to death all weakly individuals, that only the strong might live to become the fathers of future generations.  In this light, of course, parasitic diseases would be an assistance rather than a detriment to the human race.  Of course such principles will never again be dominant among men, and our conscience tells us to do all we can to help the weak.  We shall doubtless do all possible to develop preventive medicine in order to guard the weak against parasitic organisms.  But it is at all events well for us to remember that we can never hope to develop the strength of the human race by shunning evil, but rather by combating it, and the power of the human race to resist the invasions of these organisms will never be developed by the line of action which guards us from attack.  Here, as in other directions, the principles of modern humanity have, together with their undoubted favourable influence upon mankind, certain tendencies toward weakness.  While we shall still do our utmost to develop preventive medicine in a proper way, it may be well for us to remember these facts when we come to the practical question of determining where to draw the limits of the application of methods for preventing infectious diseases.

Curative medicine.

Bacteriology has hitherto contributed less to curative than to preventive medicine.  Nevertheless, its contributions to curative medicine have not been unimportant, and there is promise of much more in the future.  It is, of course, unsafe to make predictions for the future, but the accomplishments of the last few years give much hope as to further results.

Drugs.

It was at first thought that a knowledge of the specific bacteria which cause a disease would give a ready means of finding specific drugs for the cure of such disease.  If a definite species of bacterium causes a disease and we can cultivate the organism in the laboratory, it is easy to find some drugs which will be fatal to its growth, and these same drugs, it would seem, should be valuable as medicines in these diseases.  This hope has, however, proved largely illusive.  It is very easy to find some drug which proves fatal to the specific germs while growing in the culture media of the laboratory, but commonly these are of little or no use when applied as medicines.  In the first place, such substances are usually very deadly poisons.  Corrosive sublimate is a substance which destroys all pathogenic germs with great rapidity, but it is a deadly poison, and can not be used as a drug in sufficient quantity to destroy the parasitic bacteria in the body without at the same time producing poisonous effects on the body itself.  It is evident that for any drug to be of value in thus destroying bacteria it must have some specially strong action upon the bacteria.  Its germicide action on the bacteria should be so strong that a dose which would be fatal or very injurious to them would be too small to have a deleterious influence on the body of the individual.  It has not proved an easy task to discover drugs which will have any value as germicides when used in quantities so small as to produce no injurious effect on the body.