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sised by the bacterial protoplasm is often disturbed by purely 

 physical causes. 



A good example of (6) is the production of a variant by growth 

 in homologous immune serum. Here the conditions are simpler 

 and it is possible to explain the stimulus which initiates the 

 variation as being directly due to the specific antibodies in the 

 serum. In this case it is reasonable to postulate that, when 

 digested food particles are being synthesised into protoplasm, 

 the antibodies find in some of those particles their appropriate 

 antigen and " pick them off " ; the new bacterium is synthesised, 

 but it is an impoverished bacterium (a variant), because it has 

 been robbed of some of its antigenic components. To take 

 another example, the products of bacterial growth are sometimes 

 highly specific for the bacterium in question ; when these products 

 are removed by filtration, growth of the bacterium in this medium 

 (the filtrate) may initiate the production of a variant. How this 

 comes about is not really known; one can only guess. Perhaps 

 it may be said that particles in the bacterial filtrate have receptors 

 identical with those of the living bacterial protoplasm and there- 

 with " pick up " particles ready for assimilation by bacteria 

 in the nascent stage of growth ; such bacteria are impoverished, 

 i.e., they are a variant; they may be so much impoverished 

 that their offspring are non- viable. 



I have said that the initial stimulus may be (a) non-specific, 

 or (6) specific. But these are not the only alternatives. It may 

 be partly the one and partly the other ; or it may act like (6) on 

 one occasion and like (a) on another. To revert to the last 

 example, a bacterial filtrate may be so highly specific that it 

 can only produce a variant when acting on its homologous strain ; 

 other filtrates may act on all strains of the same species, others 

 on allied species, others, again, on some species which have no 

 relationship to the bacterium producing the active principle 

 contained in the filtrate. 



II. When once a new variant is formed, its propagation is 

 obviously due to a spec 'fie influence, which is the special attribute 

 of the modified bacterial protoplasm. The new arrangement 

 of particles and side-chains usually tends to repeat itself auto- 

 matically, from generation to generation and from culture to 

 sub-culture, without the need of any external influence. But 

 in the type of variation which is associated with " transmissible 

 autolysis " an external influence is still in operation. Two pro- 

 cesses are at work simultaneously ; there is renewal of an external 

 stimulus (now specific) for the initiation of the variant, and there 

 is the tendency for the variant to multiply automatically, though 

 with the production of many non- viable forms. To put it briefly, 

 the starting point is a stimulus, either non-specific or specific, 

 causing the formation of variants, some of which are non- viable. 

 These latter disintegrate into particles which act as specific stimuli 

 (like the bacterial filtrates mentioned above) and change more 

 normal bacteria into variants ; many of these are non-viable and 



