SEXUAL SELECTION 211 



when it plays about the crowns of the pahn-trees ? We might indeed 

 suppose that they are warning signs of unpalatableness, like those of 

 the Heliconiides or of the gaily coloured caterpillars, but, in the first 

 place, these gay creatures are by no means inedible, and are indeed 

 much persecuted ; and, secondly, the females have quite diflTerent and 

 very much darker and simpler colours. The gleaming splendour of 

 all these birds of Paradise and humming-birds, as well as that of 

 many butterflies, is found in the male sex only. The females of the 

 birds just mentioned are dark in colour and without the sparkling 

 decorative feathers of the males ; they are plain — just like the 

 females of many butterflies. Alfred Russel Wallace has suggested 

 that the explanation of this lies in the greater need of the females for 

 protection, since, as is well known, they usually perform the labours 

 of brooding, and are thus frequently exposed to the attacks ofi 

 enemies. It is undoubtedly true that the dark and inconspicuous \ 

 colouring of many birds and butterflies depends on this need for I 

 protection, but this does not explain the brilliant colours of the males i 

 of these species. Or can it be that these require no explanation 

 further than that they are, so to speak, a chance secondary outcome of 

 the structural relations of the feathers and wing-scales respectively, 

 which brought with it some other advantage not known to us ? 

 Perhaps something in the same way as the red colour of the blood in 

 all vertebrates, from fishes upwards, cannot be useful on the ground 

 that it appears red to us, but because it is the expression of the 

 chemical constitution of the haemoglobin, a body which is indis- 

 pensable to the metabolism, which here has the secondary and 

 intrinsically quite unimportant peculiarity of reflecting the red rays 

 of liofht. 



No one can seriously believe this in regard to butterflies who 

 knows that their colours are dependent on the scales which thickly 

 cover the wings, and the significance of which, in part at least, is 

 just to give this or that colour to the wing. They are degenerate or 

 colourless among the transparent- winged butterflies, and their colour 

 depends partly on pigment, partly on fluorescence and interference 

 conditioned by the fine microscopical structure of a system of inter- 

 crossing lines on faintly coloured scales. The scales of our male ' blue ' 

 butterflies [Lyccena] only appear blue because of their structure, while 

 the brown scales of their mates are due to a brown pigment. If the 

 pigment be removed from the scales of the female by boiling 

 with caustic potash, and they be then dried, they do not look blue like 

 those of the male ; the scales of the male, therefore, must possess 

 something which those of the female do not. 



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