198 THE FUNDAMENTAL ASPECT OF COLORATION 



reference as to whether true melanism is involved or not, but 

 from what is known of the occurrence of melanin in insects as a 

 group, it is extremely probable that the presence of this pigment 

 accounts for the phenomena just alluded to. Frequent reference 

 occurs in literature to the prevalence of melanic forms of 

 Lepidoptera in the neighbourhood of manufacturing towns and 

 urban areas. They have appeared in species among which they 

 tvere previously unknown, and the melanisation has in some 

 cases increased in prevalence. On the other hand, it should be 

 mentioned that melanic forms also occur in areas far removed 

 from the atmospheric conditions of industrial centres. 



The incidence of melanism in several species appears in some 

 way to be bound up with the industrialisation of certain areas of 

 England. This suggests that the presence of metallic salts 

 contaminating the food-plants in such districts might not only 

 affect the germ-plasm, so as to produce an inherited melanism, 

 but also might stimulate it when once developed to attain its 

 maximum expression. It was on premises of this kind that 

 Harrison and Garrett (1926) and Harrison (1928) conducted a 

 series of experiments with certain Geometrid moths, using strains 

 from localities where melanism in the species involved is unknown. 

 In these exiDcriments the larval food was artificially charged with 

 lead nitrate, manganese sulphate or manganese chloride in the 

 different series. The most conclusive results apply to Selenia 

 bilunaria, material of which was obtained from both England and 

 Germany. A certain proportion of the progeny of these moths 

 proved to be melanic forms, of a type stated to be unknown in 

 Nature, and which did not appear in the controls. It was further 

 shown that the melanism, thus apparently induced, was inherited 

 as a mendelian recessive, the insects being reared upon untreated 

 food. The experiments were carried out on three different strains 

 of the insect, with the application of three different metallic 

 compounds, and melanic forms appeared in each of the series. 

 Results comparable with those of Harrison have not been obtained 

 by McKenny Hughes (1932), who similarly attempted to induce 

 melanism in Selenia bilunaria. Larvse were fed upon hawthorn 

 treated with lead nitrate and with manganese sulphate, and six 



