252 THE entomologist's record. 



rainfall, yet in some species the melanic variation has spread far 

 beyond the area where these conditions prevail. But in most oases, 

 although melanic individuals may become frequent in such normal 

 districts, they do not become predominant ; and when they have 

 largely or completely replaced the type in localities far removed from 

 their place of origin, these localities are generally affected by similar 

 urban conditions. As examples of this may be mentioned the preva- 

 lence of black A. betularia and P. pilosaria in manufacturing districts 

 of south Wales, and of A. betularia in Belgium and Crefeld. 



The relation between melanism and urban conditions has given 

 rise to much speculation and discussion*. It has been pointed out 

 above that we are not justified in assuming that there is an immediate 

 relation of cause and effect between external environment and a given 

 variation of this kind. But it is often stated that, when the dark 

 variety has arisen, it is preserved because it is less conspicuous to 

 enemies, and so replaces the type through a proces of natural selection. 

 There is certainly much to be said in favour of this view. A dark 

 variety is undoubtedly less conspicuous on a smoke-blackened tree- 

 trunk than the paler type, and there are other well-known cases of 

 variation in lepidoptera which seem to be in agreement. For example, 

 ■Gnophos obscurata has, in the New Forest, a nearly black variety, which 

 is said to rest on cut peat, and in the north of England it has a yellow- 

 brown form which rests on the clay cliffs of the coast. Some species, 

 e.g., B. rhomboidaria, have a grey ground when found in chalk 

 districts in the south of England, and are brown when occurring on 

 other rocks. It is not easy in such cases to imagine any other cause 

 than protective coloration to account for these local forms. 



On the other hand, in many species there seems little reason to 

 believe that such protective coloration is likely to be effective. In 

 moths which do not usually rest on trees or walls, but live concealed 

 in grass or herbage, and fly only at night, it seems incredible that the 

 rapid change witnessed can be due to natural selection of colour. 

 There is some reason to believe that colour- variation may be related to 

 constitutional hardiness, e.g., in Angerona jjrunaria it seems that the 

 var. sordiata is less hardy than the type ; and it is conceivable that 

 melanic varieties which have arisen in urban conditions may be con- 

 stitutionally more fitted to the changed environment than the type. 

 Such an idea is at present purely speculative, but might be tested by 

 rearing numbers of both varieties under identical conditions. 



One further point should be mentioned. The species about which 

 information has been collected were chosen because of their frequency, 

 or as illustrating certain special aspects of melanism. But it should 

 be noted that, although melanism is widely distributed among moths, 

 yet it is apparently confined to certain families, and is absent in others; 

 within the family also some species are afi'ected, and others nearly 

 related are not. This subject has been dealt with by Prout {Entom., 

 1904, p. 151). 



In conclusion, while expressing our gratitude to the numerous 

 ■correspondents who have sent information, we wish to emphasise the 

 fact that more evidence is urgently required, especially from the 



* See, especially, Tutt's long series of papers, and discussion in Ent. Record, 

 1890-1893 (reprinted as a separate brochure under the title of "Melanism and 

 Melanochroism in Lepidoptera " (Swan Sonnenschein & Co.j. 



