MODIFICATIONS OF LONGER DURATION 113 



present differences in pigment production ; the majority re- 

 produce the type, but some are even deeper in colour than the 

 original, others paler, others colourless. If from one of these 

 colourless colonies fresh potato growths be made, a large number 

 of coloured colonies develop, but also a large number of colour- 

 less, and the more frequently this selective process is repeated 

 the greater becomes the proportion of the resulting colourless 

 colonies. Yet even after a very great number of growths made 

 in this way there is always a tendency for certain individuals 

 and their progeny to revert to type. Slight alterations in the 

 nutrition of the individuals probably induced the first differences 

 in pigment production, and the induced modifications tend to 

 reproduce themselves through many " generations." x 



The same occurs if a culture of one of these non-pathogenic 

 forms be heated for a few minutes to a point approaching that 

 at which immediate death of the special microbe is brought 

 about. After this treatment there may be numerous " genera- 

 tions " produced, incapable of pigment production. Illustrating 

 this and the preceding order of appearances, I showed a series 

 of specimens at the Nottingham meeting of the British Medical 

 Association in July (1892). The series included cultures of the 

 Bacillus ruber, Plymouth ; Bacillus ruber, Kiel ; Microbacillus 

 prodigiosus, Bacillus indicus, and Sarcina erythromyxa. To yet 

 more marked effects of high temperature I shall revert at a 

 later period. 



Again, old cultures of most micro-organisms become 

 attenuated, and although these attenuated forms still propagate 

 themselves, it may be weeks — that is to say, almost countless 

 " generations " — before there is a return to primitive appearance 

 and primitive properties. Thus old growths of Koch's cholera 

 spirillum, or the Finkler- Prior spirillum of Cholera nostras, 2 

 or, again, of the Vibrio metchnikovi 3 yield colonies which 

 have an absent or greatly reduced power of liquefying gelatine, 

 and only very slowly and under favourable conditions does this 

 power manifest itself once more. From old cultures of the 

 Bacillus pyocyaneus, the Pink torula, and many more chromo- 

 genic microbes, I find, in common with other observers, that 



1 See more particularly Charrin and Gessard, loc. cit. 



2 Firtsch, Archiv jiXr Hygiene, viii., 1888, p. 369. 



3 R. Pfeiffer, Zeitschr. fur Hygiene, vii., 1889, p. 347. 



