( xciii ) 



wet and dry, or of Terias senegalensis, Boisd,, $ wet and dry, 

 to see that an increase of dark pigmentation, whether in a 

 diffused or concentrated form, may characterize a dry-season 

 phase as opposed to a wet one. A further difficulty is that 

 dark pigmentation appears in some instances (as in Polyom- 

 matus phlseas, var. eleus) to be an accompaniment of heat, in 

 others (as in Vanessa urticee, var. polaris) of cold. 



" (4) Lastly, with regard to sex. It is no dovibt true that in 

 the majority of instances the female shows a darker coloration 

 than the male. This comes out well in many of the specimens 

 exhibited. But it is not difficult to find examples of the 

 reverse condition, where the female possesses less dark 

 pigment than her mate. This is the case, for instance, with 

 some forms of Colias eurytheme, Boisd., and with Dismorphia 

 thermesia, Godt. 



" It seems therefore difficult to attribute melanism, or the 

 tendency towards that condition, to the immediate action of 

 either geographical or seasonal conditions or to the direct 

 influence of sex. What explanation remains 1 I am strongly 

 disposed to think that in the vast majority of cases the 

 prevalence of dark pigmentation is adaptive, and that although 

 melanism in its various degrees may certainly originate as a 

 variation or a sport, its increase and establishment are to be 

 attributed to some form of selection. Any one of the 

 conditions mentioned may by itself, or in combination with 

 others, favour melanism ; not however by direct causation, but 

 by indirectly leading to the selection of melanic individuals. 

 Thus, there is little doubt that the dark pigmentation is in 

 many cases of advantage as aiding concealment. This is 

 probably the explanation of many instances of dull or dark 

 coloration in the female sex not only of insects but of other 

 animals, it being well known that the female sex stands in 

 special need of protection (Wallace). The writer has observed 

 that the darkly-pigmented female of Behnois seve7-ina, a 

 common African form, is far less conspicuous on the wing than 

 the lighter-coloured male. The dark border indeed of the 

 former sex is often hardly visible, and the general impression 

 given is that of a much dwarfed specimen, though the average 

 female is not really smaller than her mate. An enemy attack- 



