592 POPULAR SCIENCE MONTHLY. 



judged by its effect, is neither more nor less virulent than its early pro- 

 genitors. It has often died out in a given locality or country, it has 

 even been forced back to its original ancestral home, but still the same 

 type, the same species has perpetuated itself unchanged. If the plague 

 on its present world-wide journey does not cause such terrible out- 

 breaks as it has in the past, it will be not because the germ has been 

 altered by time, but because man has changed in so far as he has slowly 

 learned and profited by the lessons of previous epidemics. 



