THE UNIQUENESS OF THE INDIVIDUAL 



tion experiments prove that his results could not have been due 

 to inadvertent selection. Unfortunately, the results from group 

 (c) raise the new difficulty that improvement was more striking 

 in the line perpetuated by dullards than in the unselected 

 experimental stock; and there appears to have been a general 

 secular improvement in the group (group a) which had not 

 been exposed to the tank tests at all. McDougalPs case must 

 stand or fall by the empirical results, and these have not been 

 confirmed. 



(iii) Melanism in moths. ""The spread in industrial districts of 

 melanic forms of Lepidoptera is . . . one of the most consider- 

 able evolutionary changes that has ever actually been witnessed' 

 (Ford 1940). The change is widespread and has been rapid. 



It was argued by Heslop Harrison (1926, 1928) that melanism 

 is an induced and inheritable adaptive change: food plants in 

 industrial areas were held to be contaminated by metallic 

 fumes, and Harrison claimed to have induced the formation of 

 melanic mutants by feeding larvae of the moth Selenia bilunaria 

 on hawthorn leaves which had absorbed small quantities of 

 salts of manganese and lead. Hughes (1933; cf. also Thomsen 

 and Lemche, 1933) repeated Harrison''s experiments with six 

 generations comprising 3265 individual moths and found no 

 melanic forms among the treated or the untreated; he adds that 

 manganese salts are present in normal plants and are not 

 present to excess in plants of industrial areas. 



There is a clear-cut alternative explanation of the spread of 

 melanism in moths, for which we are indebted to Ford. Melan- 

 ism is a mutant of regular occurrence in many species of 

 Lepidoptera from non-industrial areas; there is therefore a 

 clear case for supposing that such mutants have been selected 

 for their superior viability in the smoke-stained countryside of 

 industrial districts. Indeed, the experience of many workers 

 has been that certain melanic mutants are tougher and more 

 viable than the ordinary paler forms; presumably they have 



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