212 SEXUAL DIMORPHISM 



phores under nervous influence. Beneath the colouring 

 elements mentioned is a layer of reflecting tissue, forming 

 a more or less continuous "argenteum" underlying the 

 skin. The yellow bands of the sides of the body in 

 the adult are, of course, due, like those in the fins, to 

 an excessive development of yellow pigment. After the 

 breeding season the brilliant coloration of the males fades 

 considerably. The yellow bands fade to a golden-brown, and 

 the blue bands become less intense. Examination shows that 

 the diffuse yellow pigment is reduced in quantity. In the 

 blue parts are noticed aggregations of brownish -yellow 

 matter, apparently derived from the degeneration of black 

 chromatophores. A reduction of the black chromato- 

 phores would, of course, result in a diminution of the blue 

 colour, since the prismatic bodies are blue only when backed 

 by black. 



What explanation can be offered of the evolution of this 

 development of colour in the breeding males ? Mr. Holt 

 considers the colour characters as due to some form of 

 sexual selection, but reasons that the behaviour of the 

 female does not strongly support the view of an sesthetic 

 selection. He concludes that the enlarged dorsals and 

 brilliant colours of the male are probably nothing else than 

 a conspicuous advertisement of his presence. He believes 

 that the male cannot see the female unless she is quite 

 close to him, and that the female would not see the male 

 unless he were conspicuously coloured. But from my point 

 of view it matters little whether the colours are necessary or 

 advantageous, either in attracting the female or rendering 

 the male conspicuous. The colours are due to a change in 

 the male which does not occur in the female, and therefore 

 I hold there must be some conditions which produce the 

 colours in the male and do not act on the female. As both 

 sexes are equally exposed to light in the copulatory ascent 





