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MELANISM AND CLTMATAL CONDITIONS. 

 By G. W. Smith. 



Mr. Harcourt-Bath enters upon this subject {ante, p. 97), and 

 contributes some new suggestions as to the origin of melanism 

 in Rhopalocera. He has omitted, however, to take into account 

 the melanic tendencies exhibited by certain British insects in- 

 habiting the environs of our manufacturing towns, and which 

 have been reported to have perceptibly increased during the last 

 fifty years. In the celebrated melanism controversy of 1893, 

 there was only one theory satisfactorily condemned with reference 

 to this particular form of melanism, namely, that of moisture 

 increasing melanic tendencies in insects ; such a view being 

 refuted by the presence of light-coloured species, such as Macro- 

 (faster arundinis, LWiosia nmscerda, &c., in especially marshy 

 districts. We may therefore disregard this idea altogether. It 

 was at that time proved by Lord Walsingham that dark-coloured 

 species were benefited in the struggle for existence by being 

 able to take advantage of intermittent gleams of sunshine ; and 

 Mr. Eobson wrote, "At present we may confidently assert that 

 whatever impedes the direct rays of the sun has a tendency to 

 create melanic forms in Lepidoptera." I think perhaps Mr. 

 Bath has mistaken what he terms the old-fashioned ideas on 

 melanism ; for no one has supposed, since Lord Walsingham's 

 theory, that the absence of sunshine has the direct physiological 

 effect of producing dark coloration : it was generally accepted, 

 according to that theory, that melanic varieties of light-coloured 

 species survived through the agency of natural selection by 

 reason of their being able to take advantage of broken, change- 

 able weather. 



I may here mention Mr. Birchall's view, which appeared 

 before Lord Walsingham's and resembled the latter in principle : 

 " As it appears certain that greater strength of constitution and 

 more powerful and acute perceptive faculties are, from some yet 

 unknown cause, associated with dark colours in the Vertebrata, 

 may we not presume that insects are subject to the same law, 

 and that dark varieties of Lepidoptera are able to spread and 

 increase under adverse conditions, whilst the lighter coloured 

 types fail to do so, and are consequently eliminated in the 

 struggle for life, and that the occurrence of melanic forms may 

 be thus reasonably explained as a simple case of the survival of 

 the fittest." 



Mr. Jenner Weir also, speaking before the London Entomo- 

 logical Society, said : "In the mountains of Switzerland and 

 the Tyrol the clearness of the atmosphere was nearly as great 

 (as in the lowlands of Italy, Spain, &c.), but constantly inter- 

 rupted by dense mists and clouds, and it is precisely in these 



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