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THE . I ME IU(\ 1 .V \\ 1 TC KALIS T ■ [Vol. LII 



steps toward diversification of species. We base this 

 opinion on the following facts : 



One of the few cases where selection in nature has ap- 

 parently been seen at work under our eyes is the much- 

 quoted case of melanic moths. We started in 1908 to 

 work out the case of the nun, Lymantria monacha. The 

 dark varieties of this moth have been known as rare oc- 

 currences for over a century. But only during the last 

 decades have they spread and almost replaced the white 

 forms. The analysis of the genetics of this case shows 

 that the dark form is a dominant mutation to the white 

 and that the many different stages of darkness, which 

 form a complete series from white to black, are produced 

 by sex-linked multiple allelomorphs. (Unfortunately, 

 the interesting details can not be given at present.) How 

 is it, now, that these combinations have come to replace 

 the original form! Many hypotheses, some of them very 

 strange, have been put forward; but it seems to us that 

 the case is comparatively simple. The dark forms are 

 stronger, more lively, better fliers, as far as we can tell 

 from our experience with the animals in captivity. They 

 are also larger (see Fig. 102, p. 267, in our "Einfiihrung 

 in die Vererbungswissenschaft," 2d ed., 1913). The 

 melanism is in this case, therefore, only the most con- 

 spicuous superficial feature of a quantitative and pro- 

 gressive change in a gene which causes a definite meta- 

 bolic condition, resulting in hardiness as well as in the 

 deposition of more pigment in the wings. The quantita- 

 tive change has here a superficial expression and is there- 

 fore easily recognizable. But this visible pigmentation 

 is not the really important character. How is it, then, 

 that these melanic forms, and other forms in similar man- 

 ner, have established themselves so suddenly? We may 

 venture to point to the facts that the selection, as has 

 often been stated, has occurred especially near the larger 

 cities, and that the period during which this selection has 

 taken place is the period of industrial development, i. e., 

 of restriction of forested areas near the cities. It is, 



