SEXUAL COLOUR-DIMORPHISM IN BIRDS. 31 



With the first part of the above passage I am in entire accord, but 

 I must most emphatically disagree with the last portion, if it mean that 

 all birds tend to acquire bright plumage but only those which are 

 best endowed for the fight for existence have been allowed to 

 acquire it. 



I do not believe that, when we see a dull hen-bird, we can assert that 

 owing to the great severity of the struggle for existence, the bird has 

 not been permitted to acquire bright feathers. It seems to me that 

 some birds tend to vary in the direction of bright plumage while others 

 do not. 



The mynas are a very successful race of birds, and I do not think 

 that it is in accordance with facts to say that the reason they are not so 

 brightly clothed as kingfishers are, is that they would have perished in 

 the struggle for existence had they been thus gorgeously arrayed. 

 Nor do I think we are justified in saying that the mynas have not 

 sufficient surplus energy for the formation of bright colours or accessory 

 plumes. 



Mynas, rather, are not brilliant birds as regards plumage, because they 

 have not chanced to vary in the direction of bright feathers. When I 

 use the words " chanced to vary ", it must not be thought that I 

 allege that variation is due to chance. I use the expression merely 

 because the laws which govern variation have yet to be discovered. 



As further proof of the comparative insignificance of colour I may 

 cite the conspicuous crows and drongos, which build open nests, but 

 which, nevertheless, flourish like the green bay tree. 



It seems to me that in nature an ounce of good solid pugnacity is 

 worth many pounds of protective colouration so far as the struggle for 

 existence is concerned. 



I do not believe that it is possible to find any one grand cause 

 accounting for all sexual dimorphism. I think it more probable that 

 there are a vast number of factors, working in different degrees on the 

 various species, which have brought about these complex phenomena. 

 Some of these factors have come to light, while others have yet to be 

 discovered. 



The direct action of the sexual organs on external appearance is, I 

 believe, an important factor, and one to which sufficient attention has 

 not been paid. 



