All inherited predispositions, it is important to remember, are aggravated, and each proclivity changed to actuality, by those influences which in individuals tend to like effects upon health. Such become exciting causes of various diseases. If these be constantly avoided, and all the surroundings and the mode of life of the individual be maintained in a manner most favorable to health, the hereditary tendency may remain inert through a long lifetime. Every physician must have seen this in scores of instances. The application of the principle through special precepts belongs to personal hygiene. But no physician can rightly ignore the study of this subject, or omit the utilization of his acquaintance with it by preventive advice to members of the families under his professional care. Our last remark in connection with pre-natal causation must be upon the effects of circumstances and modes of living on masses of men, especially in large cities and populous countries. Something has been said already of race-acclimatization by which there may be acquired a lessened susceptibility to certain endemic fevers.[2] Almost a reverse action is exhibited in the gradual lowering of vital energy under what has been called the "great-town system." While those having all the comforts of life and avoiding excesses may manifest but little of this deterioration, it is very observable in that mass of men, women, and children who become the subjects of medical charities. Closeness and uncleanliness of living, with more or less exposure to dampness and extremes either of heat or cold, with intemperance and syphilis, are the main causes of this general constitutional impairment. So important is it that it should never be forgotten, not only in our estimate of the causation of diseases, but in our anticipation of their results, and also in our adaptation of measures of treatment, medical and surgical, to different classes of patients. All that it is allowable here to suggest in this regard may be summed up (although very imperfectly) in the word hospitalism. [Footnote 2: It is important (but not before remarked in this article) that cholera does not appear to allow of any such diminution of liability to it among the natives of the country in which it is endemic.] Conditional causation has been, to a certain extent, included under what has been above said, as it is the action, in part at least, of surrounding conditions, that establishes a family- or race-proclivity and inheritance. But we must say something more about the direct action of conditions upon individuals.