80 MUSGEAVB. 



that the latter produce bodies (antibodies) which are inimical to the 

 tissue cells? Looked at from the point of view of the bacterium] as 

 well as from that of the animal host, according to the hypothesis 

 advanced, the struggle between the bacteria and the body cells in infec- 

 tions may be conceived as an immunizing contest in which each partici- 

 pant is stimulated by its opponent to the production of cytotoxins hostile 

 to the other, and thereby endeavors to make itself immune against its 

 antagonist." 



If this hypothesis be true, and it certainly seems to be a reasonable one, 

 the evolution of parasitism, even in pure cultures of bacteria would seem 

 at least in part to depend upon the influence of other agents in support- 

 ing, or hindering, the development of either of the elements — parasite or 

 host. 



Theobald Smith a in discussing the parasitism of the tubercle bacillus, 

 and bearing upon this point, "assumes as a basis for discussion a 

 complex relationship established in time by a selective adaption between 

 two living organisms, of which one is a parasite of the other." 



"Whatever pathologic processes of a constant character are the expres- 

 sion of this parasitism * * * are regarded as the result of an 

 interaction of two organisms rather than the work of one alone." 



"Viewed from this standpoint, this tendency toward a state of equi- 

 librium between host and parasite is disturbed by any change of condition 

 which influences either parasite or host." Again, the same author 

 believes "that the tendency of infectious disease, is toward a balanced 

 parasitism, with a reduced mortality, but not necessarily a reduced 

 morbidity as a result." 



This, Theobald Smith considers to be due to a selective action of both 

 host and parasite. In the case of the tubercle bacillus, the very attenuated 

 forms would become, as it were, the parasites of the sick lungs, rather 

 than of the normal ones and would take the same position which pneu- 

 mococci, streptococci and staphylococci occupy in the upper air passages. 



Klein * in discussing the question of the evolution of bacteria from 

 saprophytes to parasites concludes that "there is evidence to show that 

 an ordinary saprophyte under altered conditions can produce pathogenic 

 effect, thriving in the tissues of an animal; and that a microbe, under 

 one condition an ordinary saprophyte, can raise itself not only to 

 parasitic life, causing pathogenic action, but further to a parasitic action 

 which is as specific as that of the Bacillus anthracis or Bacillus influenza'." 



There are numerous additional facts which tend to support the 

 evolutionary theory of parasitism. For example; under different cir- 

 cumstances the tubercle bacillus may manifest any role from the 

 saprophyte to the most exquisite and highly specialized tissue parasite, 



3 Ibid. (1906), 46, 1247. 

 'Lancet (1904), 2, 1477. 



