102 THE CASE OF PAPILIO POLYTES [ch. 



Prussia in 1888 ; to-day it is much more abundant than 

 the older type. There, too, it is spreading eastwards 

 and southwards to Thuringia, to Saxony, to Silesia. 

 What advantage this new dark form has over the older 

 one we do not know 1 . Some advantage, however, it 

 must have, otherwise it could hardly supplant behdaria 

 in the way that it is doing. From our present stand- 

 point two things are of interest in the case of the 

 peppered moth — the rapidity with which the change 

 in the nature of the population has taken place, and the 

 fact that the two forms exhibit Mendelian heredity, 

 doubledayaria being dominant and behdaria recessive 2 . 

 Moreover, mixed broods have been reared from wild 

 females of both sorts, and so far as is known the two 

 forms breed freely together where they co-exist. This 

 case of the peppered moth shews how swiftly a change 

 may come over a species 3 . It is not at all improbable 

 that the establishing of a new variety at the expense 

 of an older one in a relatively short space of time is 

 continually going on, especially in tropical lands where 



1 From the experience of breeders it would appear that the melanic 

 form is somewhat hardier, at any rate in captivity. 



2 Intermediates may also occur in some strains (cf. Bowater, Journal 

 of Genetics, vol. 3, no. 4, 1914). 



3 An interesting case of a similar nature has recently been published 

 by Hasebroek (Die Umsckau, 1913, p. 1020). A melanic form of the 

 moth, Cymalophora or, suddenly appeared near Hamburg in J 904. 

 This new form, to which the name albingensis was given, rapidly became 

 the predominant one. In 1911-1912 over 90 % of the moths reared 

 from caterpillars taken in the open were of the albingensis form ; nor 

 were any intermediates found between it and the typical form. Some 

 experiments were also made which shew that the albingensis form 

 behaves as a dominant to the original type form. 



