268 THE FACTORIAL HYPOTHESIS 



generations. Of course this demonstration could not 

 have been made with heterozygous individuals. 



3. It has also been suggested that one factor may 

 sometimes contaminate its allelomorph, when the 

 two meet in the hybrid. There is no a priori reason 

 why this might not occur so far as we can see. The 

 question is whether there is any evidence to establish 

 or even make probable such a view. The great 

 body of Mendelian evidence points unmistakably 

 to the conclusion that as a rule contamination does 

 not occur. It will require equally clear evidence to 

 show that contamination does sometimes take place. 

 Until this evidence is forthcoming the facts w^hich 

 have been said to support the hypothesis of contami- 

 nation find a more consistent explanation on the 

 hypothesis of multiple factors. 



4. Bateson has recently argued from the visible 

 differences between characters that a process of 

 fractionation of factors takes place. The argument 

 is given in the following quotation: 



''Some of my Mendelian colleagues have spoken 

 of genetic factors as permanent and indestructible. 

 Relative permanence in a sense they have, for they 

 commonly come out unchanged after segregation. 

 But I am satisfied that they may occasionally undergo 

 a quantitative disintegration, with the consequence 

 that varieties are produced intermediate between the 

 integral varieties from which they were derived. 

 These disintegrated conditions I have spoken of as 

 subtraction — or reduction — stages. For example, 

 the Picotee sweet pea, with its purple edges, can surely 



