314 THE STRUCTURE AND LIFE OF BIRDS cHapP. 
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 
