314 THE STRUCTURE AND LIFE OF BIRDS chap. 



required the hen should admire each particular 

 ocellus, or the delicate pencilling of each feather, but 

 only the grandeur of the whole display, since the 

 theory assumes not that her admiration is the cause 

 of the fine plumes, but only the cause of their not 

 being weeded out by Natural Selection. The con- 

 stancy of the colours, and the markings of the feathers 

 as one generation succeeds another, is no doubt a 

 difficulty, since one kind of brilliancy might be as 

 pleasing to the hen-bird as another. But the patterns, 

 as I have said, are simple, and much may be explained 

 by what is called correlated development, of which 

 a good instance, illustrating our present subject, is 

 given by Darwin. In all breeds the males have the 

 elongated feathers called hackles on the neck and 

 loins. In cases where both sexes have a topknot, 

 that of the cock-bird alone consists of hackle-shaped 

 feathers. Thus there is in the cock-birds a tendency, 

 due to causes as yet unknown, to produce hackle- 

 shaped feathers in certain parts of the body. We 

 need not then assume that in these species female 

 taste demands hackles in one place and not another. 

 With regard to the second objection, it may well be 

 imagined that where grand plumage and pugnacity 

 are combined the hen-birds admire the splendour of 

 feathers as the natural accompaniment or corollary 

 of warlike prowess. The third objection was well 

 met by Darwin. That the Robin sings nearly the 

 whole year round does not prove that the power of 

 song was not originally developed to charm the hen. 

 Animals take a delight in the exercise of their powers. 

 A Gull, for instance, delights in its evolutions in the 



