SEXUAL COLOUR-DIMORPHISM IN BIRDS. 33 



had but little colour on his head. In order to account for the colour 

 of some species, Darwin has to suppose that the taste of the female has 

 undergone a somewhat violent change. This of course is a gratuitous 

 hypothesis totally unsupported by any evidence. 



Sometimes females acquire at a late period of life " certain characters 

 proper to the male. " Darwin himself admits that this phenomenon 

 cannot be explained by any kind of sexual selection. He gives exam- 

 ples, on pages 178-180 of Vol. II of the Descent of Man. He asserts 

 that " the laws of inheritance can alone account for such phenomena. " 

 This is but another way of saying that we cannot explain the 

 phenomena in question. 



As is well known to everybody, some birds assume a nuptial garb at 

 the breeding season. This I would attribute, not to the action of 

 sexual selection, but to the direct effect of the organs of generation 

 upon the general system. 



In this connection the nuptial plumage of the heron tribe is most 

 interesting. It is unnecessary to remind you, that in many of these 

 birds, such as cattle egrets and paddy-birds, both sexes assume nuptial 

 plumage. 



Darwin thus expresses himself upon this subject — 



" Some members of the heron family offer a still more curious case 

 of novelty in colouring, having apparently been appreciated for the 

 sake of novelty. 



The young of the Ardea asha are white, the adults being slate- 

 coloured ; and not only the young, but the adults of the allied Buphus 

 coromandus in their winter plumage are white, this colour changing 

 into a rich golden buff during the breeding season. It is incredible that 

 the young of these two species, as well as of some other members of the 

 same family, should have been specially rendered pure white and thus 

 made conspicuous to their enemies ; or that the adults of one of these 

 two species should have been specially rendered white during the winter 

 in a country which is never covered with snow. On the other hand, we 

 have reason to believe that whiteness has been gained by many birds as 

 a sexual ornament. We may therefore conclude that an early progeni- 

 tor of the Ardea asha and the Buphus acquired a white plumage for 

 nuptial purposes, and transmitted this colour to their young ; so that 

 the young and the old became white like certain existing egrets ; the 



