236 BACTERIOLOGY 



verify; but we may assume without immediate fear of experimental 

 contradiction that a century of incomplete protection may have 

 worked some changes in the smallpox organism. In any case, it is 

 obvious that our thesis implies, in addition to the natural decline of 

 virulence, also a gradual rise in virulence whenever the resistance 

 of susceptible individuals is raised on a very large scale. Either the 

 micro-organism, if a true parasite, will perish, or else it will aug- 

 ment its invasive powers to meet those of its host. 



Another problem has been created for the diphtheria bacillus by the 

 extensive use of diphtheria antitoxin. Will the thorough protection 

 of one group of human beings lead to the decline or to the increase in 

 virulence of the diphtheria bacillus circulating among the individuals 

 of this group? What effect will the transfer of such bacilli to unpro- 

 tected groups have? These and similar queries may be answered not 

 many years hence, for a generation of microbes represents a very 

 short space of time. 



It may not be out of place to call attention here to the bearing of my 

 thesis upon the recent attempts to utilize parasitism in ridding us 

 of undesirable or noxious animals. In bacteriology there have been 

 attempts to destroy field-mice and rats with certain species of bacteria. 

 In entomology, parasitism is such a familiar phenomenon that it has 

 been seized upon on a number of occasions to destroy otherwise un- 

 assailable insect pests. 



Leaving out of consideration the presumptive dangers of introduc- 

 ing new species into a locality or country, which must always be taken 

 into consideration, although they may be of no significance, we have 

 to consider the chances of success as compared with the cost of intro- 

 ducing and maintaining the parasites. In any event, we need not 

 expect a destruction of the noxious species, for that is not the end of 

 parasitism. A reduction in numbers is all that need be looked for. 

 The new parasite will probably fail to become acclimated at first, 

 and it may be necessary to reintroduce it for a number of years. During 

 this period some few may become adapted to their environment, and 

 continue as parasites. Whether the equilibrium finally established 

 will be of economic value, must be observed rather than predicted. 

 In bacteriologic experiments of this kind the continued vigorous activ- 

 ity of the bacteria from year to year need hardly be expected. The 

 disease will either die out or continue on a low level of mortality, 

 in accordance with the general laws I have detailed, unless bacteria 

 whose destructive powers are maintained and carefully gauged in 

 the laboratory are distributed at definite intervals. 



In conclusion, I will simply call attention to another problem affect- 

 ing the future well-being of mankind, the possibility of new infectious 

 diseases arising in the flux and change incidental to human progress. 

 We have assumed that the capacity for a parasitic existence probably 



