126 I The Process of Evolution 



of northern Europe and North America. In many of these areas the 

 melanic forms have become predominant, replacing or partially re- 

 placing protectively colored "typical" forms. These changes have 

 taken place primarily in heavily industrialized areas and have been 

 especially spectacular in England where they have been studied 

 extensively. It has been estimated that in the area of Manchester, in 

 1848, the dark form of the moth Biston betttlaria made up a maxi- 

 mum of 1 percent of the population and that in the same area, in 

 1898, it made up more than 99 percent of the population. In most 

 of the known cases the melanism is produced by a single dominant 

 gene. 



The following hypothesis has been developed to account for the 

 phenomenon of "industrial melanism." The spread of melanic forms 

 seems to be intimately connected with the pollution of woods by soot 

 in industrial areas. Apparently in unpolluted areas the dark forms 



Fig. 7.1 I Two individuals of Bt'sf on fot'iuZaria, one typical, one melanic, 

 resting on an unpolluted, lichen-covered tree trunk. (After Kettlewell, 

 1958, Proc. X. Int. Cong. Ent. 2.) 



\ 



