The induction of melanism in the lepidoptera and its subsequent inheritance

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Abstract
The authors note that one of the few recognizable contemporary evolutionary processes is the replacement of a light-colored type by a melanistic variety in several species of moths in recent years, apparently in relation to local industrial development. Experiments, using 3 geometrids, intended to determine whether contamination of foliage by metallic salts might be the cause are described. Wild melanics have never been reported in the common Selenia bilunaria; none has been reported of Tephrosia bistortata in England and Scotland although found in Wales, while melanics of T. crepuscularia prevail in many industrial districts in England and on the continent. In preliminary experiments, typical S. bilunaria were fed on plants near munition works. The 2nd generation contained 2 blacks, 5 melanochroics in addition to 84 types. Black behaved as a simple mendelian recessive in later generations on uncon-taminated food. Type T. crepuscularia reared for 2 years on the contaminated plants yielded a brood containing 1 black to 22 types. The black bred as a (homo-zygous) dominant (like the wild melanics of its species). A batch of eggs from type S. bilunaria yielded 70 types and a 2nd generation 59 types. Two 3rd-generation broods were each divided into control and test lots and fed hawthorn from a tree with unusually little manganese content. The hawthorn twigs were treated with lead nitrate for one test lot and with manganous sulphate for the other. One control lot bred true to type for 3 inbred generations, the other was a failure. Both test lots also developed only normals (F1, but the Pb lot gave F2 broods with ratios of 26 types to 1 black, and 29 types to 2 black; and a Pb F3 brood gave 16 types to 3 blacks, the Mn lot gave an F2 brood of 12 melanochroics and 8 blacks. In both cases, in extensive later experiments on pure food, black was transmitted as due to a single incompletely recessive mendelian factor (heterozygotes melanochroic) and crosses showed that the same black factor was involved in both. A different experiment was conducted with type T. bistortata from uncontaminated localities. One strain inbred for 10 generations on pure food yielded only types. The 5th generation on contaminated roadside hawthorn contained 1 black in a brood of 97. Another strain gave a single black barred [female] in the 4th generation, on the roadside hawthorn. Full black was obtained in F2 from this [female] and behaved thenceforth as a simple recessive as did the black of the preceding strain in extensive experiments on pure food. It was proved by crosses that these 2 mutant blacks involved the same factor. The authors consider that the results establish a genuine Lamarckian effect.

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