No. 613] EXPERIME. 



explanation for them which both does justice to the facts 

 themselves and falls in line with modern genetic views. 



Furthermore, we now see the exact meaning of Dar- 

 win's view, which he had to express in a somewhat am- 

 biguous way on account of the lack of experimental data 

 which would have permitted clearer expression. His 

 essay of 1842, the forerunner of the "Origin of Species," 

 begins with the words: "An individual organism placed 

 under new conditions sometimes varies in a small degree 

 and in very trifling respects, such as stature, fatness, 

 sometimes color, health, habits in animals and probably 

 disposition. . . . Most of these slight variations tend to 

 become hereditary." This statement shows clearly what 

 Darwin had in mind. If he assumes that some variations, 

 which are produced by change of conditions, are some- 

 times non-heritable, but tend to be inherited, we can now 

 explain what this means. The variations which, as geo- 

 graphic races, form the first steps in the formation of new 

 species are indeed exactly the same whether or not they 

 are inherited. Their direct physiological cause is also 

 identical, being a change in the rate of a definite process 

 during differentiation. Only the ultimate cause is differ- 

 ent ; in the one case the original quantity of the gene de- 

 termines the rate of differentiation — which then is heredi- 

 tary—from the beginning; in the other case an outside 

 factor is active, retards or accelerates the same reaction to 

 the same degree. With this additional bit of interpreta- 

 tion, Darwin is right, after all. 



The other group of facts includes certain details of 

 mimicry (mimetism). We believe that the general prin- 

 ciple of mimetism has been fully explained genetically by 

 Punnett. But there are certain details which his selec- 

 tionist opponents point out which constitute strong evi- 

 dence against Punnett 's view. We think that the most 

 valid argument against the Mendelian view of mimetism 

 has been derived from the facts about the parallel geo- 

 graphic variation of model and mimic. If our genetic 

 conception of geographic variation is correct, this point 



