from the lungs; the process and its manifestations, as well as the final outcome, are matters in which nature plays an impartial part. It cannot be admitted that there is any form of nervous disease which does not come under this generalisation. It has been argued by some authorities that because insidious forms of insanity are marked only by the slightest variation from the normal course of mental life, and that because the mental abnormalities are only modifications, and often easily explainable modifications, of normal mental processes, that the so-called insanity originates in these processes, and not in the material substratum of the organism. The fallacy of such an interpretation is obvious; it is tantamount to saying that slight albuminuria is the cause underlying early disease of the kidneys, or that a slight ódema may have something to do with the origin of circulatory disease. It is only natural that in the milder forms of mental disease the abnormal manifestations of brain activity should resemble normal mental processes; and even in the most advanced forms of mental disease there must be a close resemblance between abnormal ideation and conduct and perfectly normal ideation and behaviour. Even in advanced cases of Bright's disease the urinary elimination is more normal than abnormal; the abnormal constituents do not differ so much in kind as in degree from those of urine from healthy kidneys. It is not to be expected that in kidney disease bile or some other substance foreign to the organ would be the chief constituent of the eliminated fluid. The signs of insanity in any given case are the natural products of normal brain action mingled with the products of abnormal action. This does not, of course, preclude the possibility that under certain circumstances these abnormal products, such as delusions, hallucinations and perverted conduct, may not themselves be the direct stimuli to further abnormalities. The suicidal character of pathological processes is well seen in other organs of the body. A diseased heart, for example, is its own worst enemy; it not only fails to supply sufficient nutrition to the rest of the organism, but it starves itself by its inability to contract and expand properly, thereby increasing its own weakness. In the same way, certain phenomena of abnormal brain processes are in all probability due to the recoil on the brain of its own abnormal products in the matter of ideation and conduct.