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SEXUAL COLOUR-DIMORPHISM IN BIRDS. 



By D. DEWAR, i.c.s. 

 {Read before the Bombay Natural History Society on 

 18th August 1904.) 



Probably more than half the species of birds display sexually dimor- 

 phic plumage. This colour dimorphism varies from an almost imper- 

 ceptible difference, as in many woodpeckers and some parakeets, to a 

 divergence so great that the male and female were originally supposed 

 to belong to different species. As an extreme case of sexual dimor- 

 phism, the Indian paradise flycatcher (Terpsiphone paradisi) may be 

 cited. 



We are still almost completely in the dark as regards the causes of 

 this sexual differentiation, and we are likely to remain so until more 

 light has been shed on the causes which determine the origin of 

 variations. 



It is needless to say that Darwin attributed such dimorphism to 

 sexual selection. His theory is that in the great majority of species, 

 there is competition among the males for females, and that the latter 

 are therefore able to, and actually do, exercise a selection. They are 

 able to pick and choose their mates, and they select the most brilliant 

 of their suitors. Thus have arisen the beautiful plumage and all the 

 accessory plumes of cock birds. 



These decorations have in many cases not been transmitted to 

 females, because natural selection tends to obliterate all conspicuous 

 colours, and in the case of females there is no opposing force, in th 

 shape of sexual selection, at work. It is, however, mere waste of time to 

 enunciate Darvin's theory of sexual colouration, since my listeners are, 

 one and all, doubtless better acquainted than I am with the writings of 

 the most illustrious of naturalists. Wallace declines to accept Darwin's 

 theory, and it seems to me that we have no option but to do likewise. 



There is insufficient evidence (1) of feminine selection, and (2) that 

 females select the most beautiful males. 



In those cases in which females have been known to choose their 

 mates, their selection has been very capricious. 



Darwin, with characteristic fairness, quotes much evidence which 

 goes to show that the female, when she does select, chooses, not the 

 most beautiful, but " the most vigorous, defiant and mettlesome male," 



