136 PROBLEMS OF GENETICS 



by Mr. L. Doncaster. 21 From that account and from the state- 

 ments n Barrett's British Lepidoptera 22 this description of some 

 of the more notable cases is taken. 



The most striking and familiar case is that of Amphidasys 

 betularia, of which only the ordinary type was known in any 

 locality until about 1 848-1 850, when the totally black var. 

 doubledayaria first appeared in the neighbourhood of Manchester. 

 This black form was subsequently recorded in Huddersfield 

 between i860 and 1870; Kendal about 1870; Cannock Chase, 

 1878; Berkshire, 1885; Norfolk, Essex and Cambridge about 

 1892; Suffolk, 1894; London, 1897. For the Southern Counties 

 of England, except in the London district, there are still very few 

 records. It cannot of course be asserted positively that the 

 variety spread from its place of first appearance into the other 

 localities, and that it did not arise de novo in them, but there 

 can be little doubt that the process was one of colonisation. 

 On the European Continent the first records are from Hanover 

 in 1884, Belgium 1886 and 1894, Crefeld 188- Berlin 1903, 

 Dresden about the same date. 



As regards the increase of the variety we have the fact that 

 in Lancashire, Cheshire and the West Riding of Yorkshire the 

 black is now the prevalent form ; and in some places, as for example, 

 Huddersfield, the black alone is now found, though it was un- 

 known there till between i860 and 1870. About 1870 at New- 

 port, Monmouth, the two forms were in about equal numbers, 

 but a few years later the type had almost vanished. Similarly 

 in Crefeld, where the black form was still very rare in the eighties, 

 it now forms about 50 per cent, of the population. In the 

 London district the black remains scarce and at the date of the 

 report it was still very scarce. From Ireland there is only one 

 record and there are hardly any from Scotland. 



Boarmia repandata is another species which is behaving in a 

 somewhat similar way. Unlike betularia, however, the species 

 is a variable one, and has several colour-forms, amongst them 

 the banded var. conversaria, and many others. In addition 



21 Ent. Rec, XVIII, No. 7, 1906. 



22 This evidence was largely collected by Mr. G. T. Porritt, who has given, 

 much attention to the subject. 



