( xcii ) 



" (4) As exemplifying the influence of sex, we have Delias' 

 inferna $ and ? as above, B. descombesi, Boisd., D. eucharis, 

 Drury, Terias tominia, Yoll., and T. zita, Feld. 



"It seems obvious that in all these cases there must be 

 some relation or other between the increase of dark pigment 

 and the conditions that have been named. But is the bond 

 between them one of direct causation 1 It will be found that 

 the attempt to rank any one of these conditions as an actual 

 and immediate cause of melanism is attended with great 

 difficulty. 



"(1) Melanism cannot be purely a matter of different 

 climatic conditions dependent on horizontal distribution, 

 because in any given region it often happens that only one 

 sex shows a tendency towards melanism, the other sex remain- 

 ing unaffected. Moreover, as in the case of Belenois mesentina 

 and B. raffrayi, we may have two closely allied butterflies 

 from the same locality, one nearly white and the other almost 

 black. 



" (2) Nor is it easy to consider melanism as a direct 

 consequence of high altitude, for here again it is often only 

 the one sex that is affected, as in G. hryonia\ Besides this, 

 female G. napi of the ordinary kind may be found in the same 

 localities as G. hryonise, ; and ' white ' butterflies which are met 

 with at enormously high altitudes, such as Baltia shawii, Bates, 

 S in Central Asia and Bhulia nymphula, Blanch., in the 

 Andes, are by no means melanic. Again, the female of 

 Tatochila demodice, Blanch., shows dark pigmentation whether 

 taken at high altitudes or at the sea-level, 



" (3) There is obviously a strong connection between 

 seasonal conditions and melanism. This is clearly shown by 

 the series of seasonal forms in the present exhibit. As a rule, 

 the deeply-pigmented form belongs to the wet season, and the 

 paler form to the dry. But here again, if we attempt to make 

 wet-season conditions a direct cause of melanism, we are met 

 by the fact that sometimes the same conditions are associated 

 with an exactly opposite result. The instance of Precis octavia- 

 natalensis and sesamus will occur to every one as a case in 

 point; but without travelling beyond the Pierinx we have 

 only to compare the under-sides of, say, Ixias pyrene, Linn., $ , 



