No. 642] VARIATION IK INDIVIDUAL GENE 47 



uals. In this way relatively slight variations in muta- 

 tion frequency, caused by the special treatments, can be 

 determined, and from the conditions found to alter the 

 mutation rate slightly we might finally work up to those 

 which affect it most markedly. The only methods now 

 meeting this requirement are those in w^hich a particular 

 mutable gene is followed, and those in which many 

 homozygous or else genetically controlled lines can be 

 run in parallel, either by parthenogenesis, self-fertiliza- 

 tion, balanced lethals or other special genetic means, and 

 later analyzed, through sexual reproduction, segrega- 

 tion and crossing over. 



V. Other Possibilities 



We can not, however, set fixed limits to the possibilities 

 of research. We should not wish to deny that some new 

 and unusual method may at any time be found of directly 

 producing mutations. For exanii^le, tlie phenomena now 

 being worked out by Guyer may Ix' a case in i»oint. There 

 is a curious analogy between tlu^ i-^'actioii^ of immunity 

 and the phenomena of heredity, in apparently funda- 

 mental respects,' and any results that seem to connect 

 the two are worth following to the limit. 



Finally, there is a phenomenon related to immunity, of 

 still more striking nature, which must not be neglected by 

 geneticists. This is the d'Herelle phenomenon. D'Herelle 

 found in 1917 that the presence of dysentery bacilli in 

 the body caused the production there of a filterable sub- 

 stance, emitted in the stools, which had a lethal and in 

 fact dissolving action on the corresponding type of bac- 

 teria, if a drop of it were ai->plied to a colony of the bac- 

 teria that were under cultivation. So far, there would 

 be nothing to distinuuisli this i.hoiiomenon from im- 



