PASSIVE AGGRESSIVITY 37 



formation, however, can scarcely have anything to do with this 

 phenomenon in itself, while we can well imagine that nutritional 

 factors may play an important role. We can thus conceive that 

 during the primary infection, which in turn may be possible owing to 

 capsule formation, a certain group of organisms may have become 

 lodged in a certain organ, and that their vegetative functions here 

 become so modified that the particular juices which are there avail- 

 able can be utilized especially well. If, then, members of this strain 

 are subsequently introduced into another animal, those will develop 

 with special readiness which are placed in contact with the same 

 nutriment to which they had become accustomed in the first host, 

 while the remainder, from lack of this special nutriment, may not 

 develop at all. As a consequence that organ will become the special 

 seat of infection and disease in which conditions for the growth of the 

 organism are most favorable. The affinity for such an organ ,may of 

 course be a natural one, and exist already in an organism which has 

 not been passed through an animal for many generations, but there 

 can be no doubt that it may also be acquired. 



Attenuation. The influence of animal passage upon the aggressivity 

 of an organism can thus be twofold i. e. y it may lead to capsule 

 formation, on the one hand, and to a general increase in its func- 

 tional efficacy as a consequence of especially favorable nutritional 

 conditions, on the other, the outcome being an increased virulence 

 for the infected animal. The reverse will be caused by those 

 agencies which prevent the development of these aggressive 

 forces. We have already pointed out that the ability to form 

 capsules disappears when an organism is grown on ordinary 

 media, and we know that this deficiency may become permanent; 

 this in itself does not interfere with the viability of the organism 

 as a saprophyte, to be sure, but makes its parasitic existence in the 

 animal body an impossibility. Such a decrease in the virulence of an 

 organism can be brought about in many other ways, although it has 

 not been ascertained to what extent impaired capsule formation is 

 responsible for the change; in some instances this may be the case, 

 while in others this explanation is hardly admissible. Such attenua- 

 tion in virulence can be brought about by exposure to temperatures 

 which are unfavorable to the growth of the organism; prolonged 

 exposure to the air; exposure to sunlight; increased atmospheric pres- 

 sure; an electric current; certain chemicals, such as glycerin, carbolic 



