36 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, Vol. XVI. 



But there is, I think, a tendency to overrate the action of natural 

 selection in this respect. 



While considering the question of sexual colouring it is important to 

 remember that the struggle for existence waxes exceedingly fierce 

 among the young of a species. The mortality among young birds even 

 after they are fully fledged, is enormous. When a bird has attained 

 the age of sexual maturity, the odds are in favour of its surviving long 

 enough to give birth to a family. Now, in most species, the male does 

 not acquire his full plumage until he is ready to breed. This is 

 precisely what we should expect if these colours and appendages are 

 in some way or other connected with the sexual organs. 



The mere fact that they appear so late allows them to be developed 

 to a greater extent than they could possibly be were the young born 

 like the adult. 



I am aware of the unsatisfactory nature of this paper in that its 

 tendencies are mainly destructive. Throughout it I have tried to 

 destroy, but have made no attempt to create. I can offer no hypothesis 

 in place of those which I seek to disprove. To say that sexual dimorphism 

 in a bird's plumage is in many cases due to some occult action of the 

 sexual organs, is, I am aware, no explanation. If we accept the view 

 herein enunciated we have to admit that in some species the sexual 

 organs exercise little or no effect on the external plumage, while in 

 others the effect is very great indeed. Again, in some the effect is but 

 slight, while in a few cases the effect is considerable, but of short dura- 

 tion. Why this should be, we do not know. But, if I am able to give 

 no explanation of the phenomena under consideration, the theories of 

 Wallace and Darwin display the same defects. The former makes no 

 attempt to discuss isolated phenomena ; it is merely a rough generalisa- 

 tion. The theory of Darwin deals with details, but in a most unsatis- 

 factory manner. Let me, for example, quote Darwin's explanation of 

 the fact that the young of the scarlet ibis are brown while both the 

 adults are red. It is as follows : " When the adults are brightly coloured, 

 we may conclude that such colours have been acquired through sexual 

 selection by the nearly mature males ; but that the transmis- 

 sion, though limited to the same age, has not been limited to the same 

 sex. " When we ask why the one limitation and not the other, the 

 theory of sexual selection remains silent. 



