50 MENDELISM AND 



manent and indestructible, he is satisfied that they 

 may occasionally undergo a quantitative disintegra- 

 tion, the results of which he calls subtraction or 

 reduction stages. For example, the Picotee Sweet 

 Pea with its purple edges can be nothing but a 

 condition produced by the factor which ordinarily 

 makes the fully purple flower, quantitatively 

 diminished. He remarks also that these fractional 

 degradations are, it may be inferred, the con- 

 sequences of irregularities in segregation. 



Bateson, however, proceeds to urge that the 

 history of the Sweet Pea belies those ideas of a 

 continuous evolution with which we had formerly 

 to contend. The big varieties came first, the little 

 ones arose later by fractionation, although now the 

 devotees of continuity could arrange them in a 

 graduated series from white to deep purple. Now 

 this may be historically true of the Sweet Pea, 

 but I would point out that once the dogma of the 

 permanent indivisible unit or factor is abandoned, 

 there is nothing in Mendelism inconsistent with 

 the possibility of the gradual increase or decrease 

 of a character in evolution. I do not suggest 

 that the colour and markings of a species or variety 

 were, in all cases, due to external conditions, but if 

 the effect of external stimuli can be inherited, can 

 affect the chromosomes, then the evidence concerning 

 unit factors no longer contradicts the possibility 

 of a character gradually increasing, under the 

 influence of external stimuli acting on the soma, 

 from zero to any degree whatever. 



