366 POPULAR SCIENCE MONTHLY. 



sinister intruders, and did time permit I should like to call your 

 attention somewhat in detail to the mimic warfare which is main- 

 tained, 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 be- 

 lief that several of these unknown mechanisms are largely useful 

 in disposing of or rendering harmless numerous poisonous sub- 

 stances 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 suc- 

 cumb upon the entrance of its allies. 



It has been shown that the same germ, under varying condi- 

 tions, 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, espe- 

 cially 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 genera- 

 tions which come and pass within the range of a single experi- 

 ment, 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. 



