30 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, Vol. [XVI. 



Take, for example, the case of the common sparrow, or better still 

 siuce the nest is built in more open places, the rufous-backed sparrow 

 (Passer pyrrhonotus). Is it possible that the slight amount of sexual 

 differentiation exhibited by the species can render the female so much 

 less conspicuous when sitting on the nest as to render the dimorphism 

 necessary ? 



If we answer this question in the affirmative, how is it that female 

 orioles, ioras, and minivets have been allowed to become so conspicuous ? 

 If it be alleged that these birds build very carefully concealed nests, 

 and the female can therefore afford to wear showy plumage, I would 

 refer to the bulbuls. Both the white-cheeked and red- whiskered forms 

 build open nests in the most exposed situations, such as a raspberry 

 bush, or croton plant — yet the two sexes are alike and far more con- 

 spicuous when sitting on the nest than any cock sparrow would be. 



Speaking of such slight sexual differentiation as that exhibited by 

 sparrows, Darwin says " such differences in colour must be accounted 

 for on the principle of some of the variations in the males having been 

 from the first limited in their transmission to the same sex ; as it can 

 hardly. be maintained that these differences when very slight, serve as 

 a protection to the female." If I have learned anything from studying 

 nature, it is that minute differences of colour are of very small import- 

 ance to a species. 



If every slight variation in the shade of its plumage appreciably 

 affected the chances of a bird in the struggle for existence, there 

 would be none of that diversity of colour exhibited by individuals 

 of the same species. 



As conspicuous examples of species of which the individuals of the 

 same sex vary greatly in colour, I may cite the common bee-eater 

 ( Merops viridis)- and the Madras white-headed babbler ( Crater opus 

 griseus). 



Then, again, many species which have similar habits and live in 

 identical environments, exhibit very great diversity of plumage. 



Wallace writes : " Mr. Darwin has taught us that natural selection 

 cannot produce absolute, but only relative, perfection, and, as protective 

 colour is only one out of many means by which the female birds are able 

 to provide for the safety of their young, those which are best endowed 

 in other respects will have been allowed to acquire more colour 

 than those with whom the struggle is more severe," 



