140 PROBLEMS OF GENETICS 



continuous varieties. The question, however, must be asked 

 whether the dark form can reasonably be supposed to have 

 an advantage by reason of their darkness. Some naturalists 

 believe that the darkness of the colours does thus definitely con- 

 tribute to their protection by making the insects less conspicuous 

 and hence more likely to escape the search of birds. In support 

 of this view it may be pointed out that it is in the manufacturing 

 districts of Lancashire and Yorkshire, and again in the London 

 area that the melanics have attained their greatest development. 

 Consistently with this argument also, it is in the neighbourhood of 

 Crefeld and Essen, the black country of Germany, that they have 

 chiefly established themselves on the Continent, and Phigalia 

 pilosaria in the black form is now at home in South Wales. Thus 

 superficially regarded, the evidence looks rather strong, but it 

 is difficult to apply the reasoning in detail. We have first the 

 difficulty that the black form of betularia for instance has estab- 

 lished itself in thoroughly rural districts, notably near King's 

 Lynn in Norfolk, and in the neighbourhood of Kendal and 

 Windermere. The black form of consonaria and the dark 

 consortaria appeared in a wood near Maidstone, far from town 

 smoke, and the black rhomb oidaria was first found at Norwich, 

 which, as towns go, is clean. Then again the spread of the 

 melanics is very irregular and unaccountable. The black pilo- 

 saria is found both in the West Riding and in the Swansea 

 district, but not yet elsewhere. It rapidly increased at Hudders- 

 field, but made no noticeable progress at Sheffield though re- 

 corded there for ten years. It is also a remarkable fact that no 

 similar melanic development has been observed in America, 

 and, so far as I am aware, comparable melanic varieties have not 

 appeared on the European continent except in the case of the 

 few sorts which possibly may have come from England. 



The whole subject is beset with complications. It must 

 not be forgotten that in a few species of moths there is an obvious 

 and recognised conformity between the colours of the perfect 

 insect and that of the soil on which they live, comparable with 

 that which is so striking in the case of some Oedipodidae and 

 other grasshoppers. Of this phenomenon the clearest example 



