CLONAL PHENOMENA 



infective dilution transfers from a culture in a slightly higher 

 concentration of TMG are made into the maintenance 

 medium, tubes showing growth will be sharply divisible into 

 those containing much /^-galactosidase and those with negli- 

 gible amounts. Since a bacterial mutation is normally defined 

 as one which can be shown to differ from the parent form in 

 some observable characteristic when both are grown under 

 the same circumstances in the same medium, and the 

 character is transmitted to the offspring, this could be 

 labelled a mutation. Admittedly the conditions required 

 are complex and highly artificial. In addition to those 

 mentioned, the phenomenon would be much less striking if 

 it were not for the fact that induced organisms grow more 

 slowly than non-induced, so that with minimal concentra- 

 tions very slow induction is insufficient to keep pace with the 

 faster multiplication rate of the non-induced bacteria. 

 Nevertheless it provides an important warning that what we 

 tend to regard as unanalysable biological processes like 

 replication and mutation may, in some or all instances, be 

 analysable into a series of processes more in line with normal 

 ideas of biochemistry. Or in Novick and Weiner's words: 

 ' Some differences which arise in a clone of organisms may 

 be the result of changes in cellular systems other than the 

 primary genetic endowment of the cell'. This warning is 

 especially necessary where the cells concerned are not as 

 accessible to detailed physiological analysis as are bacteria. 

 At the same time the biologist must insist that, until possi- 

 bilities of biochemical analysis can be exploited, genetic 

 methods of approach and interpretation are still legitimate. 



SUMMARY. The following points are probably those which 

 are most likely to be relevant to any discussion of clonal 

 selection in other fields. 



(i) Adaptive changes not due to mutation are of frequent 

 occurrence; those normally studied represent quantitative 

 development of latent but genetically determined properties. 



