On the Seasonal Dimorphism of Butterflies. 6 1 



similar nature. The former are characterized by 

 a strong black dusting of the base of the wings, 

 and by a blackish or green sprinkling of scales on 

 the underside of the hind wings, while the latter 

 have intensely black tips to the wings, and fre- 

 quently also spots on the fore-wings. 



Nothing can prove more strikingly, however, 

 that in such cases everything depends upon the 

 physical constitution, than the fact that in the same 

 species the males become changed in a different 

 manner to the females. The parent form of Pieris 

 Napi (var. Bryonice) offers an example. In all 

 the Pierince secondary sexual differences are found, 

 the males being differently marked to the females ; 

 the species are thus sexually dimorphic. Now the 

 male of the Alpine and Polar var. Bryonicz, which 

 I conceive to be the ancestral form, is scarcely to 

 be distinguished, as has already been mentioned, 

 from the male of our German winter form (P. 

 Napi, var. Vernalis), whilst the female differs con- 

 siderably. 3 The gradual climatic change which 

 transformed the parent form Bryonice into Napi 

 has therefore exerted a much greater effect on the 

 female than on the male. The external action on 

 the two sexes was exactly the same, but the re- 

 sponse of the organism was different, and the 

 cause of the difference can only be sought for in 

 the fine differences of physical constitution which 

 distinguish the male from the female. If we are 



1 See Figs. TO and 14, n and 15, Plate I. 



