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



whiteness having afterwards been retained by the young whilst ex- 

 changed by the adults for more strongly pronounced tints. But if we 

 could look still further backwards in time to the still earlier progenitors 

 of these two species, we should probably see the adults dark-coloured. 

 I infer that this would be the case, from the analogy of many other 

 birds, which are dark whilst young, and when adult are white ; and 

 more especially from the adult of the Ardea gularis, the colours of 

 which are the reverse of those of A. asha, for the young are dark- 

 coloured and the adults white, the young having retained a former 

 state of plumage. It appears therefore that the progenitors in their 

 adult condition of the Ardea asha, the Buphus and of some allies have 

 undergone, during a long line of descent, the following changes of 

 colour : — firstly a dark shade ; secondly pure white, and thirdly, owing 

 to another change of fashion ( if I may so express myself }, their 

 present slaty, reddish, or golden-buff tints. These successive changes are 

 intelligible only on the principle of novelty having been admired by the 

 birds for the sake of novelty." 



This is very magnificent but it partakes of the nature of a piece of 

 special pleading rather than of science. If the nuptial plumage of 

 herons can be explained only on the principle of novelty having been 

 admired by the birds, then it is impossible to account for it. 



It seems to me that the sexual dimorphism of the plumage of certain 

 cuckoos can be explained only on the theory that the organs of gen- 

 eration affect the external anatomy of the bird in some unknown and 

 not understood way. 



These birds do not incubate their own eggs, hence there would 

 seem to be no reason, so far as natural selection is concerned, why 

 the female should not be arrayed in the same kind of plumage as the 

 male. 



Darwin would doubtless say that there is a reason, viz., that the male 

 must tend on account of the tastes of the females to secure brioht 

 plumage, even though it handicap them in the struggle for existence, 

 whereas the females are under no such necessity. 



Unfortunately for the Darwinian theory the sexual dimorphism 

 displayed by some species of cuckoo is very slight. It would, I sub- 

 mit, be absurd to believe that these slight sexual differences are due to 

 the preference of the females for showy males. 



