PASSIVE AGGRESSIVITY 35 



long been recognized by bacteriologists, but an adequate explana- 

 tion for it had long been wanting. By starting with a laboratory 

 culture that has been grown for many generations on artificial, 

 non-albuminous media, it may be absolutely impossible to produce 

 an infection at all, even though enormous numbers of bacteria be 

 injected. If infection, however, results we may imagine in the case 

 of one of the organisms in which capsule formation occurs that even 

 though the majority of organisms have lost the power of forming 

 capsules, and of thus resisting the offensive forces of the host, a 

 certain number still possessed this property, and that these escaped 

 destruction and multiplied to a greater or less extent. 



If then at the height of the infection the animal is killed, or if it 

 succumbs to the infection directly, the now capsulated bacteria will 

 be found capable of successfully infecting the next animal, to which 

 they should be transferred without being first replanted on ordinary 

 media. As a result of the increased degree of resistance which the 

 organisms have acquired in the first animal, they are now in a much 

 better position to maintain themselves and to multiply in the second, 

 and as the transfers are continued through a series of animals, it 

 will be observed that the number of organisms which is necessary 

 to kill the animal becomes progressively smaller, and the period of 

 incubation, i. e., the interval elapsing between infection and the first 

 evidence of the resulting disease, shorter, until finally a strain is 

 obtained in which the degree of virulence can no longer be increased 

 by animal passage this constitutes the virus fixe in the sense of 

 Pasteur. 



Potential Virulence. While we have thus gained a material basis 

 for our concept of the actual virulence of an organism, it is important 

 also to recognize a certain potential virulence, viz., the ability of an 

 organism actually to form capsules when placed under conditions 

 which, cceteris paribus, are favorable to their developement. Evi- 

 dently only those organisms of the capsule-forming group, or at any 

 rate those in which an hypertrophy of the ectoplasm can occur, are 

 capable of acquiring a notable degree of virulence in which this poten- 

 tiality is inherent. If once this is permanently lost the organism 

 in question is manifestly non-virulent, so far as its actual develop- 

 ment in the infected animal is concerned. We have had an excellent 

 illustration of this in Horiuchi's experiment, referred to above. To 

 determine this potentiality it is sometimes only necessary to grow the 



