244 SEXUAL DIMORPHISM 



I am maintaining, external influences only excite variation 

 and determine its direction, the result of the variation, the 

 coloration and marking produced may depend very largely 

 on the physiological processes of growth and the development 

 of tissues. 



It is very interesting and important to notice that in a 

 large number of instances the difference in coloration between 

 male and female of the same species is precisely of the same 

 kind as the difference between the upper and lower surfaces 

 of the wings in the individual. This is a correspondence 

 which surely is not without significance. For example, 

 Darwin quotes the following case from Prof. Weismann 

 {Descent of Man, second edition, p. 312) : " The female of 

 one of the Lycsense expands her brown wings when she 

 settles on the ground, and is then almost invisible ; the male, 

 on the other hand, as if aware of the danger incurred from 

 the bright blue of the upper surface of his wings, rests with 

 them closed, and this shows that the blue colour cannot be 

 in any way protective." Now if the exposure, i.e. the light 

 reflected from habitual surroundings, determines the colora- 

 tion, then this difference of habit in the two sexes will 

 explain why the upper side of the wings in the female is 

 brown, in the male not. It does not exactly explain why 

 the upper surface in the male should be blue. But, as is 

 well known, in many butterflies which rest with their wings 

 vertically raised and the upper surfaces in contact, the 

 lower side which is thus exposed in the resting insect is 

 coloured so as to resemble with remarkable exactness the 

 surrounding surface, while the upper surface, exposed only 

 during flight, is conspicuously and beautifully coloured. 

 This is strikingly the case in the famous butterfly Kallima, 

 whose closed wings resemble a dead leaf, while the upper 

 surface is brilliant with orange and other colours. In moths, 

 on the other hand, which rest with the wings depressed, the 



