THE LIMITATIONS OF PEKMANENCY 119 



frequency, will yield microbes which, will all produce typical 

 symptoms when inoculated into a series of rabbits, guinea-pigs, 

 or other animals, the microbes of the last of the series of succes- 

 sive cultures possessing the same degree of virulence as the 

 first of the series. 



Where this is the case, are we justified in stating that the 

 species is permanent ? Upon first consideration one is inclined 

 to say that undoubtedly we are, yet further thought brings in 

 doubt, for this is far from being all. There is another and 

 remarkable order of appearances to be considered, which can 

 be exemplified from every case that I have given of the establish- 

 ment of permanent races. 



Take, for instance, the races of anthrax bacilli produced by 

 Pasteur's method. These, undoubtedly, when kept upon our 

 approximation to a common soil, namely, upon beef -broth, 

 preserve a well-defined permanency. But select any one of 

 these, and pass it through a series of animals of increasing re- 

 fractoriness to the disease, and there is evolved a race growing 

 equally well upon the beef-broth, whose virulence has been 

 greatly increased. Or take Gessard's " permanent " modifica- 

 tions of the Bacillus pyocyaneus and the Bacillus cyanogenus 

 respectively ; grow any of the former upon the rich culture 

 medium formed of agar-agar, glycerine, and peptone, and all 

 develop equally into a race producing large quantities of an 

 intense blue pigment ; grow any of the latter upon broth con- 

 taining 2 per cent of glucose, and all give rise to the specific 

 blue pigment of the typical bacillus. Again, take Laurent's 

 colourless race of the " Bacillus rouge de Kiel," and place it 

 upon potatoes at a temperature of about 18° instead of 25° 

 to 35° C, and the coloured form reappears. 



Or to epitomize. It is possible with bacteria to bring about 

 modifications which persist when a return is made to the ordinary 

 media of the bacteriologist (whether these be only our approxi- 

 mation to the normal — beef-broth, potatoes, etc. — or whether 

 they be the bodies of one or other species of animal, mice, sheep, 

 etc.), so that now cultivated upon these " ordinary " media, the 

 modification preserves its characteristics, and remains markedly 

 different from the initial or type growth upon the same media. 

 And here we have a distinction between the races of bacteria 

 and the races of animals under domesticity, which, returned to 



