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individuals which are just viable tend to subdivide into non- 

 viable elements and may, eventually, be eliminated. This 

 artificial method of producing, and finally eliminating, a " sensi- 

 tive " variant, under the influence of what has been called a 

 " lytic principle," calls for careful consideration, because it may 

 have its counterpart in the animal body, as one of the many and 

 highly complex factors in the mechanism of resistance to bacterial 

 invasion. 



As bacteriologists are familiar with the more important work 

 on bacterial variation and transmissible autolysis, there is no 

 need for me to reproduce a detailed review of the literature. 



For the immediate purpose of this report a brief historical 

 note will suffice. Opinion on the significance of bacterial variants 

 seems to have been passing through the following phases : — 



(1) In ordinary routine work, when an organism is 

 plated out for the purpose of isolating and propagating 

 a pure culture, the bacteriologist selects what he knows from 

 experience to be a typical colony and does not trouble about 

 other colonies which look more or less different from his 

 idea of the normal. The question of variants he regards 

 as negligible. 



(2) But patient comparative study of different colonies 

 derived from a single strain, which is known to be pure, has 

 shown that, with most bacterial species, the occurrence of 

 so-called " aberrant " forms is not exceptional but is the 

 general rule and that, therefore, the term " aberrant " 

 may not be justifiable. This is disconcerting, because it 

 means that the forms with which the bacteriologist actually 

 has to deal are not " ear-marked " by nature as " normal " 

 and " variant " but are simply " varieties," each of which 

 may be equally normal. Consequently, one's ideas of what 

 is " typical " and what is a " variant " need justification 

 and may require revision. At this stage it is realised that 

 the question of variants is not negligible ; it has to be taken 

 seriously, because much confusion arises from failure to 

 recognise actual, or potential, differences in individuals 

 amongst the bacteria comprised in an ordinary " pure 

 culture." 



(3) The study of variants was diverted into a new channel 

 by Twort, who discovered what is now known as " trans- 

 missible lytic substance." He first found this substance 

 in growths of micrococci cultured from vaccine virus; it 

 consisted of glassy, transparent material which lysed the 

 micrococci and was transmissible from culture to culture. 

 Subsequently d'Herelle discovered similar material, which 

 he obtained, in the first instance, from the intestinal excreta 

 of a dysentery patient. These observations have aroused 

 much controversy as to the nature of the lytic substance. 

 D'Herelle, for example, has always maintained that it is a 

 living, ultra-microscopic virus and calls it a " bacteriophage." 



