138 PROBLEMS OF GENETICS 



grey, but that of consonaria is a full black, and nothing like 

 either has been found anywhere else. 



These examples are all taken from the Geometridae but 

 others, though of a less conspicuous kind, could be given from 

 the Noctuidae or the Micro-Lepidoptera. Acronycta psi, for 

 instance, has a suffused form which is believed to be becoming 

 more frequent in the London district. Polia chi has two dark 

 forms, olivacea, a yellowish grey with dark markings, and suffusa 

 which is a darker, blackish-slate colour. Both occur in the North 

 of England, sometimes together, sometimes separately, or mixed 

 with the type and many intermediates. The distribution is 

 peculiarly irregular. At Huddersfield, where the very dark form 

 appeared suddenly about 1890, some 30 per cent, are said to be 

 now dark and about 6-7 per cent, very dark, but at Saddleworth, 

 12 miles away, only the pale forms occur. 



Several questions of interest arise in regard to this evidence. 

 This progressive Melanism has arisen in certain families only, 

 and may be confined to certain species only, within those families. 

 As in almost all other examples in which variation has been much 

 observed, its incidence is capricious and specific. A collateral 

 line of inquiry relates to the degree of discontinuity which the 

 variation manifests. Here again there is no rule. Generally 

 speaking, in A. betularia, to take the case most fully studied, the 

 variation is discontinuous. Real intermediates between betu- 

 laria and doubledayaria are in most localities absent or rare. 

 The black spots of betularia may often be larger or more numerous 

 than in the normal, but this variation has nothing to do with 

 doubledayaria, and is not an intermediate stage towards it, 

 though sometimes wrongly so described. Doubledayaria owes its 

 characteristic appearance to a factor which blurs the surface 

 of the wings with a layer of black. Sometimes this blurring is 

 slighter than in the real doubledayaria, and these forms are real 

 intermediates. Occasionally the forewings alone are thus blurred. 

 These intermediates are clearly due to reduction-stages of the 

 doubledayaria factor, and are related to it as a blue mouse is to 

 a black, or a dutch rabbit to a self-colour. It cannot positively 

 be asserted that the full doubledayaria existed before the inter- 



