No. 613] EXPERIMENTS CONCERNING EVOLUTION 39 



views in regard to the variability of the sex-factors 

 a priori inadmissible. We believe that this intellectual at- 

 titude toward the problem is the result of Johannsen's 

 doctrine of agnosticism in regard to the nature of the 

 gene, which resulted in a kind of mystic reverence, ab- 

 horring the idea of earthly attributes for a gene. (Our 

 distinguished opponents will excuse this somewhat ex- 

 treme statement.) If, however, it can be proven that 

 genes are substances with the attribute of definite mass, 

 it would be illogical to deny their variability. Nobody 

 will claim that a gene is a substance that passes unaltered 

 from generation to generation. The elementary facts of 

 development and regeneration show that this substance 

 grows, at least, and increases in quantity. If, now, the 

 substantial basis of heredity in the sex-cells is established 

 by the assembling of all the factor-substances in their 

 characteristic quality and their correct quantity, the sit- 

 uation is the same for the gene as for any other organic 

 process: the varying conditions of the surroundings of 

 the gene cause a certain amount of fluctuation in its quan- 

 tity. This conclusion entirely changes the logical aspect 

 of the question, whether or not a change of the gene by 

 selection of variants is possible. 



The strongest point-of the anti-selectionists was that it 

 is absurd to assume that a selection of somatic fluctuation 

 has anything to do with the characters of the germ-plasm. 

 With the quantitative view, however, which we believe to 

 have proven in two elaborate cases, this situation changes. 

 The somatic character in question, say amount of pig- 

 mentation, can only change toward a plus or minus side. 

 This change is caused directly by a difference in the 

 velocity of the reaction of some metabolic process which 

 results in the deposition of pigment. Such a change of 

 velocity of reaction, however, can be produced either by 

 the action of the medium, and then it is a modification, or 

 by fluctuation in the quantity of the gene, causing increase 

 or decrease in the velocity. The resulting variation is of 

 course, phenotypically, the same. Selection, therefore, 



