114 ON VARIABILITY AND ADAPTATION 



colourless or faintly-coloured growths are obtainable, which for 

 long show no reversion to type. 



It would seem that, without there being necessarily any 

 attenuation, there may be modifications persisting for long 

 periods. It has been noted by many observers that for some 

 considerable period after Staphylococcus pyogenes aureus has 

 been separated out of pus and cultivated, the colonies are colour- 

 less, and that only eventually may the golden-yellow pigment 

 become produced. During the last long vacation I was a victim 

 to this order of affairs, for having obtained a micrococcus from 

 some pus gained from a case at the Addenbrooke Hospital, I 

 made two successive cultures of the pure growth during the 

 course of a week, and the second of these I employed to distribute 

 to the bacteriological class as the Staphylococcus pyogenes 

 albus. The cultures' made by the class developed the golden- 

 yellow pigment, and proved that we were dealing, not with the 

 " albus," but with the " aureus." It may be added that, save 

 for colour difference, the two forms are indistinguishable. 



The Production op Eaces 



From such cases as these we can pass on almost imperceptibly 

 to the development of what may be termed races. I will presently 

 discuss the qualifications that must be applied in employing this 

 expression. For immediate purposes it is enough to state that 

 a slight modification of environment acting over a sufficient 

 number of " generations " — that is to say, acting for a sufficiently 

 long period — or a powerful stimulus applied temporarily to the 

 individual bacteria of one " generation," may lead to modifica- 

 tion of the properties of any species which, so far as we can see, 

 are permanent so long as the microbes so acted upon are cultivated 

 under ordinary conditions upon the usual media of bacterio- 

 logical research. 



To make this statement perfectly clear, I may give an example. 

 The Bacillus ruber of Kiel grows well upon a piece of sterilized 

 potato at the ordinary temperature (15-25° C), the culture 

 becoming a deep crimson red. If now the culture be kept at a 

 temperature of 37° C. for two days, and a new piece of potato 

 be inoculated from this and kept another two days at the same 

 temperature, and a series of six to ten cultures be made thus ; 



