116 ON VARIABILITY AND ADAPTATION 



either which is present, so are the daughter cultures of particular 

 degrees of virulence. These asporogenous races appear to retain 

 their characters indefinitely under normal bacteriological 

 conditions. 



Here again it must be pointed out that " permanent " races 

 are by no means necessarily attenuated races. Starting from 

 a culture of the Bacillus anthracis, so weak that only young 

 mice are affected, it is possible, by passage of the virus through 

 a series of animals whose resistance to the disease is in an ascend- 

 ing scale, to gain a series of races of gradually increasing virulence, 

 and as Malm has shown 1 it is possible to pass beyond the 

 virulence of the type and obtain a race that will kill not only 

 sheep but the very refractory dog. 2 



Another example to the same effect may be gained from 

 Pasteur and Thullier's researches into swine erysipelas (Rouget 

 des pores, or Rothlauf). 3 This fatal and most infectious 

 disease is caused by a minute bacillus first isolated by Loffler. 

 Rabbits also succumb to the malady, but not so very readily. 

 It is found that passage through these latter animals causes the 

 virus steadily to augment in strength until a stage is reached 

 most fatal to rabbits, but cultures obtained at this stage induce 

 but a transient illness in pigs, which, consequently, can be in- 

 oculated or " vaccinated " against the disease. Here exaltation 

 of virulence for one animal and attenuation for the pig can be 

 produced by keeping cultures at 37° C. for a long period. 



Equally instructive cases may be gleaned from researches 

 upon non-pathogenic micro-organisms. Let me take first the 

 Microbacillus prodigiosus. It was found by Wasserzug 4 that 

 the addition of antiseptics to the medium of culture, in quantities 

 sufficient to retard growth, led not only to the loss of colour 

 production, but also to accompanying remarkable changes in 

 form : some individuals become much elongated, others develop 



1 Malm, Annates de VInst. Pasteur, iv., 1890, p. 520. 



2 I may here call attention to a statement in Professor C. Frankel's 

 Bakterienkunde (edition of 1891, p. 177) : " Es ist . . . bisher noch nicht 

 gegluckt, eine dauernende Verstarkung der Bakterien, eine haltbare Zunahme 

 ihrer naturlichen Virulenz herbeizufuhren . . . auf Tiere . . . eine raschere 

 und andere Wirkungsweise an den Tag zu legen, als sonst an ihnen bemerkt 

 wurde." While these statements, in face of French researches, were, to say the 

 least, already very debatable a year ago, they certainly now cannot be regarded 

 as other than quite incorrect. 



3 Pasteur and Thullier, Comptes rendus, xcvii., 1883, p. 1113. 



4 Wasserzug, Annates de VInst. Pasteur, ii., 1888, p. 75. 



