PROGRESS IN THE STUDY OF VARIATION. 63 



As regards these cases the history is far too complex to 

 be treated as a whole in the present paper. Difficulty arises 

 not only from the great extent of country inhabited by the 

 species and their varieties, but also in the case of napi from 

 seasonal dimorphism. P. napi in this country is too well 

 known to need description. Here as a rule the summer 

 brood do not differ greatly from the spring brood, though 

 exceptionally they may do so. The summer form when it 

 is distinct is called napcea. It is chiefly characterised by 

 the large size of the two spots on the fore wing in the 

 female, and by the great reduction in the amount both of 

 dark venation on the upper surface of the fore wing, and ol 

 green venation on the lower surface of the hind wing. 

 Such a distinct summer form abounds in the lower parts 

 of the valleys in the Alps and of course in most of the 

 warmer regions of Europe. 



At and above 3000 feet or rather less, bryonies may often 

 be met with in those valleys which descend from the high 

 mountains. It is in the female that this variety is clearly 

 marked, being characterised by the presence of bands of 

 brown scales following the nervures or the upper side of both 

 wings, especially in the fore-wing. The male closely 

 resembles the spring male of napi, and is not generally, 

 if ever, distinguishable from it with certainty. Bryonia 

 extends up to 6000 feet. Whether this alpine form ever 

 occupies any considerable area without any admixture of 

 napi I do not know. As regards particular hillsides I 

 think it does, but my experience is that colonies of napi 

 may occur up to nearly 6000 feet, especially near chalets, 

 appearing in summer broods identical with those of the 

 lowlands. That in such places the two forms occupy 

 more or less distinct spots, is, I think, probable. 1 Lower 

 down, however, napi and bryonies may in many places be 

 taken flying freely together, but in such localities, according 

 to my experience, intermediates are of exceptional occur- 



1 It is possible that the two forms in such cases live on different plants. 

 The chief food of bryonies at Tosa is Biscutella laevigata, but both forms 

 eat many kinds of Cruciferse, and I cannot find that the colours are changed 

 by difference in food. 



