40 OUTLINES OF BACTERIOLOGY 



have lost this capacity ; one culture produces a red pigment, another 

 has not the power to do so, and so on. We could multiply similar 

 instances indefinitely. This will be understood more clearly when it 

 is remembered that precisely the same variations are found among 

 mankind. Thus, one man having been brought up in such a way that 

 his system has been hardened, can stand an amount of exposure that 

 would kill another man who had been differently nurtured, and yet 

 outwardly these two men might appear very much the same. Bacteria 

 are very sensitive to their environment, and great changes can be 

 effected in them in a comparatively short time, without their external 

 appearance being altered. They are probably in this respect more 

 sensitive than any other organisms. This being so, it can be readily 

 understood that bacteriologists look forward to the time when the 

 energies of these organisms can be directed far more than at present 

 towards furthering the well-being of mankind. 



The physiological variations of bacteria have been responsible for 

 much confusion in their nomenclature, and there can be no doubt that 

 of the thousand odd species of the genus bacillus, a large number to 

 which names and descriptions had already been given have been 

 described and named anew, owing to differences under different con- 

 ditions. The greatest variability is seen in the colours of cultures of 

 bacteria. Sarcina aurescens, for example, has been known to assume 

 such different colours as light orange, yellow, or gray, when culti- 

 vated by different observers. The exact nature of these variations is 

 unknown. 



Again, a species normally motile does not always exhibit motility. 

 As the motility is due to the lashing of the cilia, the absence of 

 motility is due to one or other of the conditions which prevent 

 the development of the cilia, or cause them to be thrown off when 

 developed, or to conditions which cause the development of an undue 

 amount of mucilage round the cell-walls. 



Again, it is possible, within a comparatively short time, to effect the 

 formation of a new physiological variety with regard to heat, by sub- 

 mitting a particular species to a process of artificial selection. Cultures 

 of a species are submitted to different degrees of heat, the " breeding " 

 for the next generation being effected from that culture which has 

 suffered most heat without destruction. Thus, suppose that a series of 

 six culture-tubes, after inoculation with a particular species, were heated 

 respectively for 1, 2, 3, 4, 5 and 6 minutes at 100° C, and suppose that 

 1, 2, 3 and 4 showed growth, while 5 and 6 did not. The next series 

 would be selected from the number 4 culture, which, of those in which 



