246 PHYSICAL BASIS OF HEREDITY 



is speculative, and would have little importance were it not 

 that, by imputing to the advocates of Mendelian heredity 

 the assumption of absolute fixity to the gene, attempts 

 have been made to throw the burden of proof that the 

 genes are ''constant'' on the advocates of Mendelism. 



So far as the genetic evidence is involved, I see at pres- 

 ent no way of deciding whether the gene has a definite 

 molecular constitution, or is only something that fluctuates 

 under the condition of its occurrence about a mode. Inter- 

 esting as it might be to speculate about these alternatives, 

 it seems futile to do so at present, but there is one impli- 

 cation that I should like to examine. If the gene is a 

 chemical molecule it is not evident how it could change 

 except by altering its chemical constitution. Its influence, 

 i.e., the chemical effects it produces, might, however, be 

 altered by changing other substances with which the mate- 

 rial it produces reacts. This is the idea involved in the 

 theory of ''modifying genes.'' 



But if the gene is a fluctuating amount of something it 

 might seem that any "fluctuation" that is present at one 

 time might be perpetuated by selection, and that a further 

 fluctuation in the same direction might be utilized for a 

 further advance, etc. It may be pointed out that this 

 picture of the process is quite fanciful, and its success 

 would depend largely on a denial of the premise as to the 

 nature of the gene, viz., that it is of a fluctuating amount. 

 Johannsen's facts contradict an interpretation of the 

 fluctuations of the character being due to a new modal 

 position of the gene standing for that character. And his 

 facts furnish the only crucial evidence we have at present. 



