sinister intruders, and did time permit I should like to call your attention somewhat in detail to the mimic warfare which is maintained, often in extreme hazard, under conditions which belong especially to modern life.
There are still several tissues and organs about whose uses we are, in the main, ignorant; but there is much reason for the belief that several of these unknown mechanisms are largely useful in disposing of or rendering harmless numerous poisonous substances which in their complex metabolisms the varied body cells get free; and the recent revelations in the relationship which microbes bear to disease have made it seem probable that the agencies which the body has developed for its own protection against itself are the same agencies which it employs to protect itself against these small invaders.
We have learned of late that not infrequently more than one germ species may be at work in producing the cell disturbance which we call disease, and that an individual whose inherent protective mechanism promises a favorable outcome to the fight when one enemy alone is within the citadel, may speedily succumb upon the entrance of its allies.
It has been shown that the same germ, under varying conditions, can induce maladies hitherto thought to be different; so that the number of our unseen enemies in this silent realm may, as our outlook becomes more commanding, prove to be smaller than we have been led to think.
Several of the most fateful germs are constantly with us, especially in towns, but doing no harm, unless through some breach in the complex and curious line of the body's defenses they enter the sanctuary. Even then they are often harmless, as we have seen, except in an organism whose defensive mechanism is already weakened by excesses or disease.
The extreme rapidity with which these organisms multiply has made it possible to study experimentally, in the countless generations which come and pass within the range of a single experiment, the effect of environment upon their morphological and biological characters, and the existence of races and varieties within the limits of what we are pleased to call bacterial species has been well established.
It has been possible by changes of environment to so alter the metabolism of disease-producing germs that, though apparently growing as vigorously as ever, the poisons by which their evil effects on man are caused have lost their power. In fact, we now know several germs quite similar to those which are of most sinister import to man which, apparently, under what we call natural influences, have lost their power to harm. These are some of the lines of study with which bacteriology to-day is busy.
